What do I have in here? (JTM45 content)

Ramhead

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I would be caredul of using RFT EL-34s in that amp! They are great sounding tubes, but have a max plate voltage well below what that amp likely will throw at it. Also, where did you get the 10uF 160V Philips caps? I have had a hell of a time trying to get some for a restoration.
Yes but I'm sure he will measure everything before he put any kind of valuable tube in there.

I've bought those Philips caps in here:
 

Ramhead

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Derrick111

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I've checked on Mouser and they have 0 in stock, but I've found these:


do they look ok?
The ones I bought look absolutely spot on! But the ones in that link you provided are different. Thank you for the eBay link for the bias caps. I'm in the USA, but it looks like free shipping to here.

EDIT: Oh boy... they no longer seem to be available through Mouser or Digikey 😢
 
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Ramhead

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The ones I bought look absolutely spot on! But the ones in that link you provided are different. Thank you for the eBay link for the bias caps. I'm in the USA, but it looks like free shipping to here.
Glad I could help :)

EDIT: Oh boy... they no longer seem to be available through Mouser or Digikey 😢

Yeah, that's why I'm having trouble finding them. But, apart of being different, those Welwyn FA89 1K0 5W should work?
 

Derrick111

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Yes, I think so... they just need to be 1k ohm and 5 watts or more, but the only thing I am not 100% sure on is if they need to be non-inductive resistors in that position or not. I will let others chime in on that.
 

neikeel

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I search out the Welwyn/RS 1k 5w mo coated we for these 470-500R for JTM45s.
The RS ones are hard to find but 1k2 and 1k5 are ok too.
 

neikeel

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They will do the job well, they look the part (originals are radials) but with neat wiring they will fit in well.
 
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I
Are they bad? I wouldn't replace unless they've actually tested as such (although it seems we re-litigate this debate every month or so around here!)

I'd be more inclined to replace the bias caps (the smaller 8uf RS caps), but I see @Ned B has kept his.

Edit: BTW, that first shot - with the pots and input jacks hanging - is a cool perspective @tomsvintage. Were you moving between chassis or something? It belongs on a T shirt with "the guts of rock 'n roll" or some such!
Cool find ! How is the amp coming along?

My advice when it is completed is to run it on a variac/step up on 240/250 .
 
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I'm searching for information about the Drake 1203-43, and I've found this:


How much different this info is from the 1203-43?:

1203-80.jpg
So your 1203-43 works fine then? I would not be alarmed by the heat. These all run hot especially cranked up but a small fan will get the heat out and keeps them cool. Maybe leave the back panel off, that helps if you use a clip on fan.

What was the OT figured out to be? Being that the back panel has 4 ohms and no 100v),maybe a later el34 transformer I need to reread again and will .

The one below is mine but a pa. It this distracts I will delete no problem. I could use some education on these.

Pics are crappy but I need take some of it anyhow. I have yet to take any good pics of it . Never had any trouble with the original output transformer or anything with it, all the stock transformers are unstamped but original (except the pt in amp currently .)

This one never had the 110 tap on the original working pt . Not made for 105-115 from factory. A tech before i owned did replace the original PT for the guy that owned it it with Merren PT , so the modern kt66 did not melt down by by using the lower voltage taps.

As the original was like 560-80 volts. However the original power transformer according very trustworthy tech/guy I bought it from still works great. I want to learn how to test it and possibly put it in . Bias it up with some gecs which I can do. However the transformer I am still well trying to study how to do correctly. Biasing not a problem .

It also have the old brimar/Mullard el34s that the amp once contain‘d . They came in the spare valve art kt66 boxes that where used & biased in the amp . I guess someone was using it like that for a bit with the original 3 stack. It’s cool to get a old amp someone did not rob of the tube to sell apart from it.

However stock straight kt66/from factory amp I believe . The serial number is 7 away from a cool non pa amp. All caps are early to mid 66 except what you see in bias section and rectifier part.

I still want to put the original pt back in sometime for a little while with some help maybe from someone who can help me on here.

I have 1 good quad of gecs kt66 I have in another amp i could use In this with the original transformer back in. I run the old ones at 240 with a variac/step up anyway. So not having the 105-115 is not a problem .

However it is my favorite amp the way the tech guy set it up before I got it. Tubes are biased very very hot when I checked it using shunt method however b+ is lower . So maybe that is why . However 0 problems and put a lot hard time thru it. Because the lower tap used. So it handles it well.

To me this one is my best sounding/feeling amp I have ever had . So I do not really want to mess with it. I really Don’t understand why except how I explain with b+ lower and hot bias. Even with the Chinese kt66 it sounds better than one I have that has gecs in .

I was kinda doubtful of keeping the 120 volt Merren PT ( I do still run it at 240v )in there because I am all about the high plate voltage so I thought . However this guy really set it up is really amazing. I have been messing with using a 330pf cap or 500pf /570pf rs cap on the 470k mixer . However not sure about keeping like that . Still messing with it. I do think it sounds better using 8 ohm tap into 2 cabs.

Like mentioned the working original pt pictured(no tap for 110 )but not interested in selling it. As maybe someday I want to put it back in if only for a little while with the gecs. I don’t really see a reason to now as I couldn’t be happier with it (strange I know). I just don’t think I would like as much but who knows. Again no red plating or any problems but biased really hot and b+ is lower.

I have the original red pilot light that went out also.
Tried this today, unhooked all the connections and checked for voltages. They are all in the ballpark. It was just a quick test before it headed to his shop where extensive testing will follow.
The only thing that was worrying, you could feel that PT was lightly warm while idling for about 15 minutes.. You could barely feel it but it wasn't just a cold metal like the OT beside it. Hopefully that's it's working temp, we'll see. Fingers crossed.
 

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I did pull V1 and set the other volume not used higher up . I have no light picture type light right now so pics here are not great. These are pics of the original pt that works from according the tech. I really did not want to post it but what the hell. I enjoy seeing and hearing them from others.
 

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Really cool how you found that amp ramhead. Should be interesting to hear what those transformer sound like with el34. Curious about that output transformer. Curious to know what b+ will be with that power transformer of yours.

I don’t want to crash your thread . Last pics . I thought it might be a valid thing here. With the transformers in yours and all. I really don’t like posting pics of gear but I will never sell it so I don’t care. I look forward to seeing progress of yours and learning about it.

This pa compared (pictured) to a early 67 jtm45/100 (not pictured here)white panel aluminum block end 45/100 with el34s dual rectifier is really interesting sound wise. That one is a early 5a one lead with a stock bright cap on the one volume pot . That amp is a little louder . Shake your balls loud into 2 cabs lol.
 

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playloud

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Nice amp! Thanks for sharing @Whatwhatringrang

Interesting serial # too. Very close to 7026!

Appreciate the dilemma with the PT. As you have the GECs, I would definitely be tempted to try out the original. Definitely the sort of thing you want to test with a variac (and, ideally, current limiter) though.
 
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Nice amp! Thanks for sharing @Whatwhatringrang

Interesting serial # too. Very close to 7026!

Appreciate the dilemma with the PT. As you have the GECs, I would definitely be tempted to try out the original. Definitely the sort of thing you want to test with a variac (and, ideally, current limiter) though.
Thanks . Yeah I know the serial number thing is kinda all over with these but I still thought that cool . Dates of the caps are all pretty much right in there time line wise. On the original stuff that tested right and good.

I have a few nice variacs I use all the time but never built the current bulb limiter yet . I really doubt I would like it better with higher b+ colder bias though . I don’t want to ruin a good thing and that is last time i will ever probably get gecs again nowdays . Plus any gains of it would be a compromise with tubes i can not get anymore. I actually prefer the way it is set up now .

I am no tech or anything but have worked on tube mid-fi rebuilds ,Leslie’s and other musical instrument amps Over the past few years . I Have to be absolutely certain it will not fizzle the output transformer . I did have to replace one of 2 pin selector black plastic shorter piece on back. Those are good idea to solder and keep a close eye on all that. I need to learn a lot more before doing doing it. I like learning about electronics,history and amps here.
 
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playloud

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Thanks . Yeah I know the serial number thing is kinda all over with these but I still thought that cool . Dates of the caps are all pretty much right in there time line wise. On the original stuff that tested right and good.

I have a few nice variacs I use all the time but never built the current bulb limiter yet . I really doubt I would like it better with higher b+ colder bias though . I don’t want to ruin a good thing and that is last time i will ever probably get gecs again nowdays . Plus any gains of it would be a compromise with tubes i can not get anymore. I actually prefer the way it is set up now .

I am no tech or anything but have worked on tube mid-fi rebuilds ,Leslie’s and other musical instrument amps Over the past few years . I Have to be absolutely certain it will not fizzle the output transformer . I did have to replace one of 2 pin selector black plastic shorter piece on back. Those are good idea to solder and keep a close eye on all that. I need to learn a lot more before doing doing it. I like learning about electronics,history and amps here.

All good points, and there are several people with generally-nuanced opinions that agree the lower B+ makes no perceptible impact on tone.

Minor point, but shouldn't "colder bias" really refer to power, not current? In this case, you could keep it constant after the switch.
 
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All good points, and there are several people with generally-nuanced opinions that agree the lower B+ makes no perceptible impact on tone.

Minor point, but shouldn't "colder bias" really refer to power, not current? In this case, you could keep it constant after the switch.

All I know is like better after having done to ltd park 45/100 that has same style merren pt transformer. I notice techs and some amp makers recommend it especially with the new kt66 . I was always in the “no it had to be the highest” camp purist about it. I thought maybe just so the amp does not burn cheap tubes up which they will do lol. Thought it would not Like it as much you know.

It was lowered and that is where gecs live in right now. When that came back from him (it need some other debugging as I bought it used ,had some issues previous owner/ and in shipping) to my surprise I actually preferred the sound that way immediately. Different sounding like to be expected but really nice. I tend to like the sound of ones that are on the lower end of tolerance for b+ of it on different amps. Like one example runs a little higher b+ with the same exact transformer taps used and another runs towards to low end of tolerances.

Your minor point is a good point there. Made me use what’s left of my brain matter today and makes a lot sense .
 
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Derrick111

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... there are several people with generally-nuanced opinions that agree the lower B+ makes no perceptible impact on tone.

Huh? Differences in B+ absolutely does make a difference in an amplifier's tone. A huge difference.

Minor point, but shouldn't "colder bias" really refer to power, not current?

The bias voltage effects the current draw of the output tubes in a class a/b amplifier (what we're talking about here).
 

playloud

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Huh? Differences in B+ absolutely does make a difference in an amplifier's tone. A huge difference.

I'm talking here specifically about the Drake 1202-43 repros with multiple B+ taps in a 45/100 circuit (Marstran does 490/560 and Chris Merren does 460/500/560). I've spoken to Merren about this, and he reckons any difference is negligible. There are also members on here who have built clones, tried the different taps, and concluded pretty much the same.

I actually do think there's a (slight) difference - and recorded some clips to that effect a while back - but given it involves buying a set of GECs, it's also probably one of the least cost-effective paths to tone improvement I've found (and I've tried a few!)

The bias voltage effects the current draw of the output tubes in a class a/b amplifier (what we're talking about here).

I know, I'm just wondering what is more relevant to subjective impressions ("warmer/"colder") once there is another variable involved (B+/plate voltage). Is an idle current draw of 35mA with a plate voltage of 500V really "warmer" than 31.3mA at 560V? They both result in a plate dissipation of 17.5W.
 


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