I wanted to post this as I was messing around. When I was younger everyone said there really id not anything to be done about making the JCM 900 2500/2100 sound improved. I disagree. Now the 4500/4100 are another story I have not played with so far. Of course more can be done but I will finish at this point. 1.) R5 3k3 add .68uf bypass cap. (Do not decrease resistor value). 2.) R8 1k5 add .68uf bypass cap. 3.) D2 Remove single 1N4007 diode and replace with set of 5.1 volt zener's (1N4733). Stripes (Cathodes) Outward. 4.) R4 Plate = 320k. 5.) R7 Plate = 320k. Straightened out some issues on this old 1990 JCM 900 50 watt 2500 head. It belongs to my son. It has had a lot of issues over the last 18 years. At one point it was no kidding on fire and in pieces. He blew a transformer as a kid after redplatting and burning up a lot of components. It blew the pico fuse and harmed the Effects Loop to boot. He was still complaining about the loop over heating so I had another go at it. It seems even in 1990 Marshall still could not stick to their schematics. I made some modifications to the circuit as there is only so much you can do with the ic based 900 series but it sounds a lot more Marshall to my ears now. Better diodes and values, plate and cathode values as well. Plugged straight in.