# Not another JTM45 build!



## jessemhopkins

This build has been a long time coming. It’s only my second amp build but it’s been a bucket list amp to own since I first played through one of the limited edition offset reissues at a local shop almost 20 years ago (2002). I went with the Mojotone offset head and chassis since it most closely matches my favorite look, and thanks to Neikeel, I’m getting a coffin logo badge to pin on it. I also ordered Mojo’s small parts since it was easier than ordering everything a la cart, but I plan on ordering tubes and Sozo caps from CE Distribution, and transformers are still tbd.



So here’s where I need some input. I have seen some minor differences in schematics, mainly in the power filtering. I have circled all the values I have questions about in the attached schematic, but basically it comes down to the capacitor values in the bias supply (8 or 10 uf?), the values of the last two stages of power filtering (16 or 32 uf?), and the role of the 0.047 non-polar cap just before the standby switch. Even Mojotone’s other jtm45 kit has an additional 32 uf cap coming off of the rectifier. Is that necessary? As I said, this is only my second build so I’m still learning. Go easy!



I’m also looking for recs on transformers and preamp tubes. I’ve already decided on Gold Lion KT66s for the power tubes, but what do people like in the preamp? I’m only looking at new production, not planning to drop NOS money… yet. And transformers? I had decided on Classictone, but I’m open to suggestions as long as it’s something in stock somewhere I can order and have in a week or so.



This is gonna be fun! I’m looking forward to getting started in a few weeks when I have everything in hand and will be doing my best to take my time and get it right.



Cheers!

ps pics attached of some of my hoard













Schematic Quesions



__ jessemhopkins
__ Jul 26, 2020


















IMG_1094



__ jessemhopkins
__ Jul 26, 2020


















IMG_1095



__ jessemhopkins
__ Jul 26, 2020


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## Mars

bias supply: it does not make a particular difference if amp will be properly built and set-up
power caps: 16uF but I prefer 32uF for more headroom and clear sound, this amp is very bassy (I use humbuckers)
put the .05uF (on my schems its .05), regarding the 32uF I have seen it in many builds but I did not put it as it was not on the schematic I used (never had a problem).
For transformers I used merren's but it was a pita to have them and if you care the looks, well you will be disappointed... for preamp tubes in a JTM45 I like JJ, EHX, sovteks... tubes that does not shine in the midrange to maintain sound definition

good luck finding a NOS amber lamp.... its the rarest of the rarest, never saw an original one since I begun hunting vintage Marshall amps stuff 20years ago


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## Far Rider

I like Classictone for the transformers and Mullard for the preamp tubes. I also have Mullard KT66's in my JTM45. Nothing wrong with the Gold Lions though.


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## william vogel

Classictone has good transformers. Marstran are Heyboer, they are excellent. The Marstran power transformer is correct in about every way to the Drake in appearance and function, the output transformer is close in appearance and sounds great. Merren transformers are equal if not better than the Marstran but the power transformer has solder lugs for the heater winding which is more convenient but incorrect in appearance. I would use the Marstran power transformer and choke and the Merren output transformer if you want the best sounding and authentic appearance. The Classictone has flying leads and the wire colors are incorrect but they are good transformers they are also about 1/2 the cost.


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## South Park

I don’t trust all schematics That I find on line. When it comes to filter caps and choke go with original specks on all of it . People who design power supples always go bigger then needed it will last longer . As for tone caps I use a lot of Russian paper in oil caps . Up grade the pots to CTS or clearostat . Keep it all old school . My next build might be a slo 50 with a recto tube if the transformer can handle all the preamp tubes


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## neikeel




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## coolidge56

neikeel said:


>



Are you teasing Mars? Well done! lol


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## neikeel

coolidge56 said:


> Are you teasing Mars? Well done! lol



Not really. Want sure if he meant this or the more common chrome bezel type. 

Broader question is what type of JTM45 is the OP after. The offset cosmetics suggests RS OT and 5881 tubes. IMO the best sounding JTM45 is the 65-66 with Drake transformers with KT66s. 
I agree with Mr Vogel that Marstran transformers are great so I would get his 1202-55 PT an his OT or a Merren OT. The choke is 3H 250mA with DCR of 100ohms and asking as you are that spec it is not too critical. That schem is all good. 10uF bias caps are fine and the 0.047uF snubber is a good anti pop mechanism. 
I have built and restored quite a few of these and am firmly of the conclusion that Piher resistors and mustard caps are well worth using too. 
I suggest TAD selected 12AX7s if you must go new production and get a burned in Gz34 with a warranty. Main issue is early failure with modern ones. 
I always like a good build thread as I am rarely organised enough to make one myself as my building tends to be opportunistic as time permits.


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## Amadeus91

Mars said:


> bias supply: it does not make a particular difference if amp will be properly built and set-up
> power caps: 16uF but I prefer 32uF for more headroom and clear sound, this amp is very bassy (I use humbuckers)
> put the .05uF (on my schems its .05), regarding the 32uF I have seen it in many builds but I did not put it as it was not on the schematic I used (never had a problem).
> For transformers I used merren's but it was a pita to have them and if you care the looks, well you will be disappointed... for preamp tubes in a JTM45 I like JJ, EHX, sovteks... tubes that does not shine in the midrange to maintain sound definition
> 
> good luck finding a NOS amber lamp.... its the rarest of the rarest, never saw an original one since I begun hunting vintage Marshall amps stuff 20years ago


Mine


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## jessemhopkins

neikeel said:


> Not really. Want sure if he meant this or the more common chrome bezel type.
> 
> Broader question is what type of JTM45 is the OP after. The offset cosmetics suggests RS OT and 5881 tubes. IMO the best sounding JTM45 is the 65-66 with Drake transformers with KT66s.
> I agree with Mr Vogel that Marstran transformers are great so I would get his 1202-55 PT an his OT or a Merren OT. The choke is 3H 250mA with DCR of 100ohms and asking as you are that spec it is not too critical. That schem is all good. 10uF bias caps are fine and the 0.047uF snubber is a good anti pop mechanism.
> I have built and restored quite a few of these and am firmly of the conclusion that Piher resistors and mustard caps are well worth using too.
> I suggest TAD selected 12AX7s if you must go new production and get a burned in Gz34 with a warranty. Main issue is early failure with modern ones.
> I always like a good build thread as I am rarely organised enough to make one myself as my building tends to be opportunistic as time permits.



Hey Neil,

Yes, I suppose I’m mixing styles a bit with the offset cosmetics and the later KT66s, but hey, It’s a personal build and rules were made to be broken. Appreciate your thoughts on the transformers, I will inquire about lead times on those brands. As for resistors, I’ve already got a good supply of new production carbon comps that I will be using, and feel good about the Sozos, which I can always swap later if I can acquire some vintage mustard caps.

I do have a stash of unsorted vintage carbon comps I could use, what are your (or anyone else) thoughts on old stock? They're mostly NOS but some used. Presumably Allen and Bradley but I guess I can’t be sure. The seller I bought them from listed them as “tools” so they didn’t really know what they were or the origin. I seem to remember they acquired them from a relative who used to repair radio equipment. It also came with a ton of sprague orange drops, waxy Illinois Capacitor tubular caps, chocolate drops, etc, again mostly NOS but some used. Not planning to use them here, but also not sure what to do with them at all. Open to good ideas!

Thanks again!

Jesse


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## neikeel

I think the Sozos, Mallories and CDEs are all decent caps so see how you go.

I have had mixed results with carbon comps (even NOS AB or the RS ones) and have in a couple of amps had to take them all out as I could not tolerate the hiss.

See how you go. If you use split turrets and lay the components in the slots it will be really easy to swap them over later without the wrestling match that can be involved when you bend the wires and push them into hollow turrets, but that is just my opinion.

The other comment I would make is that if you are not experienced at building these (if you are I apologise!) then use the stock marshall colour codes (green for grids, yellow for cathodes, blue for plates and black for bus and grounds) and wire your bus above the board and use perforated board to loop the wires up to the turrets so you can see the wire colour to each turret.
Marshall used this method from late 68 onwards. You maybe very experienced and very confident and want the clean board look which is cool but thought I should mention before you start.


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## Amadeus91

Will always defer to Neil and the other more knowledgeable
members here, but I think the more carbon comps you use the more
noise you can introduce into the amp. The carbon comps are fine to use in a few selected positions. You also have to measure each one of the 
carbon comps, as they can drift quite a bit. I have only ever used the 
Military spec carbon comps, as they seem to have the tightest tolerances.


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## Rotorcraft230

I built a Valvestorm JTM45 about 4-5 months ago and used the Classictone PT and OT. The amp sounds absolutely beautiful. The Classictones should be in stock somewhere, I got mine at TubeDepot. The Merren and Marstrand has high lead times. Enjoy the build.


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## jessemhopkins

I’m going to give Marstran the old college try, but if I can’t get them by the end of August I’m just going with Classictone. I used them in my deluxe reverb and they sound great.


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## Chris-in-LA

The Metro-spec iron that Metropoulos uses are supposed to have a good reputation as well. I just ordered the same kit as @Rotorcraft230 but went with the Metro-spec. They are more expensive though, about $400 for the whole set. They say they ship in 2 weeks. I ordered everything on 7/22 but have not heard a peep from either dealer.


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## Big Mike

Classictone are fine, but just fine IMO. I prefer the Metro Heyboar or the Merren OT. Haven't tried Marstran but am anxious to do so.

Worse case CT for the power is fine, IMO no tonal difference there, but the lugs on the Merren sure make it easier.

Those are all pretty much 'drake' era. Consider merren or marstran or even Mercury if you want the 'radio spares' early era JTM, which is cleaner and more tweedy.

Caps..Mallory are fine. I actually like them better at higher gain than a lot of things.
I wouldn't waste money on nos mustard or anything like that.
I do tend to prefer ceramic disc caps over silver mica, but that's because I've had a few mica's fail.


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## Big Mike

Chris-in-LA said:


> The Metro-spec iron that Metropoulos uses are supposed to have a good reputation as well. I just ordered the same kit as @Rotorcraft230 but went with the Metro-spec. They are more expensive though, about $400 for the whole set. They say they ship in 2 weeks. I ordered everything on 7/22 but have not heard a peep from either dealer.



They sound good, I built that kit stock from Metro when George was doing them. Several modifications later I smoked the OT cuz I hooked something up wrong. LOL.

I liked it with a 5Y3 and 6V6's as well, but in general I'm not a huge JTM45 fan, I end up switching the OT's out and making them JMP 1986's. mainly it's the 270K mix resistor spec I dislike.


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## Chris-in-LA

Big Mike said:


> They sound good, I built that kit stock from Metro when George was doing them. Several modifications later I smoked the OT cuz I hooked something up wrong. LOL.
> 
> I liked it with a 5Y3 and 6V6's as well, but in general I'm not a huge JTM45 fan, I end up switching the OT's out and making them JMP 1986's. mainly it's the 270K mix resistor spec I dislike.


What is it about the 270k mix resistor that you don’t like, too clean?


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## Big Mike

Chris-in-LA said:


> What is it about the 270k mix resistor that you don’t like, too clean?



Wooly. It's probably the only reason I like the EL34 era better. They don't have that mid bark 'keraaang' the 470's do, even the 1986 bass specs.
Just shifts the mid frequency I guess, but the JTm's always feel kind of muddy and weak low end for my taste.
Plus really my rig/cabs are built around the 50 watt JMP..Lead or bass works fine, (lead is my fave). buckers, scumback M75's and a lightspeed or klone pedal.

The JTM for my ear, I end up liking different speakers, cab, etc. Changes everything. I did run it straight JTM with the 470 mixers and liked it a lot.

But tinkering got the best of me...

For discussion sake...here are the 3 I built on a real 87 50 watt jubilee.
The GT350 Shelby logo'd one is a JMP 1987 spec. 100ki NFB, but metro kit stock layout for the rest (for the most part).
I think I have a .0068 at V1B instead off .022 or .0022. I usually use a 100pF PI fizz cap on all of mine. Subtle but I like it.
The lead spec has 100pF brite cap, the other two none.
The germino 'isn't. It's currently in pieces becoming a 2204. But was my first kit, Metro JTM45, but now a classictone OT, EL34's, full 1987 board.
The mercury was a JTM45, but now is a full purf'ed board, (My wiring ain't pretty like these guys) 1986 Bass spec with Merren OT running 425volts on the plates with a CT power trans.
So my 2nd, 3rd and 4th attempts to like a JTM (The other was a rockytop, good amp).
I always end up making them JMPs.
https://www.instagram.com/p/BjyTSwClIA4/?utm_source=ig_web_copy_link

My favorite all time amp is a Greer Cam18. Sort of a 6V6 spitfire / ac type thing but def has its own sound. I like chime.


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## Chris-in-LA

Valvestorm shipped the JTM45 kit, ordered on 7/22, supposed to be here by 7/29. Seems pretty fast to me.


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## jessemhopkins

I heard from Marstran, who indicated a 4-5 week lead. It’s a step up price-wise from the Classictone but I’m going to try them out. I’ll not get too hung up on authentic looks, I mostly just want a a clean looking and great sounding amp.


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## Big Mike

Good call. Chris's OT's are the best IMO.

Though I haven't tried marstran yet.


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## Big Mike

Chris-in-LA said:


> Valvestorm shipped the JTM45 kit, ordered on 7/22, supposed to be here by 7/29. Seems pretty fast to me.


He's a good dude. Pretty much where I buy all my parts from unless small one off stuff on ebay.


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## neikeel

I have Marstrans in 4 of my ‘keeper’ amps. The rest have originals. Make of that what you will. 

The 270k mixer one is interesting. I have had two amps in particular that had 270k mixers. Both had lots of grind and sounded very good each bypassed with 556pF cap on bright channel. When I replicated each of them with NOS components they both needed to have 470k mixers. 
I have just worked out why having restored others in the meantime. 
It is the pots!
The old RS 1meg pots on the volumes are around 1M2 when measured. So when added to the mixer the value is around 1400 to 1500k. 
I suggest you try for yourself, I suspect you will have Alpha pots which are nice and smooth and are usually 0.9-1M when tested. See if you can use high testing ones on the volume pots or failing that use 330k or 470k mixers.


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## Big Mike

Worth checking into. I don't use the alphas, I was using PEC at the time. CTS now for everything.


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## jessemhopkins

More questions for the audience.

Does anyone have a line on the era-correct style knobs for the coffin logo heads?

What type of wire are you all using in similar builds (links appreciated)? I have a ton of this wire and I absolutely hate it. It's pvc coated and then cloth wrapped, not at all like the vintage fender stuff I thought I was getting. It's so thick and difficult to work with, but I'm not too keen on getting a bunch more until I use it up. Trying not to get hung up on everything being 100% era correct, but I do think I'd like to get some more appropriate wire for this build.


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## jessemhopkins

Here's another one- what trim pot are others using in their builds? I got this part from Mouser but others seem to have one where the wiper is connected to two legs that fold out and get connected to ground turrets. Or am I misreading it?


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## Rotorcraft230

jessemhopkins said:


> Here's another one- what trim pot are others using in their builds? I got this part from Mouser but others seem to have one where the wiper is connected to two legs that fold out and get connected to ground turrets. Or am I misreading it?




You’re close here is what Valvestorm uses and here is a pic of my installation. Instead of the center lug it has 2 side lugs and they get soldered to the turrets with the caps with one leg going to the cap and the other leg is what I believe is the bias resistor


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## Rotorcraft230

The horizontal mount is the one that has the legs on the sides you have to bend them up to solder to the turrets


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## Rotorcraft230

Page 4 about 1/2 way down of my build thread I talk about the bias pot if you’re interested


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## jessemhopkins

Rotorcraft230 said:


> Page 4 about 1/2 way down of my build thread I talk about the bias pot if you’re interested


Took a bit of doing, but I found some with matching termination style on mouser. All good! Did you buy any additional wire or did you just use what came with your kit?


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## Chris-in-LA

jessemhopkins said:


> More questions for the audience.
> 
> Does anyone have a line on the era-correct style knobs for the coffin logo heads?
> 
> What type of wire are you all using in similar builds (links appreciated)? I have a ton of this wire and I absolutely hate it. It's pvc coated and then cloth wrapped, not at all like the vintage fender stuff I thought I was getting. It's so thick and difficult to work with, but I'm not too keen on getting a bunch more until I use it up. Trying not to get hung up on everything being 100% era correct, but I do think I'd like to get some more appropriate wire for this build.



I use what comes with the kits. The wire that comes with the Valvestorm kits is easy to work with and is like regular Marshall style. They sell it in a pack of multiple colors. I believe that my Mojotone tweed kits came with the regular cloth covered wire, not sure if they sell wire but you can check. 

Valvestorm sells this for $14.55: Plexi Amp Wiring Kit 
12 feet of Red and Black. 7 feet each of Brown, Orange, Yellow, Green, Blue, Purple and White.


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## Rotorcraft230

Chris-in-LA said:


> I use what comes with the kits. The wire that comes with the Valvestorm kits is easy to work with and is like regular Marshall style. They sell it in a pack of multiple colors. I believe that my Mojotone tweed kits came with the regular cloth covered wire, not sure if they sell wire but you can check.
> 
> Valvestorm sells this for $14.55: Plexi Amp Wiring Kit
> 12 feet of Red and Black. 7 feet each of Brown, Orange, Yellow, Green, Blue, Purple and White.
> 
> View attachment 77186


I’d buy that wire. But watch out if your sourcing it from somewhere else Jesse, there is 300V and 600v rated wire. Definitely want the 600V. I found the 600V wire at Digikey but they were selling it in100 foot spools I would never need that much of one color that’s why the Valvestorm source is the best.
Mark


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## jessemhopkins

Chris-in-LA said:


> I use what comes with the kits. The wire that comes with the Valvestorm kits is easy to work with and is like regular Marshall style. They sell it in a pack of multiple colors. I believe that my Mojotone tweed kits came with the regular cloth covered wire, not sure if they sell wire but you can check.
> 
> Valvestorm sells this for $14.55: Plexi Amp Wiring Kit
> 12 feet of Red and Black. 7 feet each of Brown, Orange, Yellow, Green, Blue, Purple and White.
> 
> View attachment 77186


Ace, that looks like just the ticket! I got the Mojotone small parts kit, which came with a bit of wire but not all the colors to do the proper Marshall color scheme. That’ll get me enough.

Yeah, the stuff I’ve already got is 600v rated. It’s just awful. Really wish I hadn’t bought so much, but I’ll find ways to use it by and by.


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## Rotorcraft230

I’ve tried that “Vintage “ cloth wire stuff and hate it as well. I never use it. The Valvestorm wire they call it “TopCoat” which is a name brand I think, I’ve seen other people throw that TopCoat around. It’s very nice. Stranded and presoldered.

edit- I’m listening to Buckethead


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## jessemhopkins

Rotorcraft230 said:


> I’ve tried that “Vintage “ cloth wire stuff and hate it as well. I never use it. The Valvestorm wire they call it “TopCoat” which is a name brand I think, I’ve seen other people throw that TopCoat around. It’s very nice. Stranded and presoldered.
> 
> edit- I’m listening to Buckethead


Hang on, is that stranded core? Is that what you used in your build? I assumed most builders preferred solid core.


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## Rotorcraft230

It’s stranded, With it being pretinned it is easy to work with and it holds its shape well if you want it to hold that shape. Valvestorms component selection is the best. 
Have you seen George Metropoulos YouTube videos? He is obsessed with the Marshall sound and only picks the best.


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## Chris-in-LA

I’ve used solid core cloth covered wire for tweed heaters, it is a little difficult to use. Stranded wire in general is definitely easier to work with. I actually like using cloth covered stranded wire.


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## neikeel

+1 on that wire.
I cannot get anything as good in the U.K. - it is good because of the pretinning it holds its shape very well. I would only use solid core on a Hiwatt resto (again if I could find correct gauge/rating - which I cannot). Of course I have to import the wire from Valvestorm in USA with taxes and fees which is a bit daft but best stuff IMO


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## jessemhopkins

Slight change in plans, I've got all Merren iron headed my way, courtesy in part of a fellow forum member. I should have that all within a couple weeks, then glass, caps and a few other bits soon after that. I'll probably get started end of August or early September.


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## Big Mike

the Merren OT's are fantastic. Haven't used the PT's.


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## Guitar-Rocker

I use the ValveStorm wire on all of my builds, it's the best that I have used.


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## Chris-in-LA

Got my Valvestorm JTM45 order today. Still waiting for the Metrospec transformers and need to order a cabinet.


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## Rotorcraft230

Chris-in-LA said:


> Got my Valvestorm JTM45 order today. Still waiting for the Metrospec transformers and need to order a cabinet.
> 
> View attachment 77258


Nice, are you gonna get started without the PT and OT?

nice work bench by the way. I like the window you can look out. Is it in the house or is that a garage? Here is mine. Don’t have a window but got a 65” Samsung smart out of the trash. New mother board fixed it for 70 bucks. 
Enjoy the build


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## Rotorcraft230

That’s and old pic. Here is today. I have the flag over the TV when I’m not watching it.


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## jessemhopkins

PT and choke landed today! Also- measure twice, cut once. The cutout on my chassis is for a smaller PT so I'll have to mod it a bit . Oh well, it will give me something to do until everything else comes in!

Thanks to Amadeus91 for the hookup!













Merren PT and choke



__ jessemhopkins
__ Aug 1, 2020


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## _Steve

jessemhopkins said:


> Merren PT and choke
> 
> 
> 
> __ jessemhopkins
> __ Aug 1, 2020



Wow that thing looks vintage (in a good way)!


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## jessemhopkins

_Steve said:


> Wow that thing looks vintage (in a good way)!


Yeah, I would definitely call this "big boy iron"


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## jessemhopkins

Are there any guidelines on how to space the power tube sockets? Annoyingly, this chassis does not have a cutout to hang the 32 + 32 can cap between v5 and v6 and instead is designed for both cans to mount against the wall next to the pt. Since I'm already gonna have to cut it, I might as well fix that too while I'm at it.


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## Trouble

I would be tempted to mount them to the wall, but now is the time to change it. if you desire.


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## jessemhopkins

Travis398 said:


> I would be tempted to mount them to the wall, but now is the time to change it. if you desire.


Yeah, I’m concerned I won’t have clearance above the pt, but if I do then I’ll just leave it be. I’ll have to test fit it all when I get the OT.


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## Trouble

I see what you mean, that Transformer does look big.


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## Chris-in-LA

Rotorcraft230 said:


> View attachment 77271
> 
> Nice, are you gonna get started without the PT and OT?
> 
> nice work bench by the way. I like the window you can look out. Is it in the house or is that a garage? Here is mine. Don’t have a window but got a 65” Samsung smart out of the trash. New mother board fixed it for 70 bucks.
> Enjoy the build


That’s inside my garage and you know that I started working on it. I’ll probably put in the tube sockets and maybe wire the preamp heaters. I started populating the board today. I did this wiring yesterday. 

https://www.marshallforum.com/threads/yes-another-jtm45-build.115247/#post-2013317


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## Big Mike

Looks good.
I bet you have room to do the cans inside. Which chassis are you using?


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## neikeel

I thought that was weird, then I realised you went Mojotone and not Valvestorm


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## jessemhopkins

Big Mike said:


> Looks good.
> I bet you have room to do the cans inside. Which chassis are you using?


It’s Mojotone, I’m pretty sure it won’t fit. But we’ll see.


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## jessemhopkins

Hey everyone. Got my OT on the way now. Since the cab is an offset look but I'm doing a mid-60s style electronically, Chris Merren is doing a Drake style OT in a hammertoe shell, so it will look like an RS. Maybe that's sacrilegious to some of you but I'm excited.

Does anyone know where I can purchase a mounting bracket for the PT?


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## neikeel

I'm sure the hammered finish OT will look good.
I usually get the aluminium mounting ring when I order PTs from Marstran
You can get them elsewhere.


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## neikeel

I'm sure the hammered finish OT will look good.




Do you mean one of these?

They are $16 from Valvestorm.


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## Amadeus91

neikeel said:


> I'm sure the hammered finish OT will look good.
> I usually get the aluminium mounting ring when I order PTs from Marstran
> You can get them elsewhere.


Yes Neil, the finish does indeed look very nice.
Here is a Merren RS OT.


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## neikeel

Amadeus91 said:


> Yes Neil, the finish does indeed look very nice.
> Here is a Merren RS OT.


Is Chris doing the RS OT again? He stopped offering them for a while (I thing there was a problem with lamination supplies?).
I used a Marstran RS in my split front clone and made a paxolin turret board instead of wires.







Sorry about the photobucket smear!


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## jessemhopkins

neikeel said:


> I'm sure the hammered finish OT will look good.
> 
> 
> 
> 
> Do you mean one of these?
> 
> They are $16 from Valvestorm.


Yep, that’d be it. I’ll snag one, thanks! Been struggling in my head about how I’m going to modify the chassis in order to make everything fit, but I think I just figured it out. This forum and everyone’s help has been a godsend!


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## Amadeus91

neikeel said:


> Is Chris doing the RS OT again? He stopped offering them for a while (I thing there was a problem with lamination supplies?).
> I used a Marstran RS in my split front clone and made a paxolin turret board instead of wires.
> 
> 
> 
> 
> 
> 
> 
> Sorry about the photobucket smear!


No Neil, he is still not making them.
This one was purchased years ago and is from my spare parts inventory.
Went with a real RS OT instead.


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## jessemhopkins

Curious what the group's take is here. I finally got my ot from Merren and enlarged the opening for the pt so this one will fit. Upon dry fitting, I discovered that they are right on top of each other. Is that concerning?


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## neikeel

They do look a bit closer than normal. 
Can you oval the PT holes to nudge one way and the OT the other way?
You might get 5mm extra clearance that way. 
How big is the Merren PT?


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## jessemhopkins

I can move the ot, not the pt. It's this chassis, they layed it out so horribly!


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## jessemhopkins

Well, I have finally concluded that this Mojotone chassis isn't going to work. If I were using the Mojotone iron (Heyboer) it'd be alright but with the bigger Merren iron there are too many issues. Looking for suggestions for an offset chassis, but I may wind up using a blank and punching everything out myself.


----------



## Chris-in-LA

neikeel said:


> I have Marstrans in 4 of my ‘keeper’ amps. The rest have originals. Make of that what you will.
> 
> The 270k mixer one is interesting. I have had two amps in particular that had 270k mixers. Both had lots of grind and sounded very good each bypassed with 556pF cap on bright channel. When I replicated each of them with NOS components they both needed to have 470k mixers.
> I have just worked out why having restored others in the meantime.
> It is the pots!
> The old RS 1meg pots on the volumes are around 1M2 when measured. So when added to the mixer the value is around 1400 to 1500k.
> I suggest you try for yourself, I suspect you will have Alpha pots which are nice and smooth and are usually 0.9-1M when tested. See if you can use high testing ones on the volume pots or failing that use 330k or 470k mixers.


Both my JTM45 kits have 270k mixers and sound pretty good. But the Ceriatone kit has a 100pf treble cap and it has more sizzle than the Valvestorm kit, which has a 500pf cap. I also have two tweed Super kits with 100pf treble caps. Would going up to 500pf on these kits reduce some of the high end sizzle? @neikeel


----------



## jessemhopkins

Well gang, a million months later and I'm finally getting started. I was hoping for some input from those of you who have used Merren transformers before, I don't have a wiring diagram for the OT so I'm a little in the dark about which color is which. Looking at it with a friend, we worked out that the white/brown/red side is the primary and the green/orange/yellow side is the secondary, and that on the secondary either the green or orange is the common. Can anyone confirm? I'm going to try to phone Chris Merren in the morning but he's been a bit tough to track down, so if one of you knows it would be a huge help.













Merren OT Leads



__ jessemhopkins
__ Dec 27, 2020


----------



## william vogel

Primary:
Brown is the center tap, hook it to the B+
Red goes to V4 plate
White goes to V5 plate

Secondary:
Yellow 0
Orange 8 ohm
Green 16 ohm


----------



## jessemhopkins

william vogel said:


> Primary:
> Brown is the center tap, hook it to the B+
> Red goes to V4 plate
> White goes to V5 plate
> 
> Secondary:
> Yellow 0
> Orange 8 ohm
> Green 16 ohm



Ace! Thanks so much


----------



## william vogel

jessemhopkins said:


> Ace! Thanks so much


You’re welcome 
Just FYI 
Chris used to use a black wire instead of the white just like an original drake 784-103 but he quit buying the black wire and if it had a blue wire it would be the 100 volt line tap. You’re really going to like the transformer, I’ve used many of Chris’s transformers.


----------



## jessemhopkins

Here we go!













Starting



__ jessemhopkins
__ Dec 29, 2020


----------



## jessemhopkins

That's heaters done













Heaters wired



__ jessemhopkins
__ Dec 29, 2020


----------



## william vogel

Just wondering why you started with green wire. I get it when the power transformer has green flying leads but you’ve got a Merren with solder lugs.


----------



## jessemhopkins

william vogel said:


> Just wondering why you started with green wire. I get it when the power transformer has green flying leads but you’ve got a Merren with solder lugs.



That's a great point, I didn't even think about it. I just started following the Metro pdf. Oh well, it's done now. I won't be that muck of a stickler.


----------



## jessemhopkins

Well shoot. The choke lead won't reach the filter cap. Any ideas?













choke lead



__ jessemhopkins
__ Dec 29, 2020


----------



## Chris-in-LA

jessemhopkins said:


> Well shoot. The choke lead won't reach the filter cap. Any ideas?
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> choke lead
> 
> 
> 
> __ jessemhopkins
> __ Dec 29, 2020


Extra wire and heatshrink time.


----------



## StuC

Wire it the same way as the later JTM45s: choke and centre tap of the OT to the corner turret on the board, and run a separate wire from there to the filter cap.

Edit: obviously, if you do this, angle the bias diode over to the next turret and don't link those two end turrets.


----------



## william vogel

^^^^^^THIS
I wire all plexis this way. It gives you a good point to measure B+ too.


----------



## jessemhopkins

Wiring is complete! It took a lot longer than I would have liked, and I had to make a number of compromises along the way. Lots of frustrations added by the Mojotone chassis and turret board, the layouts of which are completely their own design and different enough from the original to drive you crazy if you’re following the original layout. But it’s done. Seeing as how it’s now 1:18 am, I’ll commence testing and give it a play tomorrow. Very excited.












Wiring complete



__ jessemhopkins
__ Jan 2, 2021


















5AC8EB9D-F3EA-466A-AF53-9CE0090A031F



__ jessemhopkins
__ Jan 2, 2021


















DC40E692-D91C-4A4D-B5FB-C4B02D31C41C



__ jessemhopkins
__ Jan 2, 2021


















FFA095A2-F326-4F50-9F32-506984F91450



__ jessemhopkins
__ Jan 2, 2021
__ 2


----------



## neikeel

Well done
Fingers crossed for testing - take it slowly!


----------



## jessemhopkins

My B+ is running very high and heaters are a bit high as well.


----------



## neikeel

I presume that is with just the rectifier in?

Expect a big drop (20%) when you get all the tubes in, if heaters are below 7v like that you are probably in the right ballpark. If more than that double check that you use the correct tap on the PT. I presume you are 120v?
Another quick cross reference is to measure the HT off the PT, you should have a smidge over 700vac across the HT (maybe 0-355 between CT and each half if your meter is limited to 600v).


----------



## jessemhopkins

Ok, thanks @neikeel. When I first fired it up in standby with only the rectifier installed, I was getting a reading on pins 5 of the output tubes. Now I am not. According to testing step D the Metro documentation:

"You should also measure approx. -45V DC on V4 and V5, pin 5 (with the bias pot adjusted fully clockwise)."

When I _did_ get a reading, it was -75 fully clockwise and about -48 fully ccw, so it was out of spec. But now I am not getting a reading at all. I buzzed out the circuit by the schematic and everything appears wired correctly. Any ideas where to check? Start lifting legs and checking components?


----------



## neikeel

Take your meter.
ac first check for 350vac on the rectifer white wire
follow it to the end of the 220k - should still have 350vac
next you expect more like 100v after the 220k (where the diode starts)
flip to dc on meter
move probe to other side of diode - expect lower dc volts
then trace to the junction of the 220k in the pi (slightly hidden by your 0.1uF caps due to the interesting board design)
expect 70vdc there
then to pin 5 of the outputs
somewhere in that line you will be losing power.


----------



## Trouble

well said


----------



## jessemhopkins

neikeel said:


> Take your meter.
> ac first check for 350vac on the rectifer white wire
> follow it to the end of the 220k - should still have 350vac
> next you expect more like 100v after the 220k (where the diode starts)
> flip to dc on meter
> move probe to other side of diode - expect lower dc volts
> then trace to the junction of the 220k in the pi (slightly hidden by your 0.1uF caps due to the interesting board design)
> expect 70vdc there
> then to pin 5 of the outputs
> somewhere in that line you will be losing power.



It’s on the other side of the diode where I’m losing voltage. I’ve got 225V ac on one side, about 1.3 v dc on the other. Diode is 1kv 1N4007. I need to further investigate the bias circuit, but as best I could tell everything is in its right place.


----------



## Guitar-Rocker

In the photo, shouldnt you have a above board jumper from the bottom of the diode to feed the 15k resistor?

If not how does the DC volts get to the rest of the bias circuit


----------



## jessemhopkins

Guitar-Rocker said:


> In the photo, shouldnt you have a above board jumper from the bottom of the diode to feed the 15k resistor?
> 
> If not how does the DC volts get to the rest of the bias circuit


It’s underneath the board


----------



## Guitar-Rocker

If you are measuring AC voltage at the node of the diode and 220K resistor, referenced to ground, and no DC volts at the bottom side of the diode, (non-banded side of the diode), referenced to ground, then the diode is toast


----------



## neikeel

I agree with Guitar-Rocker. Remember those diodes are sensitive to over-heating when soldering so good, quick, technique is important.
I don't think that you need to investigate much further - if as you say that you had volts in the right region initially.
Common mistakes are missing the ground on the bias caps. reversed bias caps, damaged diodes or under board links gone AWOL.
From your preliminary measurements you may need to sub in a 56k or even 47k for that 68k that you have in series with your pot (I use 50k pots to give me more leeway with tube swapping).


----------



## jessemhopkins

Here's a more accurate recording of the voltages:

HT Winding 400 V DC / 153 V DC
5v Winding- 215V AC (!!)
B+ 546 V DC
Node of 220k Resistor and Diode 148 V AC
Node of Diode and bias cap -0.011 V DC

1. It's crazy that I'm getting those readings on the 5v winding, no?
2. The HT Fuse popped. Would that cause the lopsided voltages on the two sides? I think it may have popped around the time I lost voltage to the bias supply.
3. The low side of the HT winding is the one that is presently wired to the bias supply, which I think is the culprit for why I'm not getting anything to the bias supply, not the diode. I've had 3 diodes on hand and they've all given the exact same reading. Nonetheless, I will order some more.
4. As I said before, when I did have voltage on the bias supply it seemed out of spec (-75v fully clockwise and about -48 fully ccw) so I will play with the value of the resistor in series with the trimmer. What range should I be aiming for? According to the metro documentation, it should be approx -45V DC when fully clockwise, which I currently can't even reach when all the way down.

To my untrained eye, it would seem that I've got a problematic pt. Thoughts?

I'm bummed I wasn't able to play on my new amp this weekend, but I'm sure it will all get sorted soon enough.


----------



## william vogel

Remove all the tubes. All switches in off position. Turn on power switch only. Measure AC voltages at the power transformer solder lugs. Measure the 5 volt, meter leads to the lugs only. Measure the ht. Meter black lead to center tap, red meter lead to each of the ht taps. Measure 6.3. Black to center tap and red to each tap. What do you have. 
The Merren transformer should have about 5.2 volts @ 5 volt taps 
369-329-0-329-369 @ HT
3.29-0-3.29 @ 6.3
I’ve used many of this transformer but mine never get the gold sticker.


----------



## jessemhopkins

137 and 139 VAC on the 5V winding
363VAC on both sides of the HT from the CT
3.27VAC on both sides of the 6V winding from the CT


----------



## Guitar-Rocker

Any chance the red wires on pin 8 and pin 4 are swapped, assuming there is a red wire on 4?

With no rectifier tube installed. Pull the two yellow wires off of the socket into the air and then read voltage on the again.

Then try lifting both red HT wires into the air and read voltage across them too. That will verify the PT


----------



## jessemhopkins

Guitar-Rocker said:


> Any chance the red wires on pin 8 and pin 4 are swapped, assuming there is a red wire on 4?
> 
> With no rectifier tube installed. Pull the two yellow wires off of the socket into the air and then read voltage on the again.


No, it is all correct. Just replied to your comment directly on the photo.


----------



## Guitar-Rocker

Yeah was trying to edit that after looking closer at your photo, but fat fingered it. Sorry.


----------



## Guitar-Rocker

If you've rechecked the wires, pull the rectifier and read voltage at the pins at the socket again. The rectifier supply should be @5 volts, not above 100v


----------



## jessemhopkins

Guitar-Rocker said:


> Yeah was trying to edit that after looking closer at your photo, but fat fingered it. Sorry.


No worries, it was worth verifying. I am now satisfied with the HT voltage after realizing I should have been measuring from the CT and not from ground, but even after pulling the wires from the 5v winding it is still reading 137 and 139 from ground.


----------



## Guitar-Rocker

read your 5 volt yellow to yellow, not ground. That coil on the PT has no ground reference


----------



## jessemhopkins

‍[facepalm] Duh! 5V. I'm an idiot.

I also just realized something- with the blown fuse, the CT has no ground reference. That may account for my other issues. I'll have to wait a few days for the fuses to come in and verify.


----------



## 2L man

jessemhopkins said:


> ‍[facepalm] Duh! 5V. I'm an idiot.
> 
> I also just realized something- with the blown fuse, the CT has no ground reference. That may account for my other issues. I'll have to wait a few days for the fuses to come in and verify.


I think it is wrong place to install fuse to CT side and now it did confuse you. Fuse should be on operative voltage circuit just after rectifier tube.

Also on first page schematic mains fuse is on neutral? Does Your three prong mains blugs have polarity? If not then it is fine placement because 50% plug installations it would come to live and 50% to neutral but if mains plug has polarity I think it would be better on live?

I know guitar tube amp tube rectifier circuits often do not have current limiter resistors but when there is no Choke just after tube rectifier the resistors should be there on both anodes. Yes your schematic has a choke before power tube screens but current there is so small that it does not protects rectifier tube and choke first role there is to filter voltage which lessens hum.

Current limiters lengthens rectifier tube life and also increase "sag" which I see only reason to use tube rectifier in the first place. Resistors act as RC circuit with first filter capacitor although there is tube in between them.

If you don't believe me look rectifier tube datasheet which good ones have example voltage/current craphs for both RC filtering and LC filtering. Filter capacitor size also has a maximum capacitance limit.


----------



## neikeel

There is a choke after OT CT and first node filter cap after rectifier.
Personally I always find a matching double pole mains switch and fuse live but switch neutral and live at the same time.
Maximum first filter cap after the Gz34 is around 60uF and the kits usually have 32uF. The jtm45ri uses 50uF first node. The GZ34 is a slow start rectifier so that application is rarely an issue.
Regarding the HT fuse on the CT this is a simple set up, again modded later with the SS rectified models to be in line with the HT feed from the diodes. It works ok enough in this situation (I’m happy to use it this way but if designing my own I would probably fuse in the ac lines into the rectifier. 
Worth reading Valvewizard’s take on this.


----------



## jessemhopkins

Boy, those fuses are hard to come by. I ran around to a few places on my lunch break today and couldn’t find a 500ma fuse anywhere. I guess I’ll just have to be patient.


----------



## Trouble

jessemhopkins said:


> Boy, those fuses are hard to come by. I ran around to a few places on my lunch break today and couldn’t find a 500ma fuse anywhere. I guess I’ll just have to be patient.



I had the same fuse issue a little while ago......I ordered extra just to make sure it doesn't happen again. 

The dim bulb tester can be your friend


----------



## 2L man

neikeel said:


> There is a choke after OT CT and first node filter cap after rectifier.
> Personally I always find a matching double pole mains switch and fuse live but switch neutral and live at the same time.
> Maximum first filter cap after the Gz34 is around 60uF and the kits usually have 32uF. The jtm45ri uses 50uF first node. The GZ34 is a slow start rectifier so that application is rarely an issue.
> Regarding the HT fuse on the CT this is a simple set up, again modded later with the SS rectified models to be in line with the HT feed from the diodes. It works ok enough in this situation (I’m happy to use it this way but if designing my own I would probably fuse in the ac lines into the rectifier.
> Worth reading Valvewizard’s take on this.



Perhaps a Choke after power tube anode current take does not protect rectifier tube much because most current goes to power tubes!

This photo is from old GZ34 datasheet back when tubes are said to be better than current tubes and there for 350VAC CT is recommended minimum 100 ohm resistors to both anodes when only capacitor filtering is used. Current limiter resistor lengthens the time the rectifier tube delivers current when they "flatten" the sine wave peaks and that is how it makes rectifier tube last longer when peak current every 120Hz does not come too high.

Choke after rectifier does the same but it stores energy to its coil and then resistirs are not needed.

Rectifier slow startup was designed so that other tubes get heated ready before high voltage rise but obviously it limits starting current peak as well.


----------



## william vogel

One thing you have to remember on the series resistance to the anodes of the rectifier tube is that the power transformer secondary resistance is part of the resistance and the reflected resistance from the primary is also part of the equation. Most marginal transformers such as that used by guitar amplifiers have enough series resistance in the windings. A very strong transformer with high current reserve would stress a rectifier tube without added resistance.


----------



## 2L man

Transformers rating should mean that voltage comes about advertised when it delivers advertised current!

It is more common subject that custom build operative voltages come higher than lower what schematics show and there current limiter resistors can give some help.

If "sagging" is wanted feature it comes better when tube rectifier is used with current limiter resistors. If not I don't see the any use for tube rectifier.


----------



## jessemhopkins

Sure enough, the fuse was the solve. I have 415v on the b+ and the negative bias ranges from -75 to -47. Proceeding with the test.


----------



## jessemhopkins

Ok, so here's where we're at:

With all the tubes in place B+ is running at 470V, and so plate voltages down the line are hot by about 20-25% according to the metro documentation. I was able to bias the output tubes in range (38 and 40 mA) with room to spare, so I think the values in the bias circuit are good as is. I went ahead and gave it a play for about 6-7 minutes... when it started to hum and crackle and the ht fuse popped again.

I don't know the expected specs well enough like you all to know if the hot voltages are especially concerning or if they're in line with the pt I'm using, but obviously I've got something going on to cause 2 fuses to pop like that. Where should I start??

FWIW it was remarkably quiet and sounded pretty fantastic for the few minutes I was able to play.


----------



## william vogel

Are you using a GZ34 rectifier or a solid state plugin? I usually get about 455 volts DC with a Mullard Blackburn F32 GZ34. A solid state plugin usually gets about 465 with Merren power transformer.


----------



## jessemhopkins

GZ34


----------



## neikeel

I would expect the same readings as William with a GZ34 and SS set up with an original spec. power transformer (and Merren and Marstran are to OEM spec.) interestingly the Classic tones all come up a little high.

Have you checked you grounds carefully (output tubes and bias caps)?

Normally your B+ should drop to the correct operating point once you get the output tubes in the correct bias range (maybe a bit high if your bias is cold).

What/whose output tubes are you using?


----------



## jessemhopkins

neikeel said:


> I would expect the same readings as William with a GZ34 and SS set up with an original spec. power transformer (and Merren and Marstran are to OEM spec.) interestingly the Classic tones all come up a little high.
> 
> Have you checked you grounds carefully (output tubes and bias caps)?
> 
> Normally your B+ should drop to the correct operating point once you get the output tubes in the correct bias range (maybe a bit high if your bias is cold).
> 
> What/whose output tubes are you using?



Neil, they’re Gold Lions. As for grounding, I followed the scheme you recommended, and I believe I did so closely, but happy to check again. Anything you think I should be looking for? As I said, it was pretty damn quiet so I would assume grounding issues would reveal themselves as noise.


----------



## jessemhopkins

Also, do you think the high voltage is to blame for the blown fuses?


----------



## neikeel

Hi voltage per se should not be the issue. Fuses blow because of excess current draw. 
The fact that your plate voltages are so high is a clue that something is not quite right (and by the fact that the fuse blows). I presume it is a T500mA (slow blo) fuse that you are using?
Also, as you say, it cannot be that far off if you can play it fir a while. At the risk of sounding like our old friend AMS I presume you have the correct impedance selected and matched to your cab with a good speaker cable (sorry if that sounds patronising but in the excitement to get playing these things can get overlooked. 
With just the output tubes pulled what voltages do you get?
If you can find the Metroamp voltage chart sheet can you fill it in and take some close up and behind (caps and fuse holder) of the power section, identify your HT hook up points etc).


----------



## neikeel

I’ve looked at your OT wiring. if you have the Merren 784-103OT (Drake clone) the secondaries should be:
Yellow=Common to output Jack sleeve
Orange = 8ohm
Green=16ohm
Looks like he dropped the blue 100v tap. 
Can you confirm what you have?

You appear to be a filter cap node missing?
Mains (first red wire from switch)=32uF
screens (yellow wire) =32uF
Can’t see the pi can? Typically 16 or 32uF?
The preamp axial is on the board (btw the turrets look empty of solder - just a photo artefact?).


----------



## jessemhopkins

neikeel said:


> Hi voltage per se should not be the issue. Fuses blow because of excess current draw.
> The fact that your plate voltages are so high is a clue that something is not quite right (and by the fact that the fuse blows). I presume it is a T500mA (slow blo) fuse that you are using?
> Also, as you say, it cannot be that far off if you can play it fir a while. At the risk of sounding like our old friend AMS I presume you have the correct impedance selected and matched to your cab with a good speaker cable (sorry if that sounds patronising but in the excitement to get playing these things can get overlooked.
> With just the output tubes pulled what voltages do you get?
> If you can find the Metroamp voltage chart sheet can you fill it in and take some close up and behind (caps and fuse holder) of the power section, identify your HT hook up points etc).



Yes, T500ma fuse. Output is matched 16 ohm to my 4x12 cab (which actually reads about 14 on my meter). I'll have to measure and record specific voltages when I get home, but they were pretty consistently about 20-25% over all down the line.



neikeel said:


> I’ve looked at your OT wiring. if you have the Merren 784-103OT (Drake clone) the secondaries should be:
> Yellow=Common to output Jack sleeve
> Orange = 8ohm
> Green=16ohm
> Looks like he dropped the blue 100v tap.
> Can you confirm what you have?
> 
> You appear to be a filter cap node missing?
> Mains (first red wire from switch)=32uF
> screens (yellow wire) =32uF
> Can’t see the pi can? Typically 16 or 32uF?
> The preamp axial is on the board (btw the turrets look empty of solder - just a photo artefact?).



The switch is a 3 way, so I bridged the orange (8 ohm) across the right and middle switched lugs and wired the 16 ohm green to the left. Take a look at the closeup below. I also put an "8" label over the 4 on my backplate so I don't forget down the line!

Having said all of that, the fuse first popped before I even put the preamp or power tubes in and took it off of standby, for what it's worth. In case it offers any clues I'm missing, the two times it popped were when I:

1. Had my meter lead on the standby switch to measure B+ (initial fire-up Saturday morning, only rectifier installed and amp on standby)
2. Was play testing and attempting to insert my patch cable to jump the channels

PI filter- this stupid chassis had no room to mount 2 cans, so I got a little creative and put a 16uf axial on a small terminal strip and tucked it away under the lip of the chassis next to the PT. See below.













69720FED-60C6-49A1-A816-2E25D6D69AA8



__ jessemhopkins
__ Jan 7, 2021


















975235FD-C175-491E-AF56-4052604252CC



__ jessemhopkins
__ Jan 7, 2021


----------



## jessemhopkins

Oh, and there is solder in those lugs. it's just a bit of shading in the photo. You can _kind of_ see it better at this angle, but it's there.












5AC8EB9D-F3EA-466A-AF53-9CE0090A031F



__ jessemhopkins
__ Jan 2, 2021


----------



## 2L man

If you install at least 100 ohm current limiter resistors to rectifier anodes voltage drops perhaps 10V on idle.

I think current limiter resistors are more important when modern, often machine wound over plastic bobbin, transformers are used because their internal resistance is lower and voltage drop comes smaller when current increase.


----------



## jessemhopkins

On Thursday night, I went to fire it back up to measure voltages. I took it off of standby and about a minute later, the mains fuse blew. Had a pretty full plate the last few days, so I put it away.

This evening, I went back at it. First, I pulled up the schematic and buzzed out the entire amp from input jacks to power supply, expecting to find an issue that would jump out. Everything was a spot on match and grounds were all solid. So I moved on. As @neikeel suggested, I removed the output tubes to measure voltages and recorded everything in the chart below. Then I powered off, put the output tubes back in, and powered on again. As soon as I hit the standby switch, I heard a loud hum through the cab and the HT fuse blew. For grins, I swapped the GZ34 for another I have on hand and the exact same thing happened.

I'm at a bit of a loss here, but if the chart triggers anything that you all think is worth checking out please do let me know. TIA.













Screen Shot 2021-01-09 At 8.30.55 PM



__ jessemhopkins
__ Jan 9, 2021


----------



## william vogel

I would suggest you get another pair of kt66 tubes. I suspect you have a dud. Another recommendation is to stop using the standby switch for warmup. A GZ34 is a indirectly heated rectifier and it’s unnecessary and hazardous due to inrush current to hot switch. All the tubes warm at approximately the same rate and the current and voltage will come up slowly just turning the amp fully on.


----------



## jessemhopkins

william vogel said:


> I would suggest you get another pair of kt66 tubes. I suspect you have a dud. Another recommendation is to stop using the standby switch for warmup. A GZ34 is a indirectly heated rectifier and it’s unnecessary and hazardous due to inrush current to hot switch. All the tubes warm at approximately the same rate and the current and voltage will come up slowly just turning the amp fully on.


Thanks for the insight. As it may be worthy clarifying, is your recommendation with the understanding that the readings in the chart I posted were taken with no tubes in the output socket?


----------



## neikeel

jessemhopkins said:


> Thanks for the insight. As it may be worthy clarifying, is your recommendation with the understanding that the readings in the chart I posted were taken with no tubes in the output socket?



It does sound like your KT66s. 
Only thing I see from your chart is higher than expected plate and screen voltages on your output tubes (even allowing for no output tube load). @william vogel - do you have one of Chris's -118 PTs that you can measure in the same way?


----------



## Chris-in-LA

jessemhopkins said:


> Thanks for the insight. As it may be worthy clarifying, is your recommendation with the understanding that the readings in the chart I posted were taken with no tubes in the output socket?


Did you leave off the V4 and V5 heater voltages? I only see one on V4 pin 2.


----------



## Guitar-Rocker

Your voltage for V2 pin 6 is either written down wrong, or there is an issue there. IT should not be 370V.


----------



## william vogel

neikeel said:


> It does sound like your KT66s.
> Only thing I see from your chart is higher than expected plate and screen voltages on your output tubes (even allowing for no output tube load). @william vogel - do you have one of Chris's -118 PTs that you can measure in the same way?


I have a 1986 with solid state rectifier but I can measure it. With no power tubes in place the voltage should be about the same being it’s not loading the rectifier.


----------



## william vogel

jessemhopkins said:


> On Thursday night, I went to fire it back up to measure voltages. I took it off of standby and about a minute later, the mains fuse blew. Had a pretty full plate the last few days, so I put it away.
> 
> This evening, I went back at it. First, I pulled up the schematic and buzzed out the entire amp from input jacks to power supply, expecting to find an issue that would jump out. Everything was a spot on match and grounds were all solid. So I moved on. As @neikeel suggested, I removed the output tubes to measure voltages and recorded everything in the chart below. Then I powered off, put the output tubes back in, and powered on again. As soon as I hit the standby switch, I heard a loud hum through the cab and the HT fuse blew. For grins, I swapped the GZ34 for another I have on hand and the exact same thing happened.
> 
> I'm at a bit of a loss here, but if the chart triggers anything that you all think is worth checking out please do let me know. TIA.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Screen Shot 2021-01-09 At 8.30.55 PM
> 
> 
> 
> __ jessemhopkins
> __ Jan 9, 2021


Do you have another amp that runs 6l6 or an amp that runs EL34’s? If you do, swap in the other tubes and see if they will power on. What you really need is a light bulb limiter to check for the fault but swapping in known good tubes will allow you to rule on the KT66. A set of 6l6/5881 will run and bias properly. EL34’s will run cold without any changes to the bias circuit. Either of these tubes are safe to test.


----------



## jessemhopkins

Guitar-Rocker said:


> Your voltage for V2 pin 6 is either written down wrong, or there is an issue there. IT should not be 370V.


Might have mistyped it, along with the v5 heater.


william vogel said:


> Do you have another amp that runs 6l6 or an amp that runs EL34’s? If you do, swap in the other tubes and see if they will power on. What you really need is a light bulb limiter to check for the fault but swapping in known good tubes will allow you to rule on the KT66. A set of 6l6/5881 will run and bias properly. EL34’s will run cold without any changes to the bias circuit. Either of these tubes are safe to test.


I had the same thought late last night. I’ve got some el34s I can try. I’d hate to drop another $80 on a new set of kt66, but we’ll see what happens. I’ll get into it later today after church.


----------



## william vogel

jessemhopkins said:


> Might have mistyped it, along with the v5 heater.
> 
> I had the same thought late last night. I’ve got some el34s I can try. I’d hate to drop another $80 on a new set of kt66, but we’ll see what happens. I’ll get into it later today after church.


Okay good, there are people that play jtm45 amps with EL34’s in them. Swapping the 150k bias dropping resistor that’s next to the bias diode to a 220k will allow proper bias voltage for EL34’s.


----------



## jessemhopkins

I tried the EL34s, didn't blow any fuses. Tell me what you make of the readings below. They dropped a little, but @Guitar-Rocker pointed out V2 pin 6 last time and I thought I might have mistyped it- I did not. It's still quite high. I tried another 12AX7 and got the same readings, but noticed that I was not able to seat the tube in the socket all the way. There's a small gap. I don't see anything blocking it, but I've ordered some replacement sockets as I don't have any spares on hand.

B+ has gone up. Was 470 with the KT66, now 488 with the EL34. Is that expected or the result of excess current draw somewhere?













Screen Shot 2021-01-10 At 8.15.24 PM



__ jessemhopkins
__ Jan 10, 2021


----------



## neikeel

CAn you show a pic of the wiring of the leads to V2 and the mid board dropping 10k resisitor?


----------



## jessemhopkins

neikeel said:


> CAn you show a pic of the wiring of the leads to V2 and the mid board dropping 10k resisitor?


You can kinda see both here, I’ll get more clear pics later today.













V2 leads



__ jessemhopkins
__ Jan 11, 2021


----------



## neikeel

Good
When you do can you confirm where that purple wire to the plate of V2 comes off, that the dropper is 10k and that the filter cap next to it is linked to the turret next to it. Finally (with preamp tubes in) measure the voltage on the other end of that 10k. Some layouts have the under board link going to the cap end of that resistor and if you’ve mixed up the under board links the voltage maybe coming from the pi direct rather than through the 10k. Otherwise you need to confirm that your tone stack is grounded securely - when you test do all of your EQ controls work properly?


----------



## junk notes

500pF/250pF this is a Superbass?


jessemhopkins said:


> You can kinda see both here, I’ll get more clear pics later today.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> V2 leads
> 
> 
> 
> __ jessemhopkins
> __ Jan 11, 2021


500pF/250pF this is a Superbass?


----------



## neikeel

junk notes said:


> 500pF/250pF this is a Superbass?
> 
> 500pF/250pF this is a Superbass?



No stock JTM45 for guitar/bass.


----------



## jessemhopkins

neikeel said:


> Good
> When you do can you confirm where that purple wire to the plate of V2 comes off, that the dropper is 10k and that the filter cap next to it is linked to the turret next to it. Finally (with preamp tubes in) measure the voltage on the other end of that 10k. Some layouts have the under board link going to the cap end of that resistor and if you’ve mixed up the under board links the voltage maybe coming from the pi direct rather than through the 10k. Otherwise you need to confirm that your tone stack is grounded securely - when you test do all of your EQ controls work properly?


I snapped the pics below on my way out the door this morning. When I play tested last week, tone controls were working as expected. Saturday night I did a full circuit audit and verified every component and connection, including grounds. So it should be bang on. Having said that, I will take a look this evening with a close eye on the line from the B+ to V2 plate.

As for underboard wiring, since I had to tweak the layout of the mojo board I made the composite image below from the images in the metro doc. I flipped and reversed the underboard wiring so that the turrets were correctly aligned, and overlayed it onto the component graphic. So this should represent the layout I followed, even though I had to move a few components around. I'll verify it all this evening.

The dropper is definitely 10k, I remember as I only had 2 of those 1W 10k resistors and I chose the one that was closest.













V2



__ jessemhopkins
__ Jan 11, 2021


















10K Dropper



__ jessemhopkins
__ Jan 11, 2021


















Underboard wiring



__ jessemhopkins
__ Jan 11, 2021


----------



## neikeel

The graphic looks correct. Have you confirmed the cap to resistor under board link.


----------



## jessemhopkins

neikeel said:


> The graphic looks correct. Have you confirmed the cap to resistor under board link.


Yes, verified the link and that node is on the other end of the purple wire going to V2 plate. As you asked voltage measurement, with no output tubes there is 422V on the control side of the 10k resistor and 367 on the tube side. It is wired as it is supposed to be, with the other side linked just as on my overlaid graphic posted above. B+ is reading right at 500 with preamp tubes only.

Looking closely, though, I saw something missing. In the couple of schematics I looked at, there is a 1k resistor coming off the choke (circled in the screenshot below) that is 100% absent from the metro instructions, despite being seen in the schematic towards the end of the pdf. I'm also seeing it missing from a number of other builds as well. Is it ok that it's missing? Seems to be working for others.













Screen Shot 2021-01-11 At 9.32.10 PM



__ jessemhopkins
__ Jan 11, 2021


----------



## Guitar-Rocker

That 1K drops the screen voltage minutely. The screens need to always be lower than the plates.

I don't think the absence of the 1K has anything to do with blowing fuses, or high voltage to.pin 6 of V2


----------



## jessemhopkins

Guitar-Rocker said:


> That 1K drops the screen voltage minutely. The screens need to always be lower than the plates.
> 
> I don't think the absence of the 1K has anything to do with blowing fuses, or high voltage to.pin 6 of V2


Certainly not the latter. My tube sockets are coming in tomorrow, I'll replace v2 socket and see if it helps. I don't know why, but there's something in the back of my head saying I saw a repair video where a faulty socket was causing excess current draw. Might be nothing. I'm still pretty green, but this build has helped cement A LOT of concepts for me.


----------



## neikeel

Colloquially known as the 'flying' screen resistor (it sits flying betwen the screen turret and the spare lug on V4 it is 1k 1w in series with the two 470k screen droppers.
George felt the amps sounded more open and less compressed without it so omitted it. The early 50w with Mullard EL34s had no screen resistors at all. Sound benefit is marginal and you cannot run modern EL34s without them.

Have you tried tube rolling in V2 once you get it to seat properly?
The voltage will go down when the tube draws full current


----------



## jessemhopkins

neikeel said:


> Colloquially known as the 'flying' screen resistor (it sits flying betwen the screen turret and the spare lug on V4 it is 1k 1w in series with the two 470k screen droppers.
> George felt the amps sounded more open and less compressed without it so omitted it. The early 50w with Mullard EL34s had no screen resistors at all. Sound benefit is marginal and you cannot run modern EL34s without them.
> 
> Have you tried tube rolling in V2 once you get it to seat properly?
> The voltage will go down when the tube draws full current


I’ve only tried 2 so far, I’ll go through my stash once I get the socket swapped and the tube seated.


----------



## jessemhopkins

I replaced the socket and the tubes now seat properly. Unfortunately I’m still getting the same results on V2 plate. I’ve tried 3 different 12AX7 and they all read exactly the same.

 Does this smiley look sufficiently depressed?


----------



## neikeel

Does the amp run?


----------



## jessemhopkins

neikeel said:


> Does the amp run?


Considering I intend to run this amp with KT66s and every time I try to use them the HT fuse blows, no, it does not run. I recognize the possibility that the set I have may be “duds” but I don’t feel I have sufficient evidence to accept that full stop, or to try to get CE to replace them.

I am extremely appreciative of everyone’s time and input. This is nobody’s problem but my own, and I do not wish to come across as entitled to anyone’s time. So please do not feel any obligation or burden to keep helping me.

And even though it’s not finished yet, let me say a hearty “thank you!”


----------



## Guitar-Rocker

With all tubes out, does the amp blow any fuses? If no, great.

Then with all tubes out can you get a useable range of voltage swing on your bias?

If yes, then turn the amp off and put in only the power tubes, and set your bias.

Does any fuse blow? If no, then the tubes are probably good and so is the power section.


----------



## neikeel

Happy to help until you are sorted. If we are hindering vs helping please tell us. 
The above advice is good


----------



## william vogel

I usually buy a quad of matched output tubes for any amp that runs two output tubes. Therefore if I have question of a dud I can swap in a replacement and have a solution. Having a light bulb current limiter also allows you to be able to find the excessive current draw without ever blowing fuses because the bulb becomes a resistor and limits the current. You can easily find the “short” while powered up and running without the chance of damaging anything.


----------



## Chris-in-LA

Are you sure you checked the resistance of that 10k dropping resistor that is connected to the V2 plate? That high voltage is coming from that spot. But I’m not sure what the voltage should be at that spot without opening up my JTM45 and checking it. Maybe @neikeel knows what the voltage should be right after that resistor.


----------



## neikeel

Well if you have 470v on plates (bit high) then you expect about 400v after the first 8k2 dropper (ie before that 10k) and about 320v on the north end of the resistor. 
Things that cause an isolated elevated voltage would be part of that circuit not drawing enough current as current draw drops the voltage. As your v1 voltages look normal it appears that the issue resides on v2, most likely the cathode follower part of the tone stack.


----------



## jessemhopkins

neikeel said:


> Happy to help until you are sorted. If we are hindering vs helping please tell us.
> The above advice is good


Certainly helpful.


----------



## william vogel

This weekend I’ll open up my 1986 and get you some voltage measurements.


----------



## jessemhopkins

william vogel said:


> This weekend I’ll open up my 1986 and get you some voltage measurements.


That’d be great. I have to put it away for a few days but I’ll have some more time on Monday.


----------



## Chris-in-LA

neikeel said:


> Well if you have 470v on plates (bit high) then you expect about 400v after the first 8k2 dropper (ie before that 10k) and about 320v on the north end of the resistor.
> Things that cause an isolated elevated voltage would be part of that circuit not drawing enough current as current draw drops the voltage. As your v1 voltages look normal it appears that the issue resides on v2, most likely the cathode follower part of the tone stack.


That makes more sense than having a bad tube socket.


----------



## william vogel

View media item 12362Here’s my voltage chart. This is a 1986, so there’s another filter node in the preamp and the 8.2k/10k dropping resistors after the choke. If I jump the 10k the voltage goes up to 387.5 at the phase inverter, 331.5 node 4 the CF, 310.4 node 5 V1.


----------



## neikeel

Kudos to William for doing that, so, spurred on I opened up my original 65 JTM45 (Mullard GZ34, yellow print ECC83s and GEC KT66s. Best sounding JTM45 I have played (and therefore kept) amongst the many that have passed though my hands, either mine or worked on. Drake trannies (selectors on sides) and 354-114 choke.

V1 pin 1 219v, pin 6 218v heaters at end of chain 3.08vac
V2 pin1 183, pin 6 318v
V3 pin 1 250v, pin 6 241
V4 449 plates, 448 screens (1k flying and 470R on sockets
V5 same

On PT ac = 350-0-350vac exact
On PT ac - 3.15vac
On PT ac = 4.9vac

So HT after rectifier 453v, 446, 369 and 318 after each dropper/decoupler node

By contrast another amp almost the same except custom PT for 300-0-300 for my Mullard EL37s:

181/177
152/269
210/196
340 plates
324 screens (1k on sockets)

HT is 300-0-300 for 350vdc/344/311 and 268 in HT chain

Hope that helps?


----------



## jessemhopkins

Man, thank you @william vogel and @neikeel so much for doing that. Very kind of you. It will be massively helpful to have those as reference.


----------



## jessemhopkins

I spent some time today going back over this. Still getting the same readings, but I powered it up with the EL34s and took the readings on the HT Line:

492, 491, 415, 358

So I'm wondering if my choke may be at fault? Barely any drop between those nodes.


----------



## jessemhopkins

Here is, perhaps, a better set of questions-

1. If my readings are steadily in the range of 13 - 17% above @neikeel's with the EL34s I have been using to test, are those voltages problematic or are they safe operating voltages? If so, maybe the thing to do is bite the bullet and buy another set of KT66 (though I fear it will just be $80 to discover the exact same problems and the issue was never to do with the tubes I've got already).

2. Neil, if as you said above:



neikeel said:


> Things that cause an isolated elevated voltage would be part of that circuit not drawing enough current as current draw drops the voltage. As your v1 voltages look normal it appears that the issue resides on v2, most likely the cathode follower part of the tone stack.



How would you propose to troubleshoot that section to find where it would not be drawing proper current? I have now verified the entire circuit against the schematic at least twice and spent even more time in and around V2, even going so far as to lift legs and check values and can not find a single issue.


----------



## william vogel

jessemhopkins said:


> I spent some time today going back over this. Still getting the same readings, but I powered it up with the EL34s and took the readings on the HT Line:
> 
> 492, 491, 415, 358
> 
> So I'm wondering if my choke may be at fault? Barely any drop between those nodes.


How much cathode current are the EL34’s drawing?


----------



## Guitar-Rocker

Did you blow any fuses rechecking the voltages that you recorded? If no, then did you verify that you have proper bias voltage swing with the tubes out? If yes, then put your power tubes back in and rebias. Check for steady voltage over a couple of minutes after rebias. If things are steady, and your calculations of dissipation work out, then you saved yourself 80 bucks on new tubes. 

Verifying that there are no issues with the tubes out prepares you for inserting a "load" , the tubes, to draw current. If all this works then your power section is valid. 

Then you can focus on troubleshooting sending grid signal to your power tubes.

If you installed a fx loop, then after verifying the power section, you could inject a signal from another amp to drive your power section.


----------



## william vogel

jessemhopkins said:


> I spent some time today going back over this. Still getting the same readings, but I powered it up with the EL34s and took the readings on the HT Line:
> 
> 492, 491, 415, 358
> 
> So I'm wondering if my choke may be at fault? Barely any drop between those nodes.


The choke is usually about 100-105 ohms, so it’s not going to drop very much voltage at idle.


----------



## Guitar-Rocker

As far as the recorded voltages only being @ 1 volt difference and wondering about the choke
The resistance on most chokes is only in the 200 to 350 ohm range, close to no resistance at all in the dropping series.

At that little resistance you drop very little volts at the choke.


----------



## william vogel

jessemhopkins said:


> Here is, perhaps, a better set of questions-
> 
> 1. If my readings are steadily in the range of 13 - 17% above @neikeel's with the EL34s I have been using to test, are those voltages problematic or are they safe operating voltages? If so, maybe the thing to do is bite the bullet and buy another set of KT66 (though I fear it will just be $80 to discover the exact same problems and the issue was never to do with the tubes I've got already).
> 
> 2. Neil, if as you said above:
> 
> 
> 
> How would you propose to troubleshoot that section to find where it would not be drawing proper current? I have now verified the entire circuit against the schematic at least twice and spent even more time in and around V2, even going so far as to lift legs and check values and can not find a single issue.


Even if V2 isn’t drawing “typical current”, it’s a preamp tube. 12ax7 tubes normally draw about 1mA per triode section, so you’re whole preamp only draws about 6mA. The power tubes biased about 60-70% will draw 30-40mA each per tube plate and the screens draw 2-5mA per also. So the power section draws about 70-90mA. Because you’re reading 492 volts, either your input voltage from the wall outlet is a bit higher than the tap (120, 230 volt, etc), or your power tubes are biased really cold and they aren’t drawing much current.


----------



## william vogel

Guitar-Rocker said:


> As far as the recorded voltages only being @ 1 volt difference and wondering about the choke
> The resistance on most chokes is only in the 200 to 350 ohm range, close to no resistance at all in the dropping series.
> 
> At that little resistance you drop very little volts at the choke.


The usual choke for a plexi is a 3 Henry @ 250mA 105 ohms DCR or the radio spares 20 Henry @ 70mA 610 ohms DCR. Most people use the 3 Henry.


----------



## Guitar-Rocker

Yeah, it was an approximation to indicate why there isnt much of a voltage drop at the choke.


----------



## 2L man

william vogel said:


> The usual choke for a plexi is a 3 Henry @ 250mA 105 ohms DCR or the radio spares 20 Henry @ 70mA 610 ohms DCR. Most people use the 3 Henry.



3H/250mA Choke possibly could be used before OPT/power tubes on 50W amp. Used before Sceens it is too powerful and inductance low so most of it gets "wasted" there.

20H/70mA is better before Screens for 50W amps.


----------



## jessemhopkins

Finally got these KT66s on a tester and confirmed I have a dud. Glad to know that for sure, so I’ll get a new set this week and see what’s up. Then I’ll recheck voltages once I have a good set installed and biased.


----------



## william vogel

jessemhopkins said:


> Finally got these KT66s on a tester and confirmed I have a dud. Glad to know that for sure, so I’ll get a new set this week and see what’s up. Then I’ll recheck voltages once I have a good set installed and biased.


I suggest you get a quad, you’ll use them and you can install the other two later without needing to check bias. I’d also suggest confirming their balance if you go this route while you bias the pair you intend to use first.


----------



## jessemhopkins

william vogel said:


> I suggest you get a quad, you’ll use them and you can install the other two later without needing to check bias. I’d also suggest confirming their balance if you go this route while you bias the pair you intend to use first.


Yes, thanks to your sound advice (pun intended) I plan to.


----------



## neikeel

I have never really noticed much difference between the chokes used and their performance. My favourite JTM45 has 3H Drake (Drake OT and PT) but the similar amp with 20H RS (with RS Deluxe OT and HD PT) was very similar back to back with same cab and guitars. I would have expected the 20H to be stiffer at lower volumes and saggier when pushed but not the case, either to my ears or feel. Both mullard pres and GZ34 and GEC KT66. 
YMMV.


----------



## 2L man

If you have placed 3H/250mA like on first page schematic has its current capacity gets "wasted" when installed before power tube screen circuit because current there is much lower.

That 3H/250mA Choke could be used after rectifier tube and all amp current is made run thru it. However LC filtered power supply output voltage comes significantly lower than on RC filtered PS.

20H/70mA Choke before Screens filters 120Hz "hum" from DC voltage very well there and it also filters noise what power amp produces much more than 3H choke does on same circuit place.


----------



## jessemhopkins

neikeel said:


> Kudos to William for doing that, so, spurred on I opened up my original 65 JTM45 (Mullard GZ34, yellow print ECC83s and GEC KT66s. Best sounding JTM45 I have played (and therefore kept) amongst the many that have passed though my hands, either mine or worked on. Drake trannies (selectors on sides) and 354-114 choke.
> 
> V1 pin 1 219v, pin 6 218v heaters at end of chain 3.08vac
> V2 pin1 183, pin 6 318v
> V3 pin 1 250v, pin 6 241
> V4 449 plates, 448 screens (1k flying and 470R on sockets
> V5 same
> 
> On PT ac = 350-0-350vac exact
> On PT ac - 3.15vac
> On PT ac = 4.9vac
> 
> So HT after rectifier 453v, 446, 369 and 318 after each dropper/decoupler node
> 
> By contrast another amp almost the same except custom PT for 300-0-300 for my Mullard EL37s:
> 
> 181/177
> 152/269
> 210/196
> 340 plates
> 324 screens (1k on sockets)
> 
> HT is 300-0-300 for 350vdc/344/311 and 268 in HT chain
> 
> Hope that helps?


I got my new tubes in yesterday and tried them out. Voltages seem to have settled right into place:

V1 pin 1 232v, pin 6 217v
V2 pin1 194v, pin 6 323v
V3 pin 1 247v, pin 6 246
V4 443v screens and grids
V5 442v screens and grids

So all of those readings are extremely close to Neil's with the original drake. The same was true for each node on the ht line, although I didn't record my readings. Got it biased and had about a 15 minute play, and then it started blowing mains fuses. I'm happy now with the readings I've got, but no idea what to check for the fuse issue.

It sounded awesome when I play tested, although it broke up a lot earlier than I thought it would.


----------



## Trouble

I would suspect a tube red plating.......... but you probably was watching it while playing? 
Or maybe fuse too small? Or try colder bias? 

I know you already checked everything twice, so I hate to even comment 

But, I am pulling for you to get this one figured out.


----------



## jessemhopkins

Travis398 said:


> I would suspect a tube red plating.......... but you probably was watching it while playing?
> Or maybe fuse too small? Or try colder bias?
> 
> I know you already checked everything twice, so I hate to even comment
> 
> But, I am pulling for you to get this one figured out.


Tubes looked ok, though I admit I wasn't staring at them. Bias is on the slightly cool side already, but would be worth playing with. Fuse is 3a 250v, which is what everyone making these amps I've seen uses but like the bias, could be worth trying. And yes, I have audited the whole thing twice and certain sections about 10x, though I suppose there could be a bad solder joint under the board. I may lift it and take a look.


----------



## Trouble

jessemhopkins said:


> Fuse is 3a 250v, which is what everyone making these amps I've seen uses .


you're right I built one and the 3a fuse has never failed me.


----------



## jessemhopkins

I also had the thought to check my filter caps too, though I’m pretty much grasping at straws at this point. Though I do feel closer with the new tubes.


----------



## Chris-in-LA

jessemhopkins said:


> I also had the thought to check my filter caps too, though I’m pretty much grasping at straws at this point. Though I do feel closer with the new tubes.


How early did it break up? I can run mine at 5 or 6 on the bright channel without much breakup. And, refresh my memory, does it blow fuses when there are no tubes in?


----------



## jessemhopkins

Chris-in-LA said:


> How early did it break up? I can run mine at 5 or 6 on the bright channel without much breakup. And, refresh my memory, does it blow fuses when there are no tubes in?


The bright channel breaks up around 2, the normal channel around 4. I suppose I can roll some tubes in the preamp, but I’ll save that for once it’s stable. So far, it has only blown fuses with power tubes installed.


----------



## william vogel

Light bulb limiter and check for excess current draw.


----------



## Chris-in-LA

jessemhopkins said:


> On Thursday night, I went to fire it back up to measure voltages. I took it off of standby and about a minute later, the mains fuse blew. Had a pretty full plate the last few days, so I put it away.
> 
> This evening, I went back at it. First, I pulled up the schematic and buzzed out the entire amp from input jacks to power supply, expecting to find an issue that would jump out. Everything was a spot on match and grounds were all solid. So I moved on. As @neikeel suggested, I removed the output tubes to measure voltages and recorded everything in the chart below. Then I powered off, put the output tubes back in, and powered on again. As soon as I hit the standby switch, I heard a loud hum through the cab and the HT fuse blew. For grins, I swapped the GZ34 for another I have on hand and the exact same thing happened.
> 
> I'm at a bit of a loss here, but if the chart triggers anything that you all think is worth checking out please do let me know. TIA.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Screen Shot 2021-01-09 At 8.30.55 PM
> 
> 
> 
> __ jessemhopkins
> __ Jan 9, 2021


I haven’t checked my JTM45 with Metrospec iron but your initial b+ is about 100 volts higher than the manual shows, which is about 400v, not 500v. Didn’t you get that Merren PT from a forum member? Maybe it’s not the right PT.


----------



## jessemhopkins

Chris-in-LA said:


> I haven’t checked my JTM45 with Metrospec iron but your initial b+ is about 100 volts higher than the manual shows, which is about 400v, not 500v. Didn’t you get that Merren PT from a forum member? Maybe it’s not the right PT.


PT is correct and voltages have all settled down now. I didn’t record the exact B+ but it was somewhere right around 450. Higher than the metro but the same as what Neil and others reported on either Merren or original Drake. I’ve still got another pair of KT66 now to try, I’ll pop them in and take some measurements shortly.


----------



## Chris-in-LA

Good luck. Mine is very close to the Metro specs, almost exactly the same on the power tubes and rectifier, but 20 volts over on V1 and V2. The PI is mixed, p3 and p6 are 20 over, p3 and p8 are 10 under. Below is the Metro specs for reference if anyone is asking.


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## jessemhopkins

I’m pleased to report that I got this amp up and running stably this weekend. With the aid of a friend, we noticed the rectifier had failed in addition to a bad solder joint on one of the output tubes. Retouched the joints, swapped the recto, we’re in business. Only thing left is for me to pin the badge @neikeel sent across the pond on it. I’ll grab some pics and shoot them over later.


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## jessemhopkins

Finished Amp Front



__ jessemhopkins
__ Apr 5, 2021


















Finished Amp Rear



__ jessemhopkins
__ Apr 5, 2021






Here's the finished amp atop my 4x12. An absurd amount of lessons were learned along the way, as they always are, but I'm so glad to have this one done and it sounds fantastic. I'm still dialing in the tone for my setup, but truly superb. Not much noise unless it's cranked, and I likely won't have much occasion for that.

A massive thank you to everyone who helped me out and lent their expertise to this relative noob, especially to @neikeel and @william vogel for your long-suffering during the troubleshooting phase. It feels a bit silly now to put the coffin badge on it, knowing how far off in many ways the look is, but I'll put it on until I build a proper replica someday.

Onto the next one!


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## adew1

Looks great!


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## neikeel

Need final final pic!


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## jessemhopkins

neikeel said:


> Need final final pic!


Haha ok, I’ll have to swing by the hardware store first. I’m taking it out for a real world test at church this weekend.


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