Laney Cub 12R Tube Amp hum noise

[camarada78]

I have a Laney Cub 12R Tube Amp to fix a noise on my bench. It works normally but it has an annoying hum (120hz and 180hz are there but around 1k are the most annoying). It works normally otherwise. I know tube amps have some noise but this is too much.

- noise goes up and down with either volume or gain (as per schematic attached)
- tubes are new, same problem as with old ones
- amp has normal ground earth.
- the noise is there even without guitar, so its not pickup hum/noise.

I was suspecting some resitors (R13 and R28) but i changed them but the noise remained. Any clues?

My next suspects are R9 and R14. I will update soon.

 

[camarada78]

Its not R9 nor R14. Checking more stuff. :-/

 

[AnalogPackrat]

Check electrolytic caps in B+ and bias supplies. Is there AC on the B+ or bias rails? Or the +/-15V rails? Could also be something related to the digital FV-1 stuff. Trace backward with a scope to where the crap first appears in the signal chain.

 

[Whoops]

With a wood chopstick tap the wiring inside the amp, and try to move the wires both sides to see if the hum changes.