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So R18 and R19? The 100k resistors?

sruffinc wrote: ↑

Sat 06/13/20 3:36 pm

So R18 and R19? The 100k resistors?

Yup. Though that numbering is based on the classic schematic, and not all schematics follow it.

All resistors seem to be in spec. I had a lot of carbon comp, but replaced all but those 100k PI plate resistors with carbon film. B+ came up to 350. PI plates at 200 and grids still a little low at 34.

One thing to note - I have trouble getting a reading on the V5 plate attached to the OT. Very noisy and hard to get a reading. I re-soldered the wire, but no change. Does that mean anything?

Voltages - Amp sounds very good, but I want it to hold up. Anything else I should check?

sruffinc wrote: ↑

Sun 06/14/20 2:09 pm

All resistors seem to be in spec. I had a lot of carbon comp, but replaced all but those 100k PI plate resistors with carbon film. B+ came up to 350. PI plates at 200 and grids still a little low at 34.

One thing to note - I have trouble getting a reading on the V5 plate attached to the OT. Very noisy and hard to get a reading. I re-soldered the wire, but no change. Does that mean anything?

That's funny, the PI plates are the only place carbon comp are actually a good thing.

It sounds to me like you have an oscillation affecting your OT. Check the wires in the vicinity of the power tubes and OT wires. Make sure there's no parallel ground or anything like that (the OT black wire is negative, not ground - don't worry about that one). Chopstick them a bit and see if you can get a reading.

The PI anode ("plate") resistors are the two 100k resistors on pins 1 and 6 of the last 12AX7 (V2) before the two EL84s, labelled R18 and R19 on the standard Marshall 18W schematic. Carbon composition resistors are ideal used there, for maximum tone benefit and minimum hiss.

Seems like I solved the problem with oscillation on the V5 plate. Tightened up some things and discovered a dangling ground wire near the TMB input. The PI plates still register at around 200 and the grids at 34. Guess I can try some other tubes? Amp sounds nice. If those values are too low what would the result on the sound be?

Thanks,

sruffinc wrote: ↑

Mon 06/15/20 1:37 pm

Seems like I solved the problem with oscillation on the V5 plate. Tightened up some things and discovered a dangling ground wire near the TMB input. The PI plates still register at around 200 and the grids at 34. Guess I can try some other tubes? Amp sounds nice. If those values are too low what would the result on the sound be?

Thanks,

If you think the amp sounds nice, and the oscillation is gone, enjoy it. And post a demo.

Thanks,
Josh

Will do. Thanks for all the help!