Crackling ugly sound diagnosis

[WORMDIRT]

I have a Sundown rebel 100 that is making shitty crackling sound, mostly during palm muting. The sound is crackling, static, only when I have an input signal.
Where should I begin my troubleshooting?

What I have tried already..
-Cleaned all tube sockets
-messed with different EQ settings
-Move the amp off the cabinet (to eliminate vibration)
-Tried different guitars/cables
-Switched all the preamp tubes, 1 by 1, with known working tubes
-Cleaned all the jacks

Amp info
It has a PPIMV as well as a normal/regular master volume
It has an "RMS" control that changes the bias from 12ma to 40ma and all points between
A trusted tech just changed all the filter caps
The powertubes and PI are JJs and are 6ish months old


I would like a starting point before I open her up tomorrow.
I'm a capable tech, I just don't have the theory behind my electrical skills, if that makes any sense. My tech is out of town for another month.

[dorrisant]

Spray the pots with Deoxit, look for DC present on the wrong side of your signal caps.
If that fails, use/build an amp stethoscope and start probing for a bad component.

Sounds like you've done most of the obvious stuff.

Are your plate voltages in line with what is expected? Maybe you can post the voltages so others can notice any red flags.

Tony

[tubeswell]

Did you re-tension tube socket pin clamps?

[WORMDIRT]

Did you re-tension tube socket pin clamps?

They are a type that can't really be retensioned. I bent the pins of my preamp tubes inward every-so-slightly to make sure they have a good connection.

[tubeswell]

Pop in a fresh set of output tubes and see if that makes a difference. Random static crackling is a common symptom of output tubes on their way out. You should always keep a spare set handy.

[Stevem]

Plug a cable into the amp and with a meter set for low low DC volts do you read anything? even if you do not go to a radio shack and pick up a 5uf to 22uf cap, a electrolytic type is fine and install in between the two 68k resistors off of the input jack and pin 7 of the first preamp tube, if the cap is a electrolytic type then the end with the band / stripe gets installed at the pin 7 side and see if this cures the issue.
If it does that's ok, but this is still a band aid and if I where you I would continue the search for the root of the problem, but at least you can play the amp without the noise!

[gui_tarzan]

Even if your tech is reliable I would still double check the solder joints on the new caps. It's easy to make a mistake, I figure all of us here have done it a time or two. As tubeswell said try a different set of output tubes too. There has been enough conversation amongst the different forums about JJ's quality that it would make me suspicious.

[Leo_Gnardo]

Plug a cable into the amp and with a meter set for low low DC volts do you read anything? even if you do not go to a radio shack and pick up a 5uf to 22uf cap, a electrolytic type is fine and install in between the two 68k resistors off of the input jack and pin 7 of the first preamp tube, if the cap is a electrolytic type then the end with the band / stripe gets installed at the pin 7 side and see if this cures the issue.

I can see you're suggesting blocking any DC that may be present on the input grid, possibly due to a defective tube. Where does that grid on pin 7 get its grid leak after you put a cap between the input jacks and tube grid.? When the grid floats to wherever it can, you will definitely have some unwanted noises and/or signal blocking. A 1M resistor from pin 7 grid to ground will be needed, in a pinch you could use anything from 220K to 6M8.

[WORMDIRT]

New development. Tried to turn it on today, switch into standby, loud pop and no power. The fuse looked fine so I went ahead and threw another one in. No power still. Like none whatsoever. No pilot light etc.

[Stevem]

Well if the mains fuse is good and you have proved out that the power switch Is good but still no pilot lamp or tube filaments come on then you have a dead PT or bad connections that bring the ac voltage into it!

[WORMDIRT]

A little old but, There was an internal fuse hidden underneath the power switch that tripped. One of the power tube sockets was arching. I replaced all the tube sockets and that solved it. Damn it is always the little hidden stuff that gets you.

[Stevem]

Yop!