increase gain in the jcm 2204

[wavenator]

http://mhuss.com/MyJCM/JCM800_2204.gif
i want to mod my jcm 2204
well
i thought about some mods
i want to increase the gain
what ever, if i decrease the 10 k resistor from v1a pin 3 ? will it works?

[petemoore]

  Yes, easy enough to try.
  You could put a bypass cap on it.

[runmikeyrun]

maybe start by playing with the neg feedback wire to the speaker jack (i'm pretty sure 2204 has it, 2203 does if i'm not mistaken) sometimes that's enough by itself.  The presence control is in there to adjust the highs, but disconnecting the wire usually gets things cranking pretty freakin loud.

I found this on a website while looking for schematics.  Can't vouch for it so beware!

Gain Mods

These are gain mods aimed at increasing the distortion available.

   1. Locate the 10K cathode resistor (R6 found on V1B) and replace it with a 4.7K resistor (or just solder another 10k in parallel on PCB amps). This will beef up the gain. This seems to be the "sweet-spot" on these amps. You can also try other values like 6.8k, 3.3k, 2.7k, 1.5k If you want insane gain use a 1.5K (but I think it's too much and doesn't sound as good). The 4.7k sounds just right and the gain doesn't get mushy. Since there is no cathode bypass cap here, all the frequencies in this stage are amplified normally (flat response).
   2. You can also increase the plate resistors values to add more gain from 100k up to 150k or 220k (insane). I don't like this mod as much, it gets buzzy.
   3. Change R11 in the voltage divider to 120k, then change R10 in value from 470k thru 120k, wherever you like it best. The 120k for R10 gives a richer distorted tone.

This sounds similar to the neg feedback control i was telling you about:

Resonance Control

This mod was first posted by Mark Cameron. It works on on Fenders and Marshalls with great results. It adds an extra knob (output jack holes work well for this). Basically, it is the opposite of a presence control, allowing you to tighten up the bottom. I will try to talk you thru the wiring. 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.

[gutsofgold]

oh man, DON'T DO IT! that is such a nice amp as it is, I would build yourself a booster.

[petemoore]

  -1 on what I said, relevant answer to the original question.
  +1 on what I wanted to say and Gutsofgold said.
  Why mess with HV/tube bias etc. when you can get as good or better using say 1 Jfet @9Vdc to do the boosting, have your original amp sound, or + booster, switchable.
  Slightly higher voltage to tube input...recommended, throw in a switchable pre-booster, will do lots like diddling with cathode bias except doesn't mess with the cathode resistor etc., sounds great, can easily be taken out or diddled with, replaced by the 'other one' etc.

[Steben]

I concur.
Some amps are just to be untouched.
A booster is less difficult and dangerous too, talking about 9 volts ONLY.

The thing is, blowing up a pedal needs an easy fix: try another.
Blowing up an amp (assuming you didn't blow yourself  ) is tricky and expensive.

[slideman82]

Talkin' 'bout JCM800's... I've seen many schematics around there, but some of them have the same preamp as old Plexis... did 2203 model come with 2 preamps, Plexi's and 3 stages? I'm consfused, but I think I know old Mastervolume differs with 80's 800's in one 100p cap and output tubes, am I right?

I'm modding an old amp built in my caountry in early 70's, just making one channel sound like an 800, and it does! Just reduce 10k resistor mentioned before, or reduce 2.7k resistor, also works. Adding a cap in parallel with 10k resistor will rise higs quite much (and I used a 100n cap) but will make the amp dirtier. Try the boost idea first, before modding that great amp! A Fetzer will do the trick quite well, or any mosfet booster.

[petemoore]

 suggestion.
  Try lowering the gain of V1, use a 12a_7, such as 12au7 substitute.
  Tends to smoothe the response of my preamps, 'hard' attack response is something ~undesirable to me, and easy to get, harder to get away from IME.
  Then 'pummel' the V1 tube amp input with a boosted source.

[Dai H.]

there's no reason to be scared of modding the thing. (Assuming that you know how to work with tube amps safely) you might try soldering some small alligator clips to a 10k R and clipping that across the existing 10k cathode R. That way you can just take it off if you don't like it. Lowering the 10k seems to be a pretty popular mod. (As mentioned above) adding a bypass across will also boost gain. You could try different values to taste. Anyway there are high voltages in there (which can be present even if the amp is off and unplugged) so if you aren't comfortable leave it to someone who is competent or learn how (and pay attention to safety).

[Phorhas]

I think it's more of a "don't touch a classic" thing, more than the HV thing. Like not installing shoulder-strap seat belts on a 60's Mustang

[petemoore]

  This is a LV forum really.
  The Amp forums have the obligatory and appropriate 'at your own risk' and 'lethal voltages' warnings and literature.
  Reliability being the most important priority to amps and humans...
 

[Dai H.]

I think it's more of a "don't touch a classic" thing, more than the HV thing. Like not installing shoulder-strap seat belts on a 60's Mustang

it must be relative (age, taste, etc.) but I don't really consider the JCM800 2204 to be that much of a classic (enough to treat it as a Nigel Tufnel "don't touch or even look at it" sort of amp). Then again, I suppose you wouldn't want to get too carried away experimenting and possibly end up hacking up an amp that is already in good condition.

The reticence here is just a little funny because I see so much solid-state stuff here on a regular basis that looks far more complicated. Really, *IMO* a semi-brain dead stoner could pull off some of the more common mods as long as he/she wasn't being too stupid and careless (there is an AC connection and high voltage, after all). I can do it, you can do it. 

[wavenator]

OK I understood
any way
i have a plexi 50 1987
how can i increase the gain in it? lol so much questions

btw
i am not new to the HV scene
i know what are the safety rules

[petemoore]

how can i increase the gain in it?
  By decreasing those things which reduce the gain.
  Tubes bias like Jfets, take a Jfet and set bias points, then use different value resistors/capacitors to see what increases gain.
  Don't use the values verbatim, but as a guide to what does what in a Jfet or triode stage.
  The drain / anode resistor ...its ratio to the cathode/source resistor...adjust to maximize gain if you like.
  But still begs the question what benefit will there be by such a modification, other than more gain in that stage ?

[MarcoMike]

I made my own Marshall-inspired amp... a Marshmallow actually (pics... sooner or later)
it's an hybrid 1987 - 2204. I made it this way because I used to play on a 2203 and it was difficoult to get nice clean sounds on that one... but didn't want to loose all that nice distortion....
now that I tried the 1987 style, I'm just in love with it! never switch the 2204-mode on! you just need an LPB and you're done!

[Phorhas]

OK I understood
any way
i have a plexi 50 1987
how can i increase the gain in it? lol so much questions

btw
i am not new to the HV scene
i know what are the safety rules

You could replace the cathode follower before the tone-stack with a HV MOSFET and this will free up a triode for you to play with. You could wire one of the inputs as a 2204, or just add another gain stage in there (think SLO100 and such...).

BTW, is it an actual 1987 or a Halilit 120W MP ?

[wavenator]

phorhas hehe its wavenator form musictalk
i remember you name

actually
i've already checked it
and halilit 120 is a plexi amp which Farchi made by him self

anyway
i am just about to build one
and i am looking for more gain in it

thx for all rec's
anyone else want to add something

anyway ill upload some pics if you would like to watch in the future

[Phorhas]

I got one of those amps for free, small box, like a Sound City 50, but mine is in bad shape - so much so that even the speaker impedance selector is broken. I should probably fix it and have myself a nice Plexi clone... I just hope that the (humongous) transformers are intact - or it goes to the scrap pile 

[AL]

Just to clarify. There are about a billion "Plexi's" out there. Everyone definition is different. But, a 2204 is nothing like a plexi.

Standard amp mods should work on this one. Mess around with the gain stages. Try increasing the plate load resistors on V1.

Or, as has already been suggested, just run a booster in front of it. A tube screamer should work just fine. If you're looking for Dual Rectifier gain this is not your amp.

AL

[slideman82]

OK I understood
any way
i have a plexi 50 1987
how can i increase the gain in it? lol so much questions

btw
i am not new to the HV scene
i know what are the safety rules

Try taking out the 470k/470p filter after the volume pot... sure you'll have more gain, but more bass too, I think. Just take them out and then put a jumper or a wire instead of.