want to mod JCM800 (2210), more gain needed

[HTH]

In addition to my post above.

How did you manage to short out C1, C39, and replace C32 and C34? They're as close to the PCB as they can get. Do I need to get underneath it?

you need to unfix the pots from the front of the chassis and flip the PCB over.

[BeëlzeM]

you need to unfix the pots from the front of the chassis and flip the PCB over.

Ah alright. That a lot of work though, I have to take all the pots out and that's it? Is that the only thing holding the PCB in place?

But it's worth it right? Will those 4 caps I can't reach make a huge difference?


EDIT:
Another thing by the way, how did that extra gain stage work out for you? Where would you rewire that green wire from the gain stage that came free?

Don't think I'm going to try that though, I reckon it will take a lot to revoice the amp but I'm curious anyway.

[Chris / CMW amps]

More bass:

you can use higher value coupling- and cathodebypass-caps but that can make the low-end mushy, less defined and it will also have an impact on the od-sound at higher freqs.
Here are some things that will work somewhat better (imho):
- installing a depth-control
- using a better OT and choke
- a tiny bit more filtering in the powersupply for the outputtube-plates & -screengrids
- check speakers, wiring/cables and cabinet

More gain:

check the resistors/caps between stages, lower a cathode-resitor-value for more gain. A better OT will give OD-sounds more harmonics and sustain.

Hope this helps!

[BeëlzeM]

More bass:

you can use higher value coupling- and cathodebypass-caps but that can make the low-end mushy, less defined and it will also have an impact on the od-sound at higher freqs.
Here are some things that will work somewhat better (imho):
- installing a depth-control
- using a better OT and choke
- a tiny bit more filtering in the powersupply for the outputtube-plates & -screengrids
- check speakers, wiring/cables and cabinet

More gain:

check the resistors/caps between stages, lower a cathode-resitor-value for more gain. A better OT will give OD-sounds more harmonics and sustain.

Hope this helps!

Hey Chris,

I'll just write in English since this is an English forum. But I'm Dutch as well. I've heard of CMW Amps before, through Voodoo amps actually, it seems like you know your stuff . I've been looking for mods for my 2205 for a while and couldn't really find them, unlike mods for the single channel 800's. Couldn't find anyone here in the Netherlands that actually knew how to improve or mod 2205/2210's. Someone from Dr. Tube actually said they don't do these amps cause they don't know mods for those, the reason was they use diode clippers and so the circuit wasn't interesting enough. Thought it was a strange response but okay. They also said these amps were too rare in the Netherlands to actually develop mods for them.

I was just wondering. Do you maybe know some easier and cheaper mods that work very well? I've already done some, only with replacing resistors with other values or shorting or clipping out resistors or caps. Or adding caps etc. It improved it quite a bit. Though I'm curious if it makes such a difference if I replace or short out those caps I can't reach cause they're too close to the PCB; I have to flip the PCB over and that's a bitch cause the pots with the control knobs are fixed to the PCB. And I have to take those out in order to get underneath the PCB.

Unfortunately I'm not really looking for the more expensive things like OT replacements or a complete overhaul by a good amptech like you. I'm very sure it's worth it though.

Chris, do you know if it can do these type of tones eventually?:
http://www.youtube.com/watch?v=UdSdgOYBH88

Any help for a Doe het zelver?

[HTH]

That a lot of work though, I have to take all the pots out and that's it? Is that the only thing holding the PCB in place?

it should take no more than 5 minutes - you remove the knobs from the pots on the front panel then loosen the nuts from the pots. The pcb is only fixed in place by the pots and input jack being fixed through the front-panel, easy.

[BeëlzeM]

it should take no more than 5 minutes - you remove the knobs from the pots on the front panel then loosen the nuts from the pots. The pcb is only fixed in place by the pots and input jack being fixed through the front-panel, easy.

Alright then. Shouldn't be too much of a trouble then...

[BeëlzeM]

What caps should I use? Polypropylene? 100V is alright? What would the max voltage rating be I should use?

[HTH]

What caps should I use? Polypropylene? 100V is alright? What would the max voltage rating be I should use?

polypropylene caps are fairly transparent sounding, but I find they have a hollow midrange. I'd use (do use) polyester caps - the mids are fatter.

As for voltage, whatever the voltage rating on the existing caps is - use the same and you wont go wrong.

100v will be too low for the most part - use 400v throughout if you're not sure. 25v is more than enough for cathode bypass caps though.

[BeëlzeM]

Alright, so I managed to unfix the pots and flip the PCB, where I did some mods. It's sounding pretty nice already.

Here's what I've done so far:
- short C1 (0.047uF) with a wire
- clip out R12 (220k)
- short out C39 (0.047uF) with a wire
- clip out C20 (0.002uF)
- R4 (100k) to 220k
- R43 (470 ohm) to 820ohm
- R41 (330k) and R44 (120k) to 1M

Next up:
- C32 (0.022uF) and C34 (0.022uF) to 0.047uF
- R47 (100k) to 47k
- 10uF cap across R25

I was also considering the 'resonance'-control, or 'depth-control'. I'm supposed to clip out R46. And do the following:

Looking at the back of a 1meg pot, left to right, numbering pins one two and three. Disconnect your purple feedback wire from the impedance selector and solder to pin three. Solder a new wire from the feedback resistor (where the purple feedback wire used to be connected) to pins one and two. Solder a .0047 cap from pin three to pins one and two.

How should I wire the 4700pF cap, from pin 3 to both 1 and 2? Is that the same as soldering it from pin 3 to pin 2 and then soldering a wire from pin 2 to pin 1?

People are also speaking of a .68uF cap across the presence-pot to control mids-highs instead of just the highs. How should I solder this .68 cap? From what pins to what pins? I heard this is supposed to give more midrange gain and grind, pretty much what I like.

[HTH]

yep, clip out R46 if you're doing the resonance mod - I don't know why, but with the DI circuit still 'in circuit' the resonance mod just didn't want to work. who uses a DI out anyway?

Solder the 4700pF cap across the resonance pot as stated in your post... one end is soldered to pin three and the other end is soldered to both pins one and two.

The 0.68uF cap on the presence pot will give you more crunch in the mids for sure - just replace the existing 0.1uF cap with a 0.68uF one - no other changes needed.

[BeëlzeM]

yep, clip out R46 if you're doing the resonance mod - I don't know why, but with the DI circuit still 'in circuit' the resonance mod just didn't want to work. who uses a DI out anyway?

Solder the 4700pF cap across the resonance pot as stated in your post... one end is soldered to pin three and the other end is soldered to both pins one and two.

The 0.68uF cap on the presence pot will give you more crunch in the mids for sure - just replace the existing 0.1uF cap with a 0.68uF one - no other changes needed.

Yeah true. Never even used the DI out in studios. I think I'm going to get a pot and put it where the DI control was, replace it with a resonance control.

Any other cap values other than the 4700pF I can try to make a larger range?

That .1uF presence cap, that would be C31? Across pot VR11. Another annoying capacitor type

Oh, and that 10uF cap across R25, 100V is enough there? Or should I go with 400?

[BeëlzeM]

In addition to my post above, did you like this one:

R3 (10k) to 2k7, also remove D1 bypassing cathode and use 0.68uF cap to bypass R3. removing D1 lowers gain quite a bit, but overdrive ‘feels’ more valve-like and natural/organic, it does however take some sizzle from the tone and need to re-EQ amp to compensate for treble loss (though a cathode bypass cap would most likely take care of that). Lowering cathode resistor (R3) to 2k2 increased gain, but not a mental amount like expected. Have added 0.68uF cap across cathode resistor, added more gain and snarl in hi-mids. The diode and 10k still had more more overall gain, though the tone was more ‘transistory’.

Sounds pretty promising, though I'm concerned about the amount of gain lost. I quite need the gain, is it worth it?

I might also try out some other things recommended in this thread:

To tighten up the bass a little I would reduce the value of the 3rd stage cathode cap (C18) from 22uF down to 5-10uF. I'm not a fan of cathode followers, so I use these values quite often in my amps with good results. A 22uF cap in that position usually just sounds to flabby / farty when the stage is driving a tone stack. Too small a cap there and you will get a high pass right in the middle of your midrange which ends up sounding funky.

Any comment on this so far? The amp is coming along nicely, it's definately sounding better than stock. I liked the presence pot cap mod by the way. Nice grunt. Still have to do some work, most definately. The low end is still a bit flabby, and not smooth. It sounds a bit sterile overall as well.

What would placing a lets say 1uF cap across R16 on V2 do? More gain?

[HTH]

What would placing a lets say 1uF cap across R16 on V2 do? More gain?

yep, more gain - but that gain stage is for the clean channel, so it would only add more gain to that channel.

[BeëlzeM]

yep, more gain - but that gain stage is for the clean channel, so it would only add more gain to that channel.

Alright, will see what I will do. Aand the other stuff? Any comments on those?

Someone from Voodoo recommended these changes:
R45 (22k) to 10k
C35 and C36 from 10uf to 100uf/100v
Remove C33 and install a 47pf across the bias side of C14 and C16

So it would be more to the specs of the PI on the 2203.
What does this part mean:
- install a 47pf across the bias side of C14 and C16

[HTH]

Someone from Voodoo recommended these changes:
R45 (22k) to 10k
C35 and C36 from 10uf to 100uf/100v

R45 is the 'tail' resistor in the PI - lowering it to 10k will reduce gain/breakup (I like 22k here and actually change amps where they use 10k here)

C35 and C36 are caps in the bias supply - I see no point changing them to 100uF

Remove C33 and install a 47pf across the bias side of C14 and C16

What does this part mean:
- install a 47pf across the bias side of C14 and C16

the C14 and C16 part makes no sense with regard to a 2210 Marshall so I'll ignore that.

C33 across the plates of the PI will stop (very) high end oscillation and smooth the treble a little. I wouldn't bother replacing the existing 100pF cap with a 47pF cap, I didn't and the amp still had plenty of treble.