hum in R-12-R Reverbrocket

[jjmt]

I mentioned this over in Workshop in passing, but I though I would start a thread about it.

My Reverbrocket (a '61, blue sparkle, 6v6) has a 60Hz hum. It's not coming from the input stage - it's not volume dependent. It's not coming from the power tubes - when I pull the PI the amp is dead quiet. Just based on the audible frequency, I'm pretty sure that it's not 120Hz from the filtered high-voltage side of things. Sounds more like heater-cathode leakage.

I just discovered that the hum disappears when I activate the trem with the foot switch. I assume this is a ground loop, and I'll be chasing after it, but does anyone have any useful observations that might save me some time?

[jjmt]

To flesh out what I said above, this amp seems to have three ground point:

1. The preamp is grounded to a tab on the cap can.
2. The 6V6 cathode resister and bypass are grounded to a different cap can tab.
3. The trem is grounded to a tab on the input jack.

Also, the volume, tone, and intensity pots are grounded to themselves.

Now, here's the rub. The cathode resister for V1b (trem) runs through the foot switch and is grounded to the preamp ground. That is the correct ground for the reverb, but not for the trem. The cathode bypass cap for V1b is grounded to the trem ground: the input jack. This seems like a problem, as the V1b cathode has two grounds on opposite ends of the chassis. I'm not quite sure what the best way to address this is, so I'm going to sleep on it.

[jjmt]

Are you sure the cathode resistor on V1b is grounded to the pre-amp ground? I see it open to the switch until you ground it at the switch, turning the tremolo on.

OK. So it turns out that my description of the grounding scheme above was not entirely accurate. There is a wire that runs underneath the board that I hadn't noticed, so I think that both the cathode resister and the bypass cap are grounding to the input jack. No loop there.

This is still on my bench (I've been away for a week or so). There is another interesting piece of information. Actually, two:

1. The hum requires time to manifest (a couple minutes from a cold start). This kind of kills the whole ground loop hypothesis, in my opinion. I hadn't noticed this before.

2. The hum happens if I pull V1, but does not if I pull V2. This is super weird given the effect the trem has on it.

All the resistors on V2 test in range, but given the delay in observing the hum, I felt that it was likely that one of the plate or cathode resistors was behaving badly with heat, so I went ahead and replaced them. I didn't have time to really test it thoroughly, but it seemed better. Maybe.

I will continue to report.

[jjmt]

I usually find hum after warm up to be tube related. Have you tried a couple of different 6SN7's?

Yes, and maybe not. I have swapped a number of different tubes. But it's not like I've kept records, and I have been chasing after a number of different issues. So it's a good reminder to go back and revisit the possibility. Also, one has to remember that my stock of tubes are all early production, from only a few manufacturers, so there is the possibility that they just all suck in this spot of the circuit.

[jjmt]

Octals are prone to microphonics. The good news is the New Sensor made Tung Sol 6SL7 and 6SN7 tubes are, for the most part, very quiet.

https://www.ebay.com/itm/Tung-Sol-TWO-Fa...7526!US!-1

Yeah, that's a possibility. The thing is that I bought one of their 6SL7s last year. I was rather unimpressed. It sounded fine, can't deny that. But the triodes were woefully imbalanced. I expected more, given the price.

Anyway, I have a bit of an update on this. As I have mentioned, I am actually chasing several hums, or so it seems. My current hypothesis is that the loudest hum, the one that grows over time is actually a power tube. I've swapped those out, and we'll see (it got late).

But the ground loop issue remains, and my current point of focus is the intensity pot. This pot links the trem section to the grid stoppers of the power tubes. It is currently grounded to itself. This hum increases as the intensity is reduced. I'm planning to experiment grounding this pot to one of the other ground points (either the cap can or the input jack). We'll see if either of those options help.

[jjmt]

This is resolved. As with everything in this amp, it seems, the issue was complex. I tested several grounding points for the intensity pot, and found that the situation was much improved by grounding it to the board on the same ground circuit that the tremolo uses. With this ground, I still had hum attenuated by the intensity pot, but I only had that from 0-2. it was previously present at all ranges (0-10).

OK, so then there was the question was where all the current coming was from that was driving the noise? I figured it needed to be one of the 6SN7s. I eventually isolated it to V2, and found a tube that worked in that spot that wasn't microphonic (so many of mine are), and which eliminated the hum. Now, the interesting thing is that by swapping out V2 the trem also improved (less noise, stronger trem). I guess V2 was pulling so much current that V1 was under voltage; likewise, you may remember that activating the trem reduced the hum. I guess the additional current for the trem starved V2 to the point of reducing the audible hum. Dunno. Anyway, done.

I can't say that the amp is super quiet, but I think it's as good as it's going to get. It wouldn't be noticeable if I wasn't in a small room. The amp now sounds simply awesome. I had forgotten just how much I like it. The trem and reverb both go to "way too much", and it's lovely with both turned off. If you don't already have one of these, you really should get one.