Channel Switching JCM800 Design? + BIAS pot question

[ViperDoc]

Thanks, Phil! I did see those. I contacted CTS directly and they said they could make some dual 1 MA pots if I ordered 1000 of them . Yep, Nope.

[ViperDoc]

OMG!!!!!!

I just transformed this amp. I did two things:

1) I disconnected the 1 nF bright cap altogether (I might put it on a switch later). Thanks for the advice!
2) I soldered a 500 pF cap across the V2A 100K plate resistor.

I left the peaking networks in for now.

I CANNOT BELIEVE THE DIFFERENCE. The treble pot is now *actually usable*, I can actually stand to use it, even dimed! Before, unless I had it all the way down, it sounded like a torture device crossed with Edward Scissorhands. The distortion is so much smoother now, I LOVE IT. OMG. I got the idea from looking at the Friedman Smallbox 50 schematic I found online, perhaps here somewhere.

I'd like to replace the 10 pF V1A cap with the 2204's original 100 pF, but I don't have one at the moment. The SB50 puts another 500 pF cap parallel to the V1A plate resistor as well, but I'm not sure I would do both that and the 10/100pF plate-cathode cap.

SB50 Small Box.png

[ViperDoc]

I put my 1 nF brite cap on an on-on switch and it has a little bit of pop. I’ve heard soldering a 1 M+ resistor across the outer terminals will help. Is that right?

[ViperDoc]

I updated my bright cap mod with an on-off-on switch and threw the 1 nF on one end and a 500 pF on the other. It’s very useful and sounds great!

[ViperDoc]

I've been swapping out cap values across my V1A plate-cathode. My kit came with a 10 pF cap, I swapped it with a 500 pF and it sounded great. I thought I'd test the 100 pF as spec'd on the 2204 Marshall schematic, so I swapped that in and it sounded a little brighter, which I didn't prefer. So I put the 500 pF cap back in. At that point my high input stopped working. The low input works just fine. I thought I might have run solder down into the V1A tube socket pins and onto the tube (the tube was in when I soldered the changes. I took the tube out and it looked fine. I swapped in a brand new 12ax7 and the low input channel works like a charm. I changed out the 22 uF cap as it looked like it had a mark on it, possibly dropped solder. Low input works great, still just massive buzzing on the high input. I desoldered all the V1A wires and replaced the socket pins with new ones, re-trimmed and soldered the wires anew. Still, V1A doesn't seem to work. Just constant buzzing now. Never happened before. The 1M, 68K and 2K7 resistors and the 0.68 uF cap all look as good as the day they went in. I could replace the whole socket, but because I just replaced all the pins and the rest looks fine, I'm not sure why that would be any more effective. Not sure what to do.

[ViperDoc]

I fixed my problem. I decided to remove the V1 socket and replaced it with a new Belton socket. I put in a new 100 pF plate-cathode cap to rule that out. Works like a charm now.

Incidentally, I have been using paste flux to clean my soldering iron tip and recently picked up some liquid rosin flux by MG Chemicals to try. It didn't work very well [in my hands]. Might have been part of my problem as it runs off the iron tip.

[ViperDoc]

After some deliberation, I decided to add a Metro ZL effects loop to my 2204 build. George did such an amazing job on this loop, it comes with stellar instructions and literally everything you could possibly need to get ideal voltage on the board. And it sounds great! My layout was offset quite a bit towards the preamp side because of the differences between the Mojo and the Metro chassis', so I opted to grab my B+ from the first 10K cap node instead of the PI supply node. The voltage there was much closer to the target anyway (317 VDC vs. 439), so it worked out. After I got my holes drilled and chassis cleaned up, installing this board literally took all of 30 minutes.



I bench-tested the in-and-out levels, trimmed the loop level pot and that was it. I played some Spirit of Radio (as one does) and then screwed the chassis into its new cabinet. Red Box.



The one thing I did notice that I don't on my other amps is that the loop pops when I step on a pedal. Any recommendations? Thanks.

[pompeiisneaks]

That's usually an impedance mismatch. If you have a buffer pedal, or a pedal with buffering that you can keep on all the time, that usually helps.

~Phil

[ViperDoc]

Thanks, Phil! I’ll check that out.

[angelodp]

Great thread, but man this is making me even more nervous about the Mojo kit. I am following the Metro layout to the tee. Mojo has stood by their mistakes with my kit but there are numerous issues.

Great to see this build get sorted out so well.

[cruisemates]

I have been resurrecting my original JCM2204 as well, and I have been through many of the changes as the OP here. I found the treble bleed cap (I still have trouble with the names of each part, sorry) that goes between pins 1 & 2 on the MV is necessary, if I remove it MY amp gets very unsteady - oscillation in the tone stacks, etc. But I settled on something far lower than 22N, I think I have something in the pF range now in there. The treble peaker is almost stock a 470p with a 510R is parallel.

Here is one option someone suggested to me that I tried and liked a lot - putting (I believe) a 3.3N cap (700V) between the 16-ohm selector (purple wire) and the lead going to the presence pot. It changes the character of the amp a lot - a drop in volume, but also a much smoother overdrive but with no loss in sustain. This seems like a great option to make switchable but I do not have it installed now because I don't see it mentioned often so I am not sure if it is commonly done.

Here is another thing, I also find my presence pot (22k) is only usable on the last 30% or so - no change for the first 2/3 a roll. I have it wired up like the schematic; 22k pot with 100n cap pin 3 to ground, and the 47k resistor from 1 to ground (on some schematics this is a 4.7k R) parallel. I am not sure if I have an A or B Presence pot (whatever the amp comes with is what I have). But does beta have more working control?

Otherwise, I love the tone I am getting, tons of tight overdrive, easy to manage treble, but still startling clarity on individual notes. Especially with no reverb.

As far as treble goes, I have found that my midrange is very effective at boosting whatever I choose to push/ If I turn up the treble the midrange gives me more. But if I dial back the treble and boost the bass the midrange gives me more lower mids. That is what you want, IMO.

AS far as the bias pot. You can't beat using the 1ohm resistor to ground method measure across that resistor. You can set your bias at the exact voltage the charts specify - no guessing.

[ViperDoc]

Great thread, but man this is making me even more nervous about the Mojo kit. I am following the Metro layout to the tee. Mojo has stood by their mistakes with my kit but there are numerous issues.

Great to see this build get sorted out so well.

I would go with Metro or try an SDM build on my next 2203/4. Pompeiisneaks just uploaded the entire SDM drive contents on another thread. See www.valvestorm.com and look for SDM info.

[ViperDoc]

I have been resurrecting my original JCM2204 as well, and I have been through many of the changes as the OP here. I found the treble bleed cap (I still have trouble with the names of each part, sorry) that goes between pins 1 & 2 on the MV is necessary, if I remove it MY amp gets very unsteady - oscillation in the tone stacks, etc. But I settled on something far lower than 22N, I think I have something in the pF range now in there. The treble peaker is almost stock a 470p with a 510R is parallel.

Here is one option someone suggested to me that I tried and liked a lot - putting (I believe) a 3.3N cap (700V) between the 16-ohm selector (purple wire) and the lead going to the presence pot. It changes the character of the amp a lot - a drop in volume, but also a much smoother overdrive but with no loss in sustain. This seems like a great option to make switchable but I do not have it installed now because I don't see it mentioned often so I am not sure if it is commonly done.

Here is another thing, I also find my presence pot (22k) is only usable on the last 30% or so - no change for the first 2/3 a roll. I have it wired up like the schematic; 22k pot with 100n cap pin 3 to ground, and the 47k resistor from 1 to ground (on some schematics this is a 4.7k R) parallel. I am not sure if I have an A or B Presence pot (whatever the amp comes with is what I have). But does beta have more working control?

Otherwise, I love the tone I am getting, tons of tight overdrive, easy to manage treble, but still startling clarity on individual notes. Especially with no reverb.

As far as treble goes, I have found that my midrange is very effective at boosting whatever I choose to push/ If I turn up the treble the midrange gives me more. But if I dial back the treble and boost the bass the midrange gives me more lower mids. That is what you want, IMO.

AS far as the bias pot. You can't beat using the 1ohm resistor to ground method measure across that resistor. You can set your bias at the exact voltage the charts specify - no guessing.

The 3.3 nF cap from the 16R OT tap to the presence pot, is this parallel to the NFB coming off of the 8R tap? Do you have a schematic?

[cruisemates]

The 3.3 nF cap from the 16R OT tap to the presence pot, is this parallel to the NFB coming off of the 8R tap? Do you have a schematic?

No, my NFB purple wire is connected to 16-ohm tap, and I just put the cap in series with the purple wire to the presence pot. It is not in now, but I think it would be a great idea to make it switchable.