champion 600 rebuild - excessive hum

[camintsev]

Need help, because I'm losing my hope. I've built several amps, but last two rebuilds made me puzzling the most. Amps are champions 600, converted to PTP using stock transformers, chassis and cabinet. Schematics are pretty close to 5F1 with mods to suit my tastes. What I encounter is hum, which increases with tirning the volume knob. I've never experienced that amount of hum in previous builds. Here's what I've done to quiet the amp in the first place:

1) added 240R/22uF filter stage, 0.1uF snubber caps across primary and secondary AC, which helped a lot with noise floor
2) heater supply connected to 6V6 cathode using two 100R FP resistors, humdinger made no improvement in noise level
3) isolated preamp and power supply/6V6 bus grounds, both connected to the input jack, connecting PS bus to the end of preamp bus or to the chassis near PT with safety ground made no difference
4) moved OT farther from PT closer to preamp, but I found it caused no hum, only feedback and oscillations, which I fixed with lead dressing
5) adding more filtering to any of the stages didn't help
6) changing tubes didn't help, 6V6 biased at around 100% dissipation
7) changing position, plugging in different outlets or places of course didn't help too

Hum sounds a lot like mains hum, not really buzzy, but soft and low frequencish. Interestingly, converting filament supply to DC didn't help a bit, but feeding 12X7 heater from battery seems to cut off any hum and add hiss and other strange noises. I have no idea what the source of hum can be in first triode in my case. May stock chinese PTs theoretically cause that problem, because I left them in both rebuilds? Layouts are very different. Thanks!

[camintsev]

UPDATE: moving OT or it's wires had no effect. before adding shielded wire I tried to desolder the existing wire from pin2 and connect that pin to the ground side of the 1.5k resistor to see if there's a problem with inputs or grid resistors and that brought down the hum considerably. I double checked my input jacks wiring and solder new shielded wire - hum is back. Now I wonder what that means, looks like some grounding problem

[Helmholtz]

Please post schematic and a list of main circuit/construction changes between different versions.

Do you have a scope?

[camintsev]

UPDATE2: I solved the problem with preamp hum! Turns out the problem was in grounding, I connected preamp grounds to bus backwards so v1a cathode was the last in chain and i guess that was the main cause. So I ended up maintaining the right order: from screen grid supply all the way to the first cathode and input jack, also I disconnected first CRC from bus and soldered it right to the safety ground and speaker jack, not sure, but it may contribute to noise reduction too.

Now I'm hunting the idle hum, my first champ (4th photo) is dead quiet with volume turned down, last one with the turret board is humming a bit, I can hear it in quiet room from any corner. To localize the problem I pulled out 12AX7 and connected 6V6 grid to ground - still noisy. Swapping out tubes and biasing colder made no difference. Not so many variants there, grounding in power section seems OK, maybe proximity of cathode resistor and AC wires? Adding more filtering only reduces very low frequencies, buzz remains the same. Messing with heater supply yielded no results. I guess the answer is simple, but I can't see it at the moment, I'll be glad to read your opinions on my noise floor problem)

P.S. I don't have a scope, only planning to get one) I can post schematic, if it will help to solve remaining problem, but it's really nothing special in my power amp or power supply.

[patlaw]

I can post schematic...

Please do. We ask that schematics always be posted with requests for help.