Low frequency Popping Sounds from Randall RD45

[lbitton93]

I'm a Noob when it comes to amp repairs, but I'm from a Physics background and have done a fair bit of electronics, so I would like to be able to repair my own amps.

The amp is a randall RD45, I've attached the schematic.

There's a weird low end popping that can be heard on this recording I have made.

https://soundcloud.com/hiraeth-ambient/tube-amp-sounds/s-fFnFkk4yvLC

At the start of the recording I'm not touching anything, this displays the seemingly random temporal nature of the popping. It get's better for a minute or so and then goes through bad phases where the popping occasionally breaks into a raspy white noise sound for a second or two (don't think I managed to capture this on recording).

I am then alternating between tapping the power amp tubes, which seems to make a bit of a Sawwy sound (I think on one tube, but I couldn't get as good a whack on the second due to the position of the Transformer). The Preamp gain seems to have little effect on the popping, however it seems to be more severe or at least more audible as power amp volume is increased. The popping does come through on the XLR output too.

This started happening after I replaced the Master volume pot and poked around a bit. I Discharged the filter caps through a 5 watt resistor. I think it was only a few hundred Ohms, but the caps were only at 0.3V or something fairly low anyway, so I wouldn't have thought that would damage them. I noticed a fair amount of gammy flux around the Tubes from heat damage and what looked like some melting of the socket plastic near the power tubes, I have gigged this thing quite hard over about 6/7 years.

I pulled at a few cables that joined the pre and power amp circuit boards and realised they were soldered on. I've tried prodding said connections with a wooden tooth brush with the amp connected and this doesn't seem to induce any crackling or the problem pops.

Guitar signal is still audible and it seems to make the popping disappear at low volumes, or at least obscure it.

Any Ideas what might be wrong?

(p.s has any body tried putting a DC fan inside the amp to cool the tubes or does this cause too much noise?)

Cheers!

[vampwizzard]

physics/electronics background here too. welcome to the club.

Check the tubes. Check all solder connections. Check to see if the mod you made is causing the issue. Then start tracing out parts through the signal path to the XLR. If its present there it would have to be attached to that part of the circuit.

[Latole]

"This started happening after I replaced the Master volume pot and poked around a bit."
-lbitton93


The issue is there, look at you work, something you do wrong.

Tube amps use lethal voltage, it can kill you if you don't know what you are doing.

[lbitton93]

Cheers Vampwizzard.  I'll buy a couple of new Powertubes and double check the pot connections, hopefully it's one of them.