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!

[tubenit]

In the top two photos, you have a brown wire that is just sticking up and unconnected to anything.  What is that and where is it supposed to go?

Overall, the photo of the interior amp chassis with the layout board looks really nice to me.  Have you check voltages?  And could you mark up a schematic showing your mods? 

If you have some known "good tubes" did you try those good tubes? 

My experience is that single ended amps do have more hum (per volume level) then push/pull amps do at the same volume level.  It's one of the reasons that I like low watt push/pull  6BM8 and ECL84 tubes.

with respect, Tubenit

[tubenit]

I thought of one other thing ……….    have you tried the amp in a different room or a different house?  I've had some odd things induce hum in an amp build before with one of the more puzzling being a soldering iron that I had plugged in.  Unplugging the soldering iron, eliminated the hum problem? 

[ac427v]

Beautiful work! It seems like you did the standard mods for hum in a single ended amp. That should have worked. I'm looking at your first amp, the one with the turret board conversion and ignoring for now the amp with the point-to-point conversion. I see that the OT has been relocated and the yellow/black twisted pair routed next to the volume and tone controls. That is my first guess for the hum cause. Can you test by removing the output jack nut and temporarily playing the amp with the jack and wiring suspended in air away from the sensitive preamp stages?

[sluckey]

In the top two photos, you have a brown wire that is just sticking up and unconnected to anything.

That wire is an extra OT primary. The Hammond 1750C has a tap on the primary. It can be used as a 5K or 8K primary.

[ginger]

Looks like you didn't use shielded wire from V1 , to the volume pot

[camintsev]

I'll try all of your suggestions and we'll see if it helps)

[camintsev]

unfortunately, moving OT or it's wires had no effect. but 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

[ac427v]

In the top photo, it appears that the first three power supply caps are grounded to the input jack. If so, the preamp may amplify any noise that those  grounds carry. The first power supply cap could be grounded directly to the chassis where the bridge rectifier is grounded.
Another possible hum cause is the negative feedback connection from the speaker jack to the board. It runs next to the socket for the 6V6. Can it be moved to empty space farther from output tube noise?Now I'm really grasping at straws...but in troubleshooting desperation I reflow every turret top to leave a shiny dome clearly indicating a good connection.

[ginger]

In the last picture you posted , is that a fuse holder , jammed up against the power indicator lamp ? Looks like it... can't really tell .

[ginger]

In my past  SE builds , even with the PI filter between the rectifier and B+ , there was hum... now I use a choke there.. no hum .

[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)