# JMP 2203 Master Volume build



## adew1

Kit from Valvestorm:






Head cabinet from Tube-town:











I will be using Metro spec transformers, which I posted about here: https://www.marshallforum.com/threads/whats-your-opinion-of-the-state-of-this-ot.117293/
I have managed to clean these up a little and will post some up to date pics shortly.

Crikey, this amp is going to weigh a ton! Steel chassis plus those transformers. 

Will post some build pics as I go along.

A few thoughts regarding this build:
1. Grounding scheme. Not sure the original Metro layout is the best option.
2. Could be tempted to add a PPIMV, but not decided. 
3. Metro FX Loop is a likely addition.


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## StingRay85

PPIMV, but his thing already has preamp/MV?


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## adew1

StingRay85 said:


> PPIMV, but his thing already has preamp/MV?


I mean a Post PIMV. I always get confused with whether "PPIMV" means post or pre.


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## StingRay85

But why? You already have an excellent preamp and master volume control. You also have a volume control on your guitar. You need a fourth volume control?


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## neikeel

I would build it as stock except for the grounding scheme.
Suggest you follow the Larry grounding scheme (there are others but this is pretty clearly set out).





I would not add a PPIMV to this amp but the no loss FX loop will be a good idea, but get it built and working sweetly before you add the loop.


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## AlvisX

A PPIMV never made sense to me on a cascade amp .One of my 4010's had one when I got it , but put I put it back to stock ,sounds much better to my ears


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## adew1

StingRay85 said:


> But why? You already have an excellent preamp and master volume control. You also have a volume control on your guitar. You need a fourth volume control?


 Just an idea I'm knocking around. No decisions yet.


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## adew1

neikeel said:


> I would build it as stock except for the grounding scheme.
> Suggest you follow the Larry grounding scheme (there are others but this is pretty clearly set out).
> 
> 
> 
> 
> 
> I would not add a PPIMV to this amp but the no loss FX loop will be a good idea, but get it built and working sweetly before you add the loop.



Thanks, neikeel. I was hoping you would drop by and comment on the grounding. 

I found this on SDM's Google drive, with various alternative Metro layouts, of an alternative grounding scheme:




But looking more closely at this, the main difference compared to the original Metro build guide appears to be the bus wire arrangement on the pots. Perhaps I'm missing something.

I need to study your photo in more detail. I will have questions. 

Thanks for the other suggestions. Yes, I will build it stock and only add anything once all is up and running properly. The FX loop is working well in the JCM800, so I will definitely add it to the 2203.


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## neikeel

I’ve used Steve’s scheme too. The main thing I do is truncate the bus bar on the pots to exclude the presence pot which I ground at the presence pot. I quite like a ground wire linking the four output tubes to the bias caps too. I think that grounding the heater centre tap at the inputs is very well worth doing. As I mentioned you will see various schemes. The Larry on is simple and easy to follow and works well.


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## _Steve

neikeel said:


> I’ve used Steve’s scheme too. The main thing I do is truncate the bus bar on the pots to exclude the presence pot which I ground at the presence pot. I quite like a ground wire linking the four output tubes to the bias caps too. I think that grounding the heater centre tap at the inputs is very well worth doing. As I mentioned you will see various schemes. The Larry on is simple and easy to follow and works well.



I still don't understand grounding properly  What's the purpose of the bus wire on the pots, when the pots are already grounded to the chassis? Why doesn't this create ground loops?


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## neikeel

1) stops them moving (yes you can drill the chassis and use tabs)
2) it gives a secure ground to the pot shell to which you can attach the other grounds
3) it does work, my original Marshalls with this don't have ground loops and if I am building a clone or restoring I use that method but hide the other grounds mentioned above under the board.
I do think the other schemes may work better and the elevated bus wire method works just as well (you flip the ground wire down and run heavier gauge wire through the twisted loops.
Other alternative is central star ground for audio and another for power supply.
YMMV!


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## Trouble

I agree with 1-3 above ^^^. I have tried it without and it worked fine, but I like 1-3 myself as I have done it even when the layout didn't call for it.


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## adew1

neikeel said:


> I’ve used Steve’s scheme too. The main thing I do is truncate the bus bar on the pots to exclude the presence pot which I ground at the presence pot. I quite like a ground wire linking the four output tubes to the bias caps too. I think that grounding the heater centre tap at the inputs is very well worth doing. As I mentioned you will see various schemes. The Larry on is simple and easy to follow and works well.



@neikeel Thanks. Not sure I fully understand this "...truncate the bus bar on the pots to _exclude the presence pot which I ground at the presence pot_." 

I still haven't fully understood the Larry method you posted either, but am studying it in detail to understand the differences between the original Metroamp scheme and this.


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## neikeel

On Steve’s layout the preamp is a separate cluster of grounds. The power amp a separate cluster, power amp/pi/screens another set. The idea is to avoid contamination of signal with ac in each part of the amp.


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## coolidge56

adew1 said:


> @neikeel Thanks. Not sure I fully understand this "...truncate the bus bar on the pots to _exclude the presence pot which I ground at the presence pot_."



Basically where will you locate the chassis ground for the presence pot ground solder lug.

Location 1 - if the presence pot solder lug for ground is attached to the bus bar running across the other pot's solder lugs then it's chassis ground point is way over on the left side of the chassis at the 50uf+50uf preamp filter cap to chassis ground.

Location 2 - drill a hole in the chassis next to the presence pot and ground it there.

Location 3 - as the presence pot is part of the PI, per this schematic run a ground wire over to the 50uf+50uf PI filter cap chassis ground as shown in the schematic.


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## coolidge56

That schematic reminded me of this gotcha...from the 2203 schematic above the OT primary wires are Red/Brown and the center tap is White.




From the 2204 schematic the OT wire colors are different. The primary wire colors are Red/White and Brown for the center tap vs above Red/Brown and White. In fact both schematics are correct, it's the OT transformer wire colors that vary depending on brand/model OT.


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## adew1

Been crazy busy at work, hence not much movement on this build yet. But a few days off will allow me to get stuck in.  But, some progress has been made. I'll post a couple of photos later.

@neikeel , @coolidge56 Thanks for the additional explanations re grounding. Still hesitating a little on which way to go.

Re the OT wiring colours, yes, that almost caught me out too, the white being the CT on the 2203, but brown on the 2204.



coolidge56 said:


> Basically where will you locate the chassis ground for the presence pot ground solder lug.
> 
> Location 1 - if the presence pot solder lug for ground is attached to the bus bar running across the other pot's solder lugs then it's chassis ground point is way over on the left side of the chassis at the 50uf+50uf preamp filter cap to chassis ground.
> 
> Location 2 - drill a hole in the chassis next to the presence pot and ground it there.
> 
> Location 3 - as the presence pot is part of the PI, per this schematic run a ground wire over to the 50uf+50uf PI filter cap chassis ground as shown in the schematic.


Is there a "best" or "better" option out of these? My intention is to ground at the PI cap, ie Location 3, as per the layout.


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## adew1

If I go with the Larry ground scheme, as posted by @neikeel previously, can I just clarify if my understanding is correct? Numbers and colours as per Neikeel's image.

Ground 1 (Red): Heater centre tap, input jacks, V1 cathode.
Ground 2 (Black): Preamp cap, Gain and Master pots, V2 cathode.
Ground 3 (Yellow): Middle and Presence pots, PI cap, Speaker jacks ground.
Ground 4 (Purple): Bias caps, Screen cap ground. In the original layout, the screen cap is grounded to the chassis at the cap holder bolt. I assume this is no longer required due to the new ground point, correct?
Ground 5 (Orange): This one is confusing me.  Looks like it is tying together the rectifier ground, main caps, and the V4 - V7 sockets. Again, as per original layout the main cap is grounded to the chassis at the cap holder bolt. Is this ground kept or removed? It is not clear to me.

Yes, I admit it. Grounding schemes are a dark art to me.


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## adew1

A brief progress update. 






Trannies, choke, filter caps and tube sockets mounted. All pretty straightforward, although I did have to deal will a couple of very minor issues. Firstly, the bolt holes in the OT end bells needed to be opened up so that I could re-position the OT an extra millimetre or so to clear one of the board stand-off screws. Of course, I only noticed this after I had installed the OT . So, some time wasted removing the OT, filing out the bolt holes, then refitting. Secondly, getting the OT secondary wires through the chassis grommet was a bit of a bear, as each wire has a chunky glassfibre (?) sleeve making for a very tight fit. Second time around, I remembered that I had some silicone spray lying around, and a touch of this smeared on the grommet made the job 100 times easier. Why hadn't I thought of this before??!! I guess the more you build, the more tips and tricks you learn. 






Faceplates are quite nice, but I did have to do some trimming of the switch cutouts as they were slightly too small. The faceplates come with a contact adhesive already applied. Tip: this adhesive is very strong, meaning that fitting these is a "get it right first time" job. 

Presence pot will go in after I've soldered the 0.1uF presence cap in place. I prefer to do this before mounting the pot in the chassis.






Chassis is pre-drilled for the Metro FX loop jacks, but not for the loop bypass switch. Rear faceplate is not drilled for the loop, so I had to drill these, which was easy to do after the faceplate was mounted.






I'm still trying to decide on the grounding scheme (see earlier posts), but in the meantime will start on wiring up the switches and heaters.


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## jchrisf

Awesome build.. I love JMPs. Is this any different than a JCM 2203 build?


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## AndyD

That’s a great start. It’s a shame you’ve decided to implement a loop and drilled through the lettering on the rear faceplate.


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## adew1

AndyD said:


> That’s a great start. It’s a shame you’ve decided to implement a loop and drilled through the lettering on the rear faceplate.


Thanks. I’m an effects junkie, so I have to have a loop. Yes, it’s a shame about the lettering, but I wanted to use the holes already drilled in the chassis. I tell myself that it adds mojo .


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## adew1

@jchrisf Basically the same circuit. JCM800s are essentially rebadged JMPs with different head shells. Some minor component value differences here and there, but this was also the case during the JMP and JCM800 era.

I know you’re planning to do a build (saw your thread). You may want to check out my 2204 build here: https://www.marshallforum.com/threads/my-first-build-jcm800-2204.117303/

That was my first build. It wasn’t difficult, but I did have a few head scratching moments here and there. This forum was great for helping me past those moments. The kit I bought only had a layout, no schematic. The first thing I did was draw my own schematic based on a real JCM800 and the kit’s layout diagram. I spent a lot of time doing this ( and surfing Rob Robinette’s site) so that I had a good understanding of the circuit and its various constituent elements before I fired up the soldering iron. I also used the Metro 50 watt build booklet as an additional reference guide, specifically for the build sequence, and for things I wouldn’t have known how to do, such as wiring up the input jacks, dealing with shielded cable, and how to do the heater wiring.

If you’re methodical, working slowly and carefully, always checking your work against the layout and the schematic, building such an amp is doable.


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## jchrisf

adew1 said:


> @jchrisf Basically the same circuit. JCM800s are essentially rebadged JMPs with different head shells. Some minor component value differences here and there, but this was also the case during the JMP and JCM800 era.
> 
> I know you’re planning to do a build (saw your thread). You may want to check out my 2204 build here: https://www.marshallforum.com/threads/my-first-build-jcm800-2204.117303/
> 
> That was my first build. It wasn’t difficult, but I did have a few head scratching moments here and there. This forum was great for helping me past those moments. The kit I bought only had a layout, no schematic. The first thing I did was draw my own schematic based on a real JCM800 and the kit’s layout diagram. I spent a lot of time doing this ( and surfing Rob Robinette’s site) so that I had a good understanding of the circuit and its various constituent elements before I fired up the soldering iron. I also used the Metro 50 watt build booklet as an additional reference guide, specifically for the build sequence, and for things I wouldn’t have known how to do, such as wiring up the input jacks, dealing with shielded cable, and how to do the heater wiring.
> 
> If you’re methodical, working slowly and carefully, always checking your work against the layout and the schematic, building such an amp is doable.


Thanks for the info @adew1. I'm going to check out your other build too.


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## adew1

jchrisf said:


> Thanks for the info @adew1. I'm going to check out your other build too.


Cool! Feel free to post any questions about the 2204 on that thread. I will answer them.


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## jchrisf

adew1 said:


> Cool! Feel free to post any questions about the 2204 on that thread. I will answer them.


I'm sure I will have some


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## neikeel

He's going for that LA hotrod drilled Marshall look.

I prefer not to use the loop switch at all so that's one hole less (leave it in all the time) and put them in a line with the output jacks with equal spacing across the 4 jacks (Steve deliberately made the spacing on the loop like that so you could fit it to a 70s 100w without drilling.


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## South Park

I’m planning on building a 100 watt jtm and using only two inputs the ether input are for the FX loop .


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## coolidge56

neikeel said:


> He's going for that LA hotrod drilled Marshall look.
> 
> At first I was like oh no, then I was like never mind that's like 90% of the 2203's out there. lol


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## coolidge56

Here's some OCD inspiration for your build.


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## _Steve

coolidge56 said:


> Here's some OCD inspiration for your build.
> 
> View attachment 83350



Thats how they all look in my head just before I start working on them


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## adew1

neikeel said:


> He's going for that LA hotrod drilled Marshall look.


Exactly!


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## adew1

coolidge56 said:


> Here's some OCD inspiration for your build.
> 
> View attachment 83350


Looks at coolidge's wiring p*orn. Looks at own wiring. Suddenly feels inadequate... 

@coolidge56 Absolutely gorgeous workmanship, as usual. Love it!


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## adew1

Small progress update:

Whilst I continue to hesitate about the grounding scheme, and hence am a little blocked in moving forward with the build, I decided - for no other reason than having the urge to try something new - to build a turret board for the amp. I did buy the Metro one from Valvestorm, but I really liked the split turrets used in my 2204 build and thought why not have a go at making my own for the 2203.











G-10 board from TubeDepot. Harwin H2073Z01 turrets from Digikey.

Fun to do, though quite time-consuming.


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## junk notes

AndyD said:


> That’s a great start. It’s a *shame you’ve decided to *implement a loop and *drill*ed* through the lettering on the rear faceplate*.


 It is a kit? Hoping we never see that on a real one.


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## adew1

junk notes said:


> It is a kit? Hoping we never see that on a real one.


Yes, it's the kit from Valvestorm. As mentioned previously, the chassis comes pre-drilled for the loop jacks.


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## adew1

Can anyone help me decipher part of this image? I've decided to go with the Larry grounding, but have a few questions.






Ground #1 (red): Any reason why I can't run the heater CT along the edge of the chassis underneath the pots, rather than down the middle of the board as shown?

Ground #3 (yellow): The speaker jacks are also grounded here, yes?

Ground #5 (orange): I can see that this is connecting the main filter caps and the rectifier ground, but I can't make out what is going on at the output tube sockets. (Horizontal orange line at top of pic.) Is this linking the ground side of the 1ohm bias resistors to each other?

Ground #6 (green): This is the mains earth, yes?

Two additional questions:

1. PT bias secondary tap. Does one side still get terminated at the PI filter cap -ve terminal?
2. In the original Metro layout, all filter caps (or pair of filter caps) are grounded to a lug on the caps' mounting brackets. With the Larry ground scheme, am I correct in saying that these lugs are no longer used?

Thanks in advance for help with any of these points.


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## coolidge56

@adew1 a few things...

1. Love that you built your own board bravo! Never feel confined to the kit!

2. Seriously consider this no clean solder. Josh at 18watt.com recommended this stuff, it's amazing. I was draping off things to keep the flux spatter from polluting the boards/chassis. This stuff is pretty much ZERO spatter. Just finished a preamp board and it's completely clean and shiny. This is 20 guage, almost wish I had ordered 18 gauge instead a bit thicker. Good stuff! - 24-6337-8800 Kester Solder | Soldering, Desoldering, Rework Products | DigiKey

3. As for your grounding conundrum recommend you do this...place extra ground lugs in the chassis for both grounding schemes. This way you can experiment for lowest noise after the amp is built. Way easier to do this now vs later.

4. Place a ground lug on the chassis right next to the input jacks for the Randall Aiken RF interference obliterator mod. I do this as standard on every build now. Did I mention "right next to the input jacks" distance matters here even on Marshall builds where the star chassis ground is only 3 inches away from the jacks, even that's too far away I found on my JCM800 rebuild. RF (local radio stations) come flying into the chassis on the guitar cable shield. This mod puts a .01 to .001 cap between the guitar cable shield at the jack and chassis slamming the RF to ground as soon as it enters the chassis. Randall described this mod as killing RF "deader than hell" and he's not wrong. Here in the Pacific NW where I live radio interference is an annoying issue playing out the speaker cab. This mod obliterates it.


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## coolidge56

adew1 said:


> Can anyone help me decipher part of this image? I've decided to go with the Larry grounding, but have a few questions.



Arrgh, recommend you just use the ground scheme in your post #8. The 'Larry' also works but why make things more difficult than they need to be. Countless amps built using post #8 scheme that are so quiet you will think they are broken. Also why is there a ground bus wire on the back of the pots? More on that in a moment.

If you really want a quiet amp instead focus your attention on the AC wiring, tight twists and routing it away from signal wiring. Speaking of which in this grounding image above that's the WRONG way to route the heater wires for the preamp tubes. Yes it's duplicating the way Marshall did it in 1968 but they did it wrong.

Look at this heater wiring. Note how the heater wires arch up and directly over to the pins on the preamp sockets. WAY less AC hum with this method and if the AC wiring is well done throughout the amp, and you have quality preamp tubes there will be no AC hum at all. Well AC hum coming in on Strat single coil pickups could be an exception, but the amp will have none.




Bus wires on the back of pots - the post #8 scheme runs the bus wire instead through the ground lugs of the pots, better idea. The pot covers on the back may already be grounded to the chassis. This is true for PEC pots the rear covers and the rest of the pot not part of the resistive element are already grounded to the chassis by means of attaching them to the chassis with the front nut. Attaching the ground lug of the resistive element to the rear covers means now you have a bunch of ground locations, one at each pot attachment. This is likely why the method of running the bus wire through the resistive element of the pot ground lugs came from then attaching this to a single chassis star ground point.

While I'm at it WARNING those increasingly ridiculously expensive PEC pots...yeah the rear covers are frequently NOT grounded. (face palm) They don't crimp the rear covers on properly, kind of a 70% crimp so the crimp tabs are resting on the edge of the non-conductive pot wafer instead of being fully crimped to the front pot disc. I have to carefully finish crimping the rear covers with some channel lock pliers. Get your meter out, test for continuity between the front of the pot and the rear cover. Most likely some will not be grounded, others may be loose with intermittent ground.


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## adew1

@coolidge56 Thanks so much for your detailed reply. Much appreciated! I need to read through your comments carefully.

Re the solder, that’s the one I use. No spatter, very clean.


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## adew1

coolidge56 said:


> If you really want a quiet amp instead focus your attention on the AC wiring, tight twists and routing it away from signal wiring. Speaking of which in this grounding image above that's the WRONG way to route the heater wires for the preamp tubes. Yes it's duplicating the way Marshall did it in 1968 but they did it wrong.
> 
> Look at this heater wiring. Note how the heater wires arch up and directly over to the pins on the preamp sockets. WAY less AC hum with this method and if the AC wiring is well done throughout the amp, and you have quality preamp tubes there will be no AC hum at all.


I did the traditional Marshall style on the 2204 and don’t have AC hum, but am tempted to try the up and over method that you talk about, on this build. Heater wiring is probably my least fave part of amp building.


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## coolidge56

adew1 said:


> Heater wiring is probably my least fave part of amp building.



Said everyone who's ever built and amp brother.


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## adew1




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## adew1

Heater wiring done. First attempt at doing the up and over method (technical term, lol), and I'm quite pleased with the result. Surprisingly, I found it slightly easier to do than I expected, and perhaps easier than the traditional Marshall method, as the wire twisting of each tube to tube section can be done outside the chassis. @coolidge56 Thanks for persuading me to have a go at this method.


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## _Steve

That looks sweet - nice job!


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## coolidge56

Nicely done @adew1 did you twist the heater wires by hand or use a cordless drill? Overall your build is looking neat and tidy keep it going!!


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## adew1

Thanks, guys!
I twisted them by hand, one pair at a time, and took it slowly. As mentioned before, what I like about this method is that all the twisting can be done outside of the chassis. 
However, I didn’t share with you the first attempt that I quickly abandoned. 

Now the heaters are done, I can get on with the fun stuff! Next jobs: presence pots and input jacks.


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## AndyD

Looking good! Enjoying watching your progress!


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## coolidge56

adew1 said:


> Thanks, guys!
> I twisted them by hand, one pair at a time, and took it slowly. As mentioned before, what I like about this method is that all the twisting can be done outside of the chassis.
> However, I didn’t share with you the first attempt that I quickly abandoned.
> 
> Now the heaters are done, I can get on with the fun stuff! Next jobs: presence pots and input jacks.



On your next build cut a 20 inch length pair of heater wire. Chuck one end of the pair in a cordless drill, clamp the other end to your workbench. Pull slightly taught, the cordless drill will twist them for you perfectly in 5 seconds! A must method really for the PTFE (Teflon) top coated wire I use.


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## adew1

Thanks for the tip @coolidge56, will try that next time. 

Happy New Amp Building Year, builders! Looking forward to seeing what everyone will be building in 2021! 

Back to business... Still got a fair bit to do in the chassis before I can get on with the board, but I've been thinking ahead about a few minor mods and choices.

1. Using a 2200pF or 1800pF cap in the first treble peaker, instead of the usual 470/500pF, as suggested by @neikeel in another thread.

2. Wondering about adding a bypass cap to the 820R cathode resistor on V2. Some people swear by it, others swear at it and remove it immediately. Easy enough to try out once I've got the amp up and running, but any thoughts from you guys who've tried this?

3. TMB slope resistor as per the schematic is 33k. I've been playing around with the Tone Stack Calculator and I think I'll stick with this value for now as it gives a bit more low mids than higher values.

Why am I thinking of these mods? I would like to have a touch more gain than standard and fatter low mids. I've always liked the thick low mids that Orange amps seem to have. However, I think a lot of this is coming from their preamp design rather than the tone stack.

In the meantime, a couple of photos of latest progress:










This was my first time dealing with an OT with self-leads. Definitely more tricky to deal with, thanks to the enamelled solid core wires. I used a craft knife to scrape off the enamel for soldering, but I wonder if there is a simpler, less messy way to do this.

I need to touch up a couple of solder joints here and there, but otherwise the build is going quite nicely. Next jobs: input jacks, screen and grid resistors, and shielded cables to and from V1.


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## _Steve

You could always put the V2a bypass and treble peaker mods onto switched pots, that way you can have the best of both worlds.

Im definitely in the camp of not bypassing V2a. It introduces a lot more fizz. Valvewizard has an interesting schematic for a 'Bootstrapped DCCF' at the bottom of his page here: The Valve Wizard . I havent tried it but it might be worth thinking about if you are trying to get less fizzy gain. Horses for courses


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## neikeel

I think the cap over V2a depends on the amp. In a spilt cathode NMV amp it typically works well for that upper mids gain boost. On an MV amp it will depend on the NFB you use. If you go for 4ohm/100k (stock 2203) then I would definitely not use it. If you go 8ohm/47k (ie late plexi) then consider it - maybe on a mini toggle on the back (ground lift). Talking of ground lifts (and _Steve's talk about Bootstrapped cathode followers for gain boost a simpler method would be a ground lift switch on the tone stack.
Also consider 8k2 on cathode of second stage (unbypassed) subtle gain increase as is using 120k plate resistor on first stage.
All tone shaping rather than radical hikes as you keep the essential flavour of the 2203.


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## adew1

_Steve said:


> You could always put the V2a bypass and treble peaker mods onto switched pots, that way you can have the best of both worlds.
> 
> Im definitely in the camp of not bypassing V2a. It introduces a lot more fizz. Valvewizard has an interesting schematic for a 'Bootstrapped DCCF' at the bottom of his page here: The Valve Wizard . I havent tried it but it might be worth thinking about if you are trying to get less fizzy gain. Horses for courses


Thanks @_Steve 
Not looking for fizz, so I’ll take your comment onboard re the bypass cap. 
Yes, I thought of making the peaker switchable, but perhaps for another build. I’ve already committed atrocities to the rear panel, but the front one looks so nice and pretty as is. 
Re the bootstrap DCCF, thanks for reminding me of this. Interestingly, in his book, section 14.7 High Gain Preamp Design, he didn’t like the bootstrap in his high gain design. However, the 2203 isn’t really high gain so it might be an interesting mod. Have you done something similar in one of your builds?


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## _Steve

adew1 said:


> Thanks @_Steve
> Not looking for fizz, so I’ll take your comment onboard re the bypass cap.
> Yes, I thought of making the peaker switchable, but perhaps for another build. I’ve already committed atrocities to the rear panel, but the front one looks so nice and pretty as is.
> Re the bootstrap DCCF, thanks for reminding me of this. Interestingly, in his book, section 14.7 High Gain Preamp Design, he didn’t like the bootstrap in his high gain design. However, the 2203 isn’t really high gain so it might be an interesting mod. Have you done something similar in one of your builds?



Haha I feel I could win in a crimes-against-amplifiers competition 

No I haven't tried the bootstrapped CF yet. I was just re-reading his book (one day i'll even understand it!) right before I read your post so it was fresh in my memory. It sounds appealing though as I really hate fizz. I have put switchable bypass caps in at least 3 builds. I've stopped doing it now as I just prefer no bypass, but thats just me.


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## adew1

neikeel said:


> I think the cap over V2a depends on the amp. In a spilt cathode NMV amp it typically works well for that upper mids gain boost. On an MV amp it will depend on the NFB you use. If you go for 4ohm/100k (stock 2203) then I would definitely not use it. If you go 8ohm/47k (ie late plexi) then consider it - maybe on a mini toggle on the back (ground lift). Talking of ground lifts (and _Steve's talk about Bootstrapped cathode followers for gain boost a simpler method would be a ground lift switch on the tone stack.
> Also consider 8k2 on cathode of second stage (unbypassed) subtle gain increase as is using 120k plate resistor on first stage.
> All tone shaping rather than radical hikes as you keep the essential flavour of the 2203.


@neikeel Many thanks again for your feedback. People are so helpful here - it's great! 

In the Valvestorm/metro spec kit, the NFB is 8ohm/100k. For my 2204 build it was 4ohm/100k.


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## adew1

_Steve said:


> Haha I feel I could win in a crimes-against-amplifiers competition
> 
> No I haven't tried the bootstrapped CF yet. I was just re-reading his book (one day i'll even understand it!) right before I read your post so it was fresh in my memory. It sounds appealing though as I really hate fizz. I have put switchable bypass caps in at least 3 builds. I've stopped doing it now as I just prefer no bypass, but thats just me.


Interesting. Thanks. 
I may experiment with the bypass cap once the build is finished.


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## AndyD

Before I say anything, let me praise your efforts so far. The lead dress and general neatness of your wiring is superb! Now, having built a few amps, I know how tempting it is to tinker here and there! My advice is quite simple. I suggest you build a stock 2203 with stock values and see how it beds in over a number of hours of use. Components burn in and mellow and the same goes with the iron. After you have used it for a good length of time, that is the time to make adjustments. My 2103 from 1977 is totally stock and I wouldn’t change a thing about it!


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## harleytech

AndyD said:


> Before I say anything, let me praise your efforts so far. The lead dress and general neatness of your wiring is superb! Now, having built a few amps, I know how tempting it is to tinker here and there! My advice is quite simple. I suggest you build a stock 2203 with stock values and see how it beds in over a number of hours of use. Components burn in and mellow and the same goes with the iron. After you have used it for a good length of time, that is the time to make adjustments. My 2103 from 1977 is totally stock and I wouldn’t change a thing about it!


+1... !!


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## adew1

AndyD said:


> Before I say anything, let me praise your efforts so far. The lead dress and general neatness of your wiring is superb! Now, having built a few amps, I know how tempting it is to tinker here and there! My advice is quite simple. I suggest you build a stock 2203 with stock values and see how it beds in over a number of hours of use. Components burn in and mellow and the same goes with the iron. After you have used it for a good length of time, that is the time to make adjustments. My 2103 from 1977 is totally stock and I wouldn’t change a thing about it!


@AndyD Thanks for the kind words and encouragement!
You’re right, I will need to live with it a while, but it’s fun to speculate on tweaks here and there. I won’t be adding the bypass cap, though I may try it one day to see what it’s like.
Chassis and output tube sockets are now wired up. Time to get cracking on the board.


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## coolidge56

Agree with others on avoiding mods at this stage with the exception of the bypass bright cap on the preamp gain pot. That one cap can have a dramatic effect on tone and is much loathed by JCM800 owners. Some like the stock value, some hate it and clip it off, some replace it. I think because it's dependent on the guitar, pickups, and speaker cab being used. Boutique amps frequently install a toggle switch with 2-3 bright cap options for this reason.

On my 2204 build with a 2005 American Deluxe Strat, factory noiseless single coil pickups...

500pf was bright as the freaking sun even with the guitar tone backed off and the amp treble on like 1.
120pf was almost perfect for the strat but on the edge of being too dark.
220pf was the sweet spot for for strat/humbucker guitars.

This is a 3 way ON/OFF/ON 1/4-20 toggle switch with 3 bright caps wired in parallel. OFF is the middle cap only plus the cap on the pot. Up ON is the middle and top cap added together with the cap on the pot. Down ON is the middle and lower cap added together with the cap on the pot.

Values

*147pf* (100pf middle cap + 47pf pot cap)
*247pf* (100pf middle cap + 100pf top cap + 47pf pot cap)
*367pf* (100pf middle cap + 220pf bottom cap + 47pf pot cap)

So you could use a much larger cap in the up or down position on the toggle giving you both low and high value bright cap options.


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## adew1

@coolidge56 Looks great! I tried something similar on my 2204, using a DPDT switch to get no cap, 330pF or 800pF (330 + 470 in parallel), which had been inspired by a previous thread of your's in fact. 
I always play guitars with humbuckers, usually LPs, and found that the difference was subtle, so I usually leave the switch in the 330pF position. I'm usually playing through an attenuator with Preamp at around 8 and MV at around 5, so the bright cap probably has little or no effect.

For this build I'm keeping it simple and will clip in a few values until I find what I like.


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## adew1

Build finished!






With preamp tubes in, no output tubes, voltages look pretty good to me. Wall voltage is 232, rectifier "in" is giving 176vac, rectifier out giving 478vdc. This is a little lower than the Metro build guide figures, but probably because I'm using the 240v PT primaries with a slightly lower wall voltage.






Grid voltage of -48 is with no output tubes and trim pot set to maximum bias voltage. As mentioned, these figures are without the EL34s installed.

Time to load up the output tubes and see (and hear!) what happens.


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## coolidge56

adew1 said:


> @coolidge56 Looks great! I tried something similar on my 2204, using a DPDT switch to get no cap, 330pF or 800pF (330 + 470 in parallel), which had been inspired by a previous thread of your's in fact.
> I always play guitars with humbuckers, usually LPs, and found that the difference was subtle, so I usually leave the switch in the 330pF position. I'm usually playing through an attenuator with Preamp at around 8 and MV at around 5, so the bright cap probably has little or not effect.
> 
> For this build I'm keeping it simple and will clip in a few values until I find what I like.



Don't be afraid to try a lower value cap, say 120pf - 180pf. The effect of the bright cap is not linear. A 120pf cap while bypassing only the highest of the highs can sound less bright than a 470pf because those highest of the highs are not as powerful and will seem darker than a 470pf which seems to bypass a stronger band of highs vs a 120pf. If this was mapped to a graph the perceived brightness starting at 120pf would be lower, ramping up to a peak of brightness as you increase the cap value, then start ramping down again with even larger caps as some high mids begin bypassing and mixing with the highs.


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## coolidge56

@adew1 tell us about how you wired the fuses.


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## adew1

coolidge56 said:


> @adew1 tell us about how you wired the fuses.



@coolidge56 Uh-oh. You've spotted something I've missed, right?


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## coolidge56

adew1 said:


> @coolidge56 Uh-oh. You've spotted something I've missed, right?



No I didn't spot anything, just curious how fusing is done for 240 in Europe. Take a look at this thread...Amp Builders, do not fuse the neutral ! | The Gear Page


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## neikeel

With IEC socket you should take live to fuse immediately and switch live (not neutral).
If using solid/captive lead you should always use dpst mains switch and simultaneoulsy switch live and neutral - those are the regs. also to have a marked dedicated mains live chassis ground.
Personally I use the DPST for all new builds as it is safer and no-one really notices the 'mod'.


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## AndyD

Looks like a work of art! Hope you like how it sounds. What speaker setup are you going to use?


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## adew1

coolidge56 said:


> No I didn't spot anything, just curious how fusing is done for 240 in Europe. Take a look at this thread...Amp Builders, do not fuse the neutral ! | The Gear Page


As @neikeel said, mains Live goes direct to fuse then to DPST mains switch. Actually, I'm not 100% happy with the mains switch, which I bought from Modulus, as it is very stiff. But I can live with it.

Talking of fuseholders, although I did buy the "JMP" style ones offered by Valvestorm, I decided in the end to use ones suitable for 5x20mm fuses, as 1.25" x 0.25" fuses are hard to come by here (apart from Digikey, Mouser, etc).


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## adew1

AndyD said:


> Looks like a work of art! Hope you like how it sounds. What speaker setup are you going to use?


@AndyD Thanks!

Sounds great. There is something about the punch of a 100 watter that is very satisfying indeed. Feels stiffer (if that makes sense) than the 2204, but not in a bad way. Tonal balance - qute bright, but nothing that the EQ can't deal with. I find that what sounds good at home never cuts through live drums and bass, so this brightness will be good in a band situation.

At home, I'm playing through a 2x12 loaded with Celestion Redbacks or my Torpedo Captor X. Once I get it to my rehearsal space I'll be trying it out with a 4x12 loaded with V30s, but that won't be possible for a week or two due to other commitments.

Despite all the discussion about mods in this thread, the only deviations from the schematic are:

1. 2200pF treble peaker, instead of 500pF.
2. 120k anode resistor instead of 100k on 1st stage triode
3. 8.2k cathode resistor instead of 10k on 2nd stage.

I decided to install the standard 1000pF bright cap for now, but will experiment with lower values as suggested by @coolidge56 later.


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## adew1

All finshed apart from fitting the FX loop.


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## AndyD

adew1 said:


> @AndyD Thanks!
> 
> Sounds great. There is something about the punch of a 100 watter that is very satisfying indeed. Feels stiffer (if that makes sense) than the 2204, but not in a bad way. Tonal balance - qute bright, but nothing that the EQ can't deal with. I find that what sounds good at home never cuts through live drums and bass, so this brightness will be good in a band situation.
> 
> At home, I'm playing through a 2x12 loaded with Celestion Redbacks or my Torpedo Captor X. Once I get it to my rehearsal space I'll be trying it out with a 4x12 loaded with V30s, but that won't be possible for a week or two due to other commitments.
> 
> Despite all the discussion about mods in this thread, the only deviations from the schematic are:
> 
> 1. 2200pF treble peaker, instead of 500pF.
> 2. 120k anode resistor instead of 100k on 1st stage triode
> 3. 8.2k cathode resistor instead of 10k on 2nd stage.
> 
> I decided to install the standard 1000pF bright cap for now, but will experiment with lower values as suggested by @coolidge56 later.


That’s brilliant! I am new to 100watters and I agree that they have more “punch”. Added to that I think they have a broader frequency response and more definition. I’m quite taken with it! If you love the sound of your amp now....it can only get better with use! Thank you for sharing your project with us all.


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## adew1

@AndyD You're welcome.

I'm itching to play it in a band context, but not possible right now, thanks to Covid restrictions. I know a lot of us are in the same situation. I need to be patient. 

In the meantime I'm already thinking ahead to my next build. This amp building game is getting addictive!


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## adew1

My real job has kept me busy for the last week or so, therefore I haven't yet get around to fitting the FX loop and loading the headshell. 

However, had a couple of hours playing through the amp today (via my Torpedo Captor X) and noticed a very annoying fizz at the end of the note decay. I know this is pretty normal, as I've played plenty of amps in the past that did this to a greater or lesser extent. But this is irritatingly noticeable with the 2203, as it sounds "wrong" as the fizz fades in and out at the very tail of the note decay. Only noticeable when playing with high preamp gain and guitar volume rolled back, but I would like to fix this. As a first step, I'll swap the preamp tubes with another set and see if that makes a difference, and will recheck the bias - just in case.

If anyone has other suggestions, that would be great. Thanks.


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## Chris-in-LA

If you turn the treble and mids to zero, does the fizz go away? If so, you might be running too much highs.


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## _Steve

adew1 said:


> My real job has kept me busy for the last week or so, therefore I haven't yet get around to fitting the FX loop and loading the headshell.
> 
> However, had a couple of hours playing through the amp today (via my Torpedo Captor X) and noticed a very annoying fizz at the end of the note decay. I know this is pretty normal, as I've played plenty of amps in the past that did this to a greater or lesser extent. But this is irritatingly noticeable with the 2203, as it sounds "wrong" as the fizz fades in and out at the very tail of the note decay. Only noticeable when playing with high preamp gain and guitar volume rolled back, but I would like to fix this. As a first step, I'll swap the preamp tubes with another set and see if that makes a difference, and will recheck the bias - just in case.
> 
> If anyone has other suggestions, that would be great. Thanks.



I too would love to know what specifically causes this!? I've searched this before and always the answer is "its normal", but there must be a root cause and/or location it happens?


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## AndyD

Having just had a biasing fiasco with my 2103, I noticed that before I got the amp biased properly, the sound was definitely a bit fizzy. Maybe experiment a bit with the biasing . Also, too much bass can introduce fizziness. Speakers? Good luck!


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## dinkyguitar

@adew1 

I'm also thinking about building the JMP master volume from ValveStorm and I have a question.

On the PDF instructions for the Marshall JMP 100 watt master volume it lists 7 SOZO MUSTARD+ .022uf @ 400V.

But when I go to the website to select all the items with the "M" I see:
6 .022uf caps
1 0022uf cap

Is that an error? I counted 7 .022uf caps on the PTP board.

dinky


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## Chris-in-LA

Change the 6 to a 7 and buy them. You’re probably right.


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## junk notes

adew1 said:


> @AndyD You're welcome.
> 
> *I'm itching to play it in a band context, but not possible right now, thanks to Covid restrictions. I know a lot of us are in the same situation. I need to be patient.*
> 
> In the meantime I'm already thinking ahead to my next build. This amp building game is getting addictive!


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## neikeel

dinkyguitar said:


> @adew1
> 
> I'm also thinking about building the JMP master volume from ValveStorm and I have a question.
> 
> On the PDF instructions for the Marshall JMP 100 watt master volume it lists 7 SOZO MUSTARD+ .022uf @ 400V.
> But when I go to the website to select all the items with the "M" I see:
> 6 .022uf caps
> 1 0022uf cap
> Is that an error? I counted 7 .022uf caps on the PTP board.
> dinky



you need x7 0.022uF 400 or 630v caps for a 2203, a 0.1 for the pi on the board and (depending on your choice of presence circuit) another 0.1 on the presence pot. If you want more aggressive approach then use the 25k/4k7/0.68uF presence set up and then you need one 0.68uF for v1 cathode. The cathode and presence caps only need to be 160v rated max.


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## adew1

dinkyguitar said:


> @adew1
> 
> I'm also thinking about building the JMP master volume from ValveStorm and I have a question.
> 
> On the PDF instructions for the Marshall JMP 100 watt master volume it lists 7 SOZO MUSTARD+ .022uf @ 400V.
> 
> But when I go to the website to select all the items with the "M" I see:
> 6 .022uf caps
> 1 0022uf cap
> 
> Is that an error? I counted 7 .022uf caps on the PTP board.
> 
> dinky


I agree, I think it's an error on the Valvestorm site. The build guide shows 7 x 0.022uF and that's what I used. I almost bought the 0.0022 but VS were out of stock so I replaced it with the 0.022. Once I checked the build guide I was happy to have made that "mistake".


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## adew1

Once again work has kept me busy and out of the amp-building den. 

Thanks for the suggestions re the fizz. I don't think it is an EQ issue as it is only noticeable when playing with high preamp and low guitar volume. I don't like overly bright tones and tend to run the treble lower than middle and, on this amp, presence at only 2 or 3. As mentioned, the main note is clean, but at the tail end of the note's decay this fizz bubbles up (ha ha) kind of fading in and out as the very end of the note fades to silence. My 2204 doesn't do this at all.

@_Steve Yes, I've noticed this on other amps too and, like you, would love to know which part of the circuit is causing this. In a band context I don't think it would be noticed, but I can hear it and it bugs me! 

I wonder if the amp-building gods are trying to tell me that I really should buy an oscilloscope. 

In other news...

The lovely headshell purchased from Tube-Town doesn't fit my chassis! 
The overall dimensions are fine, but the internal bracing is wrongly sized and in the wrong place. Dirk at Tube-Town is trying to the right thing and sort it out, but this means shipping the head back to Germany - which is a PITA. I think I'm going to have to modify it myself, which will not be pretty.


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## adew1

Problem solved! 

Tinkering around this afternoon I decided to roll back a couple of the minor mods I had made, and found the culprit: I had forgotten to add a resistor in series with the Ice Pick cap around the NFB resistor. I clipped out the cap and normality was restored! I should have been more diligent as Rob Robinette had mentioned on his site that a series resistor may be needed to stop oscillations. On my 2204 build I had followed this advice, but for some reason didn't when I made the same mod to the 2203. 
I now have nice cleans when I roll back the volume, with no weird fuzzy fizziness.

Overall, I would say that this is still a brighter amp than the 2204, but it now behaves and sounds as I expect. Can't wait to crank it up through a 4x12!


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## _Steve

adew1 said:


> Problem solved!
> 
> Tinkering around this afternoon I decided to roll back a couple of the minor mods I had made, and found the culprit: I had forgotten to add a resistor in series with the Ice Pick cap around the NFB resistor. I clipped out the cap and normality was restored! I should have been more diligent as Rob Robinette had mentioned on his site that a series resistor may be needed to stop oscillations. On my 2204 build I had followed this advice, but for some reason didn't when I made the same mod to the 2203.
> I now have nice cleans when I roll back the volume, with no weird fuzzy fizziness.
> 
> Overall, I would say that this is still a brighter amp than the 2204, but it now behaves and sounds as I expect. Can't wait to crank it up through a 4x12!



Congrats! what valves you running?


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## adew1

Tung-sol 12ax7 and EL34B. I've got some JJs lying around somewhere and may give them a go too.


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## Goldguitarguy

Any chance you could post a pic of the inside of the head cabinet? I’m building a head cab for my 77’ 2203 and I’m wondering how the front panel is attached with the thicker front to fit the plastic corners. Also the build looks sweet!


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## neikeel

Goldguitarguy said:


> Any chance you could post a pic of the inside of the head cabinet? I’m building a head cab for my 77’ 2203 and I’m wondering how the front panel is attached with the thicker front to fit the plastic corners. Also the build looks sweet!



That head cab is like the earlier ones but has two vertical battens flush on the front of the inside of the cab (same thickness as the internal battens that reinforce the cab internals. The front panel with the control cutout is narrower to suit


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## adew1

Goldguitarguy said:


> Any chance you could post a pic of the inside of the head cabinet? I’m building a head cab for my 77’ 2203 and I’m wondering how the front panel is attached with the thicker front to fit the plastic corners. Also the build looks sweet!


Thanks.
I should be able to take some photos in the next day or two.


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## dinkyguitar

Does anyone know how tall the turret board standoff's are?

The turret board "looks" to be about 3/4" high off the chassis...is that right?

dinky


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## Trouble

It depends on what you have for standoffs I have 3 or 4 different ones


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## Trouble

I think all of these are shorter than 3/4


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## adew1

I used 14mm standoffs with 8mm threaded posts, same style as the one on the far right in Travis' picture. 14mm is just over 0.55". I think the ones supplied with the Valvestorm kit are 0.5".


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## dinkyguitar

Thanks....

I might have to cut the ones I ordered...but we'll see.


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## adew1

0.75" will be fine. There's plenty of vertical space in the chassis, unless you're planning to use enormous capacitors on the board. The field of view is a bit distorted in this photo, but it gives you an idea.


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## dinkyguitar

Thank you for taking the time posting the pic.

The chassis is relatively deep as you said.


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## adew1

I just measured the Valvestorm standoffs. They are 0.7". You will be fine with 0.75".


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## adew1

@Goldguitarguy About 2 weeks too late, probably, but here's a photo of the inside corner of the headshell:


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## adew1

Finally, all buttoned up in its headshell:


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## dinkyguitar

@adew1
Are JCM/JMP head cabs 1/2 plywood?

Can't tell.....

BTW...it looks official now


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## adew1

As far as I know, yes, normally plywood. Given the weight of the loaded chassis, the headshell (and carrying strap) needs to be well built! The chassis is a pretty tight fit, and manipulating it in and out several times when marking up where to drill the mounting holes was a real workout. Tip to self: next time, do this with an empty chassis.


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## Chris-in-LA

dinkyguitar said:


> @adew1
> Are JCM/JMP head cabs 1/2 plywood?
> 
> Can't tell.....
> 
> BTW...it looks official now


No. Not 1/2 inch. Either 5/8 or 3/4 inch. I need to measure mine when I get home.


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