Seymour Duncan Twin Tube classics. Same noise problem crackles and hiss

[SoulFetish]

customer comes in with two Seymour Duncan Twin Tube Classics with the same issue. When turned on, the pedals have a very noisy hiss and crackling sound like "frying bacon".
Almost sounds like noisy carbon comp plate resistor, or failing silver mica cap. Here's what's been done:
Tubes have been replaced, coupling caps have been replaced, plate load resistors have been replaced.
my suspicion at this point is maybe faulty pots? the noise is in both channels or settings, and seems to get worst as the gain pot is increased in both channels. I've read that other people have had the same problems with their Twin Tubes. Same MO, bought the pedals and loved them, but after about six months noise started making them unusable.
Anyone solve this? Any thoughts?

[Jon Snell]

I would use my oscilloscope to determine where the noise comes from.
From your description, turning the gain controls (Rhythm, Lead1, Lead 2), stops the noise.
Which one?
Have you unplugged V1 and tested the system?

[SoulFetish]

actually the noise is constant. Turning both the gain and volume modulates the noise, with the noise increasing as you turn either gain up.
These tubes are soldered in. I have not tried pulling V1

[SoulFetish]

So I did some probing with my scope. With the vertical display set at 200mV per div, in the high gain channel, I could clearly see noise on the grid of V2b and then amplified on the plate. The V2b grid is DC biased from being DC coupled to the plate of V2a. The cathode is at 74 volts or so, and the grid is -2.64V to the cathode voltage


Another thing to note is the pin numbering on the schematic I posted. I'll upload a datasheet showing the actual pinout later on

[Helmholtz]

I would try shorting grid AC to ground using a 0.1µ or larger cap.

In rare cases bad ecaps can cause noise (e.g. a cathode cap).
Of course could be a tube as well.

Do plate and cathode voltages look ok?

[SoulFetish]

I would try shorting grid AC to ground using a 0.1µ or larger cap.

In rare cases bad ecaps can cause noise (e.g. a cathode cap).
Of course could be a tube as well.

Do plate and cathode voltages look ok?

thanks, I actually did AC short the grid. Noise went away. So I slowly moved my way back through the circuit shorting different nodes. This one is a challenge. I was able to work my way back to the plate of V1. But its a new resistor, so I replaced r5 100k. No change.
It’s not the tubes because It’s still happening in both pedal on both channels, and tubes have been replaced.

something common to both units is causing the same problem. Not tubes, probably not plate resistors or coupling caps, as most have been replaced in the one Im working on. I’ve never had a e-cap do this before, but that doesn’t mean it doesn’t happen. That leaves relays, pots, and maybe electrolytic caps. Or something
entirely

edit: oh yeah, voltages look good at the plate and cathode

[Jon Snell]

"It’s not the tubes because It’s still happening in both pedal on both channels, and tubes have been replaced."
Just because it is new and shiny, doesn't mean it is perfect.

Connect pin 7 or V1 to ground ... does the noise stop? If it does, the noise comes in before that point, if not the valve is probably noisy if R9, R10, C7 & C8 have been replaced.

Just a thought, is +Ua smooth and noise free?

[Helmholtz]

^^That.

If Ua is clean, lift C2.

(I don't like that the input impedance of these things is only 100k.)

[g1]

That leaves relays, pots, and maybe electrolytic caps. Or something

These days it's sometimes worth thinking of the board as a 'component'. Or is there any kind of goop in use?

[SoulFetish]

"It’s not the tubes because It’s still happening in both pedal on both channels, and tubes have been replaced."
Just because it is new and shiny, doesn't mean it is perfect.

Sure, there is always the possibility that any replacement tube is faulty.
The chances that all 4 replacements, actually being different kinds of tubes altogether, having the same exact failure mode and having no effect on the noise problem after installation is astronomical... But not impossible. Best to test the tubes in a different application; which is what I did. Just so happens I have some effects and breadboards set up to experiment with these kinds of 8-pin subminiature tubes. His original tubes show no obvious problem outside of the Twin Tube

Connect pin 7 or V1 to ground ... does the noise stop? If it does, the noise comes in before that point, if not the valve is probably noisy if R9, R10, C7 & C8 have been replaced.

Just a thought, is +Ua smooth and noise free?

I'll check that out. But I just need to know what you mean by +Ua? It's not a nomenclature that I normally use. Do you mean the supply at the plate? or quiescent plate voltage? something else entirely?

edit: Oh, I see the label on the schematic you are referring to. thanks, I'll check it out.

[SoulFetish]

These days it's sometimes worth thinking of the board as a 'component'. Or is there any kind of goop in use?

See, this is my real fear that I've been trying to suppress. These tubes get quite hot, and they are fully enclosed in the chassis with a small vent on the back plate which sits maybe 10mm off a surface.

[Jon Snell]

Have you established where the noise comes from?

Please do not go with ideas that may or may not be your issue. If you do, you will never fix it as you will confuse yourself.

Work from the output towards the input, checking for where the noise stops. When you find the point at which the noise is no longer there, the following component is the guilty party.
It is unlikely to be the board, it is more likely to be a single component or even a parrasitic generator, cured with a simple grid stopper.
Be logical, make notes, don't get side tracked. After all, it is only a pre amp.
Drink plenty of coffee/tea, take regular breaks and when you've fixed it, a tipple of your choice to cellebrate.

[g1]

It is unlikely to be the board, it is more likely to be a single component or even a parrasitic generator, cured with a simple grid stopper.

two Seymour Duncan Twin Tube Classics with the same issue.
...
other people have had the same problems with their Twin Tubes. Same MO, bought the pedals and loved them, but after about six months noise started making them unusable.

Maybe not the board, but seems there must be something common to all these similar failures. As no one has seemed to solve the issue, I think the board is a reasonable suspect.
I also suspect SoulFetish has substantially more than adequate skills and has eliminated any of the obvious suspects.
I do like the parasitic idea, although that is also something a board could be responsible for.