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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.jpgPS 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!!
Edits: 03/30/25 03/30/25
Update, I swapped the leads going to the 6u8 right after the volume control, and the hum moves to the other channel. Looks like it's a pre- amp issue. I'd be lying if I told you I was happy about that. I was secretly hoping it would have been the less complex output section - lol..
Does anyone know if there is a simple way to use a scope to trace back from here? Or is it just keep swapping wires until you find it?
Just wanted to post an update to this post.
I was finally able to isolate the issue and fix it. It turns out I was right on top of it. There is RF getting into that channel and it definitely was in the pre amp section of V102.
In a previous response somewhere in here I mentioned that I changed out the preamp cables to higher quality ones and it slightly improved. I did some poking on those same cables with my phenolic probe and noticed shifts in the hum and I could even get the other channel to act up. -Strange I ran the cables exactly the way the original ones were.
I pulled them and cut new ones, re- routed away from the pre amp tubes and the diode ground that ran under under them. The new lengths are actually longer but take a new route into the 6u8's by running closer to the front of the amp near the control's VS near the "powered" stuff.
The shields at one end were grounded but I made a better effort to tin them and connect to the ground point (same ground as the pre-amp tubes)
Powered up and very good results, slight hum but very acceptable for a vintage unit like this. The only thing that confuses me is that those cables were original to start with and I ran the new ones the same way as the stock ones, nothing in that area changed physically. I can only surmise that the other changes to the PS and the new components created a new RF source.
Thank you guys for the help, especially Walter and Chip
Chip,
I believe you are right, trying to find it is the big chore. I am trying to narrow down where it could be. So far I have narrowed to A channel pre-amp section before the volume control at c20. I am slowly working my way back trying to find it.
Jeff
They can be buggers to find as seemingly "good looking" joints can have problems. That it is around the volume control section narrows it down quite a bit. Ground all of the shields on the input RCAs together and tie them to a good central ground. Ground your volume control to the same point. The input tube grounds should be at this point too. This will trap the volume control hum problem somewhere in this area. The volume control itself should be looked at.
Update, I swapped the leads going to the 6u8 right after the volume control, and the hum moves to the other channel. Looks like it's a pre- amp issue. I'd be lying if I told you I was happy about that. I was secretly hoping it would have been the less complex output section - lol..
Does anyone know if there is a simple way to use a scope to trace back from here? Or is it just keep swapping wires until you find it?
That Scott unit has a power switch on the back of the volume pot. Is it possible that it is inducing hum in the channel closest to the switch?
Have you tried reversing the 120v power plug? When the switch is closed, it will be at line neutral or at the full 120vAC, which will affect capacitive coupling to the adjacent volume control.
JRMA
The volume pot is new, however, the replacement is like the original in that the AC switch is mounted to the back of the pot.
I think the best suggestion that has been posted on my thread to tell if pre-amp/volume is in the mix and that back pot is affected, is to swap the the preamp wires that feed the pot.
I think even before I do that just as a curiosity I can chase the leads of that back pot and see if by chance it's the same side. But this is a possibility I didn't think of. I wonder if any other folks have had an issue like this with the pot/switch.
thanks for the suggestions and I will update...
I have the same Scott unit that I use in a second system. I never run the volume past 8-9:00 or it is too loud! Not sure why yours is running at 2-3:00, seems that something is not right there.
I'm getting ready to open mine up because the balance control is giving me trouble that normal deoxit cleaning is not fixing. Maybe we can compare notes :)
JRMA
Update, the rear pot adjacent to the AC switch is actually the clean channel. Good theory though..
JRMA,
Some of this is from my memory, but I believe the reason I reach that full useable volume at that spot, 2:00 -ish is just the physical characteristic of the new volume pot. If I re-call the original pot was more like you described.
The reason for this is because the new pot needs a good amount of clockwise turn to turn the power switch on. The new switch is "turn on -click" - not "pull on" like the original. By the time you click to "on" you are almost at 8-9:00 already with no volume.
So if my memory is right, this would put the "lag" just about where you are stating your volume is. I also think if I re-call, with very inefficient speaker load, 12:00 would be the absolutes max it could run and with efficient loads, right about where you are stating. So I think it's just a physical positioning thing.
Yes we can compare notes, I hope to have more to report...
JRMA,
Some of this is from my memory, but I believe the reason I reach that full useable volume at that spot, 2:00 -ish is just the physical characteristic of the new volume pot. If I re-call the original pot was more like you described.
The reason for this is because the new pot needs a good amount of clockwise turn to turn the power switch on. The new switch is "turn on -click" - not "pull on" like the original. By the time you click to "on" you are almost at 8-9:00 already with no volume.
So if my memory is right, this would put the "lag" just about where you are stating your volume is. I also think if I re-call, with very inefficient speaker load, 12:00 would be the absolutes max it could run and with efficient loads, right about where you are stating. So I think it's just a physical positioning thing.
Yes we can compare notes, I hope to have more to report...
One test is to put to ground the C3 and C123, versus 150kohm resistor so to understand if the pre section is involved
And swap the cable from one section of the volume with the other
Walter
Walter,
I swapped after the 150k. I went right to pin 2 of the 6u8's and switched it there... and yes the hum moves over to the other channel...
I was actually hoping it was in the the output section -lol, as it's easy to get to, more simple and easy to understand. The pre- section is cramped and now lots of controls and pots are in the mix.
So I guess I work backwards from here, and keep swapping until I hit the right section or component? Is there any way to use a scope to trace it back?
thanks
Jeff
OK
I think is a pre section that has the issue.
You can put to ground the R16 and R116 close to V2A and V102, or swap there the cables
This is the line level pre amp.
Walter
thanks Walter, yes that makes sense. This morning I swapped at C20 and it moves to the the other side. I believe if I am reading this right, this eliminates the volume, loudness, and balance as possibilities.
I'll will swap at R16 tonight and see what I get.
walge,
yes, I was thinking of trying this, it should absolutely determine if I can include or eliminate the pre-amp section as the source.
Appreciate the suggestions, as soon as I have a chance to swap the input wires, I will update.
That is, the D.C. balance 100K pot of the noisy channel.
If there is some imbalance of the current bias of output tubes then the HT hum rejection is impaired.
Omnes feriunt, ultima necat.
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