Marshall JMP MKII non master volume

[lowell]

JJ6550 is 35watts. General rule in a fixed bias amp is 70% dissipation. Some disagree with this but I've never had a problem stem from it.

35*.7 = 24.5watts

Watts = V*I
24.5 = 386*I
24.5/386 = your target current = 63ma PER TUBE

So if you have 4 power tubes double that # for each side of the OT.

[g1]

35 watts max? No wonder modern 6550's have such reliability issues. Real 6550's are supposed to be 42W tubes. Does anyone even make such a thing anymore?

[nsubulysses]

Sovtek 6550WE claim 42W max plate dissipation

[Gtr0]

Here's what I believe is the closest thing to a proper schem for that amp... I think this schem is from 1970-ish

https://drtube.com/schematics/marshall/1987u.gif

[Helmholtz]

I haven't found "unreliable connections" to be an issue with my bias probe in all my years of doing this.

Agree and would like to add that as bias probes allow voltage measurement directly across the resistors and not WRT ground, some additional contact resistance will not change readings. Tubes are high impedance current sources.