Twin Reverb crackling/static noise help

[buildAndPlay]

I've got a mid 70's silver face twin. Master volume pull switch model. Not the ultra linear model.
Anyway, it makes a crackling static noise all the time, even when not playing. It doesn't seem to affect the regular sound of the amp, as in it's not distorting when I play, it's just an additional sound that can be pretty annoying depending on how loud or quiet it decides to be at any given moment.

Turning all controls to zero does not affect the problem. I've pulled v1 through v6 one at a time and the crackling is always there. I've pulled out the inner and then the outer pair of power tubes and the crackling is still there. I just recently replaced most of the capacitors in the amp (all the electrolytics and what I guess are the coupling caps) but this problem was there before so it's probably not anything I did wrong.

I'm guessing whatever the source of the problem is, it's later on in the circuit, but I really don't know for sure. I tried re-soldering most of the points on the component board, but not much anywhere else.

Hopefully someone has some good advice that will steer me in the right direction.

Thanks!

Jim

Edit: Here's a link to a sound clip... crackling.mp3

[Justin Thomas]

Does it stop with ALL of the power tubes pulled? I would guess if not, that narrows it down to the OT & or connections to it. Only one power tube, one at a time? Elimnate the tubes first, then move on to checking connections. Dirty tube sockets? Bad solder joints?

Justin

[buildAndPlay]

There is absolutely no sound with all 4 power tubes pulled...which I guess is to be expected.

[buildAndPlay]

Here's a 10 second sound clip if it helps...
crackling.mp3

[gui_tarzan]

One of the most common issues I've found in old Fender amps is the 100K resistors in the pre-amp section back to the phase inverter or tremolo section of the board. Almost every one I've worked on had a static noise problem and replacing these cures it almost every time. Also check the grid and screen resistors on the power tubes as they are also a common source of issues. As always, make sure your caps are discharged and check the tube sockets for loose and/or dirty pin sockets and bad solder joints as Justin mentioned. That could also be a dirty jack, make sure you clean those as well and check for bad contacts on the shunts.

[pdf64]

Remove the pre-amp tubes starting at V1. When the crackling stops, you've narrowed down where the problem is.
Yes, it's likely a 100k plate resistor.

[überfuzz]

Might I suggest a more delicate approach on this matter? :-) Get an insulate stick of some sort and start picking and poking around. I've had amps behaving like that and turned out to be solderings/connections that had turned sour. With a little luck you just have to resolder some connections...

[gui_tarzan]

I'm not against a delicate approach, but if you're checking solder joints you might as well replace those resistors for the whole buck or two it might cost for them. I learned about those way back in the mid '80s from a guitar magazine article. It was on the first Fender I ever repaired. There is always a chance that the fiberboard has gone conductive, let's hope not in this case for his sake.

[buildAndPlay]

Ok, I went around tapping on things with a stick with the amp on, hoping to find a noisy part but it didn't turn up anything. Then I had an idea and I unsoldered the two wires from the outputs of the PI section to the inputs of the power section. It hummed a bit, but the crackling was still there. I was thinking this would be a good way to rule out anything in the pre-amp section, but I don't know if that's necessarily true. The bias supply is still connected to the inputs too, so I suppose that could be a source. I will mostly likely replace the 100k resistors as my next step unless someone comes up with any other good ideas to try.

Thanks for the help so far.

Jim

[52 Bill]

Ok, I went around tapping on things with a stick with the amp on, hoping to find a noisy part but it didn't turn up anything. Then I had an idea and I unsoldered the two wires from the outputs of the PI section to the inputs of the power section. It hummed a bit, but the crackling was still there. I was thinking this would be a good way to rule out anything in the pre-amp section, but I don't know if that's necessarily true. The bias supply is still connected to the inputs too, so I suppose that could be a source. I will mostly likely replace the 100k resistors as my next step unless someone comes up with any other good ideas to try.

Thanks for the help so far.

Jim

The easier way to do the same test is to pull out the phase inverter tube.

If the noise is still there after removing the signal to the power tubes, that points to the power tubes and circuitry. Try pulling one power tube at a time and see if the noise goes away.

[Jazz P Bass]

Well that 'idea' certainly proves that the problem is not in the preamp.

Pull the output tubes, pick one power tube socket & put them back in, one at a time to see if any of them is the cause.


If none of the tubes makes noise, pick one & try it in the other sockets.

If that does not bring about the noise, you may have a faulty output transformer.

[buildAndPlay]

The easier way to do the same test is to pull out the phase inverter tube.

If the noise is still there after removing the signal to the power tubes, that points to the power tubes and circuitry. Try pulling one power tube at a time and see if the noise goes away.

I had already tried pulling the PI tube previously so tried the wires since I already had it open and it only took a second. I have also already tried pulling two tubes at a time but I suppose it's possible I have two bad tubes. I'll try them on at a time.

[überfuzz]

The reason I mentioned the very scientific poke around analysis method is that you sometimes get lucky and nail it. I saw that you've been pulling tubes. Have you tried swapping them around? This would be the next method that don't drain man hours. Or even better do you have an extra set of 6L6 tubes to try?

If you do all the 'easy' stuff without finding the issue you probably have to get you hands dirty. :-/

[buildAndPlay]

Progress

Ok, things are looking up. I reconnected the wires from the PI and then removed the PI tube and all but the first power tube (v7) - no noise. I tried all 4 tubes in the v7 spot - no noise. Then I tried a tube in just the v8 socket, again, no noise. Then I went to v9 and got lots of noise. I then went to v10 and again, got lots of noise. So it's both of those sockets, which explains why my previous test of pulling just the inside and just the outside tube pairs didn't get me anywhere.

So I'm thinking it has to be one of the three resistors I highlighted in the attached image, or the solder connections to them. I will resolder them next as I don't have extra parts laying around.

[pdf64]

Good work!
Any suitable resistor with a 'close enough' value would be fine, none of those values are super critical.
The 68k seems the most likely suspect, as it's common to both V9 and V10. Consider trying a good ~100k resistor in there.