I quickly measured all the resistors in the amp (initially looking for the "problem" before I realized it was a 6550 amp) and I found that the V1b Plate Load resistor had drifted high (measured @200k --- stock is 100k)
It is an original Iskra carbon film resistor and it looks original to the amp (it's marked 100k) --- but it measures @200k
After I installed a set of 6550's in the amp and biased it up I tested it (at high volume) and realized that this amp is a F'ckn BEAST --- It is the most "alive" and the loudest 2204 I've ever played
The owner (who runs a pro backline rental service) said this amp used to be a favorite rental for touring pro musicians for many years --- then it quit "sounding right" and was put aside (EL34's in a 6550 amp will do that)
Switching Plate Load resistors from the stock 100k to 220k used to be a "quick and dirty" mod that some people did to get more preamp distortion (it does work) --- but this amp modded itself through Resistor Drift
The matching original V1a Plate Load resistor still measured 100.3k (still in spec -- not drifted)
I have another '79 Marshall 2104 head (used to be a 2x12 combo -- but same amp as a 2204) so I pulled it out and measured the V1a and V1b Plate Load resistors (both original Iskra's) --- and on the '79 the V1b resistor was still in spec (100.8k) but the other V1a resistor had drifted up to 134.7k (from stock 100k)
These are the original type Iskra carbon film resistors that some 'purist" Marshall experts will search for to do "Exact Marshall Builds" ---- but, in the real world, they tend to drift all over the place (maybe improving the sound of the amp, depending on your viewpoint)
But that '82 2204 (with the self-drifted @200k V1b plate resistor) sounds better and more alive that most 2204's (and much better than an '81 I used to own)
I think I'll try this same combination in my next 2204 build ---
![Cool 8)](./images/smilies/icon_cool.gif)
![Cool 8)](./images/smilies/icon_cool.gif)
![Cool 8)](./images/smilies/icon_cool.gif)