RE: One Channel Hum Affected by Volume

Posted by mr9iron (A) on March 30, 2025 at 13:46:23

Posted this in Vintage forum with not much success so trying here:

OK, warning a bit of a novel coming:

Trying to chase one channel hum that is affected by volume control from 2:00 to 5:00 on the control.

Amp is HH Scott 222D

Schematic:
https://rubli.net/classic_amps/files/amps/scott/222d/images/circuit.jpg

PS and Bias supply are all new and all voltages have been checked.

Bias supply is over-built and the bias to the tubes is evenly balanced. Bias holds steady.

Output tubes are Russian 6bq and test very good. All pre amp input tubes test very strong. (Wish I had the more rugged 7189A but don't have funds ATM)

All coupling caps have been replaced, all out of tolerance resistors have been replaced. The only caps I did not replace were the two on the selector switch, they are just not reach-able in a practical sense.

Ground scheme is super simple with everything running to the chassis via one 18ga ground bus. The tube grounds were left stock and also very simple, a chain that runs to the chassis.

All controls have been saturated with de-ox it and fader lube. The balance pots were also cleaned, input RCA's also cleaned. The 500k volume pot is brand new.

Tube sockets were cleaned multiple times.

Tape monitor switch is new.

The amp is very quiet at idle, typical tube amp, if you are right on top of the speaker you can pick up that the amp is on. There is no meaningful detection of any noise unless you are right on top of the speakers.

The amp sounds great over all, everything functions correctly.

The amp is quiet at idle, even if the volume is raised, until you get past 12:00. The A channel or "left" develops a hum, with a tiny buzz. Then at 2:00 it's noticeable, and at 3:00 it's blatantly noticeable and loud. The B channel is quiet and clean, with hardly any noise worth noting.

I have zero luck trying to solve and troubleshoot issues like this. I have tried to isolate the issue without much progress. It's unclear to me what kind of hum this is, ground, signal, RF, power? There is tons of conflicting information on hum out there. There are also many examples of hum issues, but nothing ever quite seems to simulate the exact situation here.

Swapped input tubes and swapped output tubes, the hum does not change, nor does it move to the other channel.

I shorted all the inputs to no effect either.

Happens no matter what input source is selected

One experiment I tried really puzzled me. I thought maybe I could narrow down if the problem is before the volume control or after. I used the amp's mode selector to switch to the "left in-put only" or "right in-put only" . I thought for sure this would move the hum to both channels or move the quiet side to both channels. It did nothing; it stays on the left side no matter what mode. It stays there even if I select mono mode.

Does this mean I can rule out looking for issues in the preamp section? I am not expecting an answer to this issue but it would be good if I could rule out some possibilities. Maybe there are some things I can do to try and isolate the area that is causing this.

Here is a link to a video of the hum. I am moving the volume back and forth from 2:00 to 5:00

https://drive.google.com/file/d/1qQUsYOmfKY9_rpSORqqikcWeBaDTkZJ9/view?usp=drive_link

It sounds like a 120hz hum? -which again according to what I have researched is power supply issue? -but that just seems unlikely as I'd expect this to be in both channels, and also not be affected by volume.

I should note that 2:00 to 3:00 is usable volume on this amp, which is why this is concerning me. I frequently listen to things in that 2-3 range (yeah I like to fill the room)

Any reply to this novel (sorry just trying to be thorough and exacting) is appreciated!!

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Topic - RE: One Channel Hum Affected by Volume - mr9iron 13:46:23 03/30/25 (19)

RE: One Channel Hum Affected by Volume - mr9iron 15:31:46 04/15/25 (0)

RE:UPDATE- One Channel Hum Affected by Volume - mr9iron 15:31:38 04/15/25 (0)

That is a ground loop. You have a cold ground joint somewhere. nt - Chip647 12:01:59 04/01/25 (2)

RE: That is a ground loop. You have a cold ground joint somewhere. nt - mr9iron 12:26:34 04/01/25 (1)

RE: That is a ground loop. You have a cold ground joint somewhere. nt - Chip647 14:30:23 04/01/25 (0)

RE: One Channel Hum Affected by Volume - mr9iron 00:32:57 04/01/25 (0)

RE: One Channel Hum Affected by Volume - JRMA 12:04:43 03/31/25 (6)

RE: One Channel Hum Affected by Volume - Paul Joppa 14:16:43 03/31/25 (0)

RE: One Channel Hum Affected by Volume - mr9iron 13:03:02 03/31/25 (4)

RE: One Channel Hum Affected by Volume - JRMA 14:02:37 03/31/25 (3)

RE: One Channel Hum Affected by Volume - mr9iron 00:26:39 04/01/25 (0)

RE: One Channel Hum Affected by Volume - mr9iron 00:10:50 04/01/25 (0)

RE: One Channel Hum Affected by Volume - mr9iron 14:17:04 03/31/25 (0)

RE: One Channel Hum Affected by Volume - walge 07:26:53 03/31/25 (4)

RE: One Channel Hum Affected by Volume - mr9iron 00:21:15 04/01/25 (2)

RE: One Channel Hum Affected by Volume - walge 04:02:04 04/01/25 (1)

RE: One Channel Hum Affected by Volume - mr9iron 12:22:28 04/01/25 (0)

RE: One Channel Hum Affected by Volume - mr9iron 13:07:49 03/31/25 (0)

I would try to adjust the R41 (or R141) trimmer... - cellailuca 03:27:09 03/31/25 (0)

RE: One Channel Hum Affected by Volume

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