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Hi everyone,
My build functions and sounds great but I cannot run power tubes (V4/V5) above 60% w/out a lot of 120Hz hum with no guitar cable in the input. It is a 50w plexi (1987) circuit with preamp section switchable between paralleled V1a/v1b and cascaded V1b/V1a (2204) with v2 driving either a 1st tone stack outputting to PI(V3) through FX loop and a pre-PI MV (800 style) or driving a 2nd tone stack, bypassing FX loop and pre-PI MV, into PI. PI outputs to V4/V5 via a Ken Fisher Type II PPIMV. Relays and heaters for V1 and V2 are on a regulated DC supply. v1 and input ground at first point; v2, gain/vol pots, fx loop and 32+32 filter ground at second point.
Presence pot and OT secondary CT ground at a 3rd pt. Power amp incorporates KOC's power scaling circuit to scale V4/V5 screen, bias, and B+. PT Heater winding supplies V3, V4 and V5 heaters and is also serving as bias supply (transformed up to ~120V, rectified, and then into power scaling board for proportioning). Both 50+50 filters, DC heater CT, bias supply, power scaling board all ground at 4th pt.
The small box chassis is obviously crowded and I don't doubt I committed lead dress malpractice to make it all fit, but I can't figure out if I have a ground loop, mutual conductance issue, or component failure. Highlights of troubshooting are:
1. hum exists in either plexi or 800 mode through either tone stack
2. hum vol changes with PPIMV
3. no change in hum across three different sets of power tubes, or rotating through 2 sets of preamp tubes.
4. grounding signal path at either v1a coupling cap or v2a grid removes all hum
5. lifting v1a coupling cap from plate side and leaving downstream side coupled to v2a grid reduces hum volume by ~50% but hum is still there. Grounding unlifted side of cap still removes all hum, so it seems v1 is only amping a hum originating downstream.
6. removing PI grid inputs reduces hum volume by a lot but doesn't eliminate it.
7. removing the neg. feedback from the output tap has no effect on hum
8. dmm registers quite a bit of AC at the bias splitters, ~250mV; however when I ran a 10uF/160V cap to ground right at the splitter there was no change in hum even though dmm then measured only about 10mV.
9. 100 ohm hum balance pot had no effect
10. checked all grounds, reflowed a lot of solder points, poked/moved a lot of things with a stick, all with no effect.
I'm all out of ideas and close to giving up and just leaving it run at 60%, which is pretty disappointing.
My build functions and sounds great but I cannot run power tubes (V4/V5) above 60% w/out a lot of 120Hz hum with no guitar cable in the input. It is a 50w plexi (1987) circuit with preamp section switchable between paralleled V1a/v1b and cascaded V1b/V1a (2204) with v2 driving either a 1st tone stack outputting to PI(V3) through FX loop and a pre-PI MV (800 style) or driving a 2nd tone stack, bypassing FX loop and pre-PI MV, into PI. PI outputs to V4/V5 via a Ken Fisher Type II PPIMV. Relays and heaters for V1 and V2 are on a regulated DC supply. v1 and input ground at first point; v2, gain/vol pots, fx loop and 32+32 filter ground at second point.
Presence pot and OT secondary CT ground at a 3rd pt. Power amp incorporates KOC's power scaling circuit to scale V4/V5 screen, bias, and B+. PT Heater winding supplies V3, V4 and V5 heaters and is also serving as bias supply (transformed up to ~120V, rectified, and then into power scaling board for proportioning). Both 50+50 filters, DC heater CT, bias supply, power scaling board all ground at 4th pt.
The small box chassis is obviously crowded and I don't doubt I committed lead dress malpractice to make it all fit, but I can't figure out if I have a ground loop, mutual conductance issue, or component failure. Highlights of troubshooting are:
1. hum exists in either plexi or 800 mode through either tone stack
2. hum vol changes with PPIMV
3. no change in hum across three different sets of power tubes, or rotating through 2 sets of preamp tubes.
4. grounding signal path at either v1a coupling cap or v2a grid removes all hum
5. lifting v1a coupling cap from plate side and leaving downstream side coupled to v2a grid reduces hum volume by ~50% but hum is still there. Grounding unlifted side of cap still removes all hum, so it seems v1 is only amping a hum originating downstream.
6. removing PI grid inputs reduces hum volume by a lot but doesn't eliminate it.
7. removing the neg. feedback from the output tap has no effect on hum
8. dmm registers quite a bit of AC at the bias splitters, ~250mV; however when I ran a 10uF/160V cap to ground right at the splitter there was no change in hum even though dmm then measured only about 10mV.
9. 100 ohm hum balance pot had no effect
10. checked all grounds, reflowed a lot of solder points, poked/moved a lot of things with a stick, all with no effect.
I'm all out of ideas and close to giving up and just leaving it run at 60%, which is pretty disappointing.
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