Hot Rod DeVille Crackle

[BungleSim]

Had a Hot Rod DeVille come in that was having a lot of noise/no signal issues. Measured flaky voltages in the power section so I replaced all of the power electrolytics. Now the amp sounds great save for crackling/rolling thunder that is intermittent. I've replaced the 330R/5W resistors feeding the +/- 16VDC rails, the 470R screen resistors, and I replaced all the plate resistors with 1W. I just realized I did not replace the PI plate resistors yet, but other than that I seem to have replaced all of the "common" issues with this amp. I've also inspected the solder joints on the main board and touched up all of the solder joints on the tube sockets. Any suggestions?

[xtian]

The solder joints on the flexible jumper ribbon cables are common points of failure.

You can reduce the possible problems areas by dividing the circuit at the FX loop.

[Stevem]

Its those PI plate resistors!
They are only 1/2 watters with over 350 volts on them and there is no 1/2 watt resistor made rated by the manufacturer to take over 350.
A very very poor choose by Fender!!!!
They had the out right Balls in there Champ 12 amps to use a 1/4 watt resistor in the power supply node string, and this is in a 12 watt SE amp!
To me that's pure distain for there customers!!!!
Oh, also make sure that the output tube pins are resoldered with good leaded solder.

[BungleSim]

Changed the plate resistors on the PI, no dice. Also thoroughly inspected the solder joints throughout the amp, even re-soldered all the tube sockets for the second time, and tested ribbon cable connections to no avail. I did measure 7.2V of ripple on the B+. Is this normal? The ripple is consistent and does not seem to follow the crackling noise at all, but the tubes seem to be getting abnormally hot. Could this ripple be heating the tubes? Seems like R70 and R71 (220K, 1/2W) in the power section are under powered. They measure fine but perhaps they are failing as the amp heats up? The noise definitely gets worse as time goes on so that leads me to believe it could be heat related.

Also, the crackling noise does not go away if I pull V1 and/or V2. It only goes away when I pull the PI. One of the first things I did with this amp was roll through new tubes, but I even tried replacing the PI and power tubes again last night just to be sure and there was no change.

[Leo_Gnardo]

I did measure 7.2V of ripple on the B+. Is this normal? The ripple is consistent and does not seem to follow the crackling noise at all, but the tubes seem to be getting abnormally hot. Could this ripple be heating the tubes?

Other way around - drawing too much current from the HV supply will increase ripple, and you said output tubes are running hot = overcurrent. You can adjust that but it's not likely the cause or result of your crackling racket.

Couple weeks ago I had a similar amp with a similar problem. A year previously I had replaced the voltage dropping 5W resistors so I figured "that can't be the problem". After looking everywhere else, that's exactly where the problem was! One of the 5W leads simply cracked in two making intermittent contact. Replaced it and everything was OK. So have a very close look at those problematic 5W R's and their connections, a loose one would cause the crackling problem you describe.

Stevem's comment on the drive tube's plate resistors is spot on - I haven't yet found one to be intermittent or noisy, but occasionally I get an amp from this series that has no output drive or half the waveform missing and in these cases it's usually an open plate R in the drive department.

[BungleSim]

I will be referencing the attached schematic. So far this is what I have done.

1. Replaced the screen grid resistors (R61, R62) with 470R/3W resistors.
2. Replaced large power caps (C31, C32, C33, C34, C35, C36) with high quality F&T electrolytics.
3. Replaced all plate resistors (R4, R11, R16, R22, R57, R58) with 1W resistors.
4. Replaced all electrolytics in bias supply (C43, C22) and op-amp supply (C39, C40, C41, C42).
5. Replaced the poly caps before and after the phase inverter (C24, C25, C30, C26, C27).

It sounds like DC is making its way onto the audio path and it also sounds like it's a function of heat. I can leave the amp on for 20 minutes or so and there's no crackling, but then it starts to creep in slowly and finally it gets to the point where it is just a rolling storm of pops and crackling. Pulling V1 and V2 does nothing to remove the noise, but pulling the PI does. Any further thoughts/ideas?

[Stevem]

My bets on the PI tube socket, or a thermally bad solder connection in the B+ feed path from that nosed filter section to r57 and 58!

Does rolling the PI tube around make the noise get worse, or better?