Marshall JMP. 100W version with 6550s and master volume. I think this is the most preferred rock amp that preceded the higher gain JCM800. It is really fun to play lead guitar on. This one is from 1979.
Seems like these amps are known to squeal when the volume and tone knobs are cranked really high. In my experience a 1000pF/.001uF cap across V2A’s plate resistor will rid that. V1A’s 68K control grid resistor is mounted on the PCB with a shielded cable going to the grid. I could still hear some radio coming through with volume cranked so I tried moving the grid resistor to the tube socket with good results.
JJ 50uF/50uF/500V caps fit perfect for recap. Don’t even need new brackets
Marshall JMP 1959 2203 schematic