JCM900 on the bench after it had been somewhat abandoned. I think it was shelved over 5 years ago for sounding too trebley and blowing a fuse. When I got it it worked actually but had weak output. Power tubes were weak and one was not working at all because the 2.2K screen grid resistor was burned open. Someone had recapped this amp in the past but decided to leave the bias supply caps original. I hate this blasphemous recap style. Why would you not replace two $2 caps that are arguably the most important ones in the amp. If you lose the bias supply you probably lose the power tubes. I always see recapped amps with original bias supply capacitors and I just do not understand.
Since this amp was too trebley I did a mod that affected the negative feedback to try to give it more bass response. Putting a .001uF capacitor in series with paralleled C6 and R17 in the negative feedback circuit creates a shelving filter that does not feed back lower frequencies. So when lower frequencies are not fed back, they are not phase canceled in the phase inverter, and the amp has more bass response. Still sounded like a Marshall gain type of sound but at least had some heavy bass to go along with it to fill in the sound.
Marshall JCM900 2100, 2101, 4100, 4101 and 4102 power supply and output section schematic
Marshall JCM900 2100 preamp