HELP jcm 800 2204 lacks OOMPH

[marshall_law]

Did a 2204 build from the ground up. Not a kit. Used carbon film resistors. Sozo yellow caps. Classictone iron. I have modified the original circuit slightly by removing the 10k on v1a and replacing it with a 2.7 and a .68 combo and also added a .68 to the 820 ohm on v2a. I also removed the first treble peaking circuit. In my estimation it has enough gain but really lacks low end. I was thinking about upgrading the output transformer from classictone to a MM. Before i spent the money i wanted opinions on if that will even help and if there is other mods i can do first to get some low end thunder.

[danman]

By removing the 10k cathode resistor and biasing with a 2.7k/.68uf, the sound may turn to mush. You are really increasing the gain with the 2.7k and boosting the mids quite a bit with the two .68 bypass caps. If you try to increase the gain too much in one stage it can start to sound bad. If you want a little extra gain, try a 5-7k cathode resistor without the bypass caps. If you want to cut back on the high end, remove the 470k/470pf treble peaker on the input and replace with a single 68k resistor. I installed a fixed resonance mod on my 2204 and it did help to increase the low end and tighten it up a bit.

[danman]

Forgot to mention that I don't believe that buying a MM OT will help to increase the low end. Try a few of the changes I mentioned first before dropping the coin on a different OT.

[DG45]

Not an expert but for what it's worth, I agree with danman on the OT. Different build but I'm guessing we probably used the same Classic Tone OT (18025 I think). I'm definitely not lacking any bottom end. Again, that's assuming the same OT.

[Haze13]

Pics, Pics, Pics...
Return to the stock circuit and start looking for a problem. I have 2204 with classic tone OT and the last thing that I'm worry about is a Bottom end...
P.S. Make sure that all your caps are 22nF and not 2.2nf...

[neikeel]

By removing the 10k cathode resistor and biasing with a 2.7k/.68uf, the sound may turn to mush. You are really increasing the gain with the 2.7k and boosting the mids quite a bit with the two .68 bypass caps. If you try to increase the gain too much in one stage it can start to sound bad.

Yup.

I would make sure that I had 0.022uF couplers. Then use 8k2 cathode resistor on second stage (maybe bypass it later with a 0.1uF - but not yet). Use a treble peaker, I suggest 470k but use a 1000pF - 2200pF cap for it and a 1000pF on the pot. By all means use the 0.68uF on V2a but you may need to use a little more NFB than stock (100k on 4ohm tap) try 47k on 8ohm tap.

If you overload first stage you will lose definition and tightness.

[syscokid]

Excuse me if you already know this, but these type of amps really come alive only when you really crank them. Not being "macho", but unless you have a real good attenuation system for the amp, these amps are like magic when you got them operating at a 50%-plus level... and there's plenty of oomph...

[SteadyEddie]

My 2204 clone through a 4x10 cab has got plenty oomph.
Try the fixed resonance mod first. That should give you some chunk.
It might be frowned upon, but don't be afraid to turn down the mids. Depending on your speakers. I find the midrange pot can have dramatic effects when turned down, it can go from bright and beamy to thick and chunky. It won't sound like no Pantera scooped mids but it can give it the "illusion" of sounding fatter when turned down.

[darkbluemurder]

Try the fixed resonance mod first. That should give you some chunk.

What he said. Try 220k in parallel with 2.2 nf (=0.0022uf) or 4.7nf (=0.0047uf) into 47k ohms, from the 8 ohms tap.

Cheers Stephan