Weird grounding issue with bright switch/treble control

[nee]

Hello all,

My 'Wreck homage has developed a weird issue with what sounds to me like a grounding buzz BUT:

if I switch the bright switch on in either position the buzz disappears OR:

if I turn the treble up full it disappears BUT:

if the bright switch is on and the treble is up full the buzz comes back!

I'm sure someone can follow these clues to the solution!

Many thanks!

[nee]

Another clue: when the volume is on zero there's a terrible howl from the speakers!

[paulster]

Does it sounds like a mains hum but at full power?

I had this on an Express and Liverpool build after putting some chassis damping material down which was great for combating microphonics but lifted the second triode grid wire about two millimetres off the chassis and allowed a parasitic oscillation to set up which modulated the power supply.

It was a rather unpleasant experience to have my head next to a 4x12 when I discovered this phenomenon.

Pay particular attention to this grid wire and try using a shielded one as part of your debugging process if necessary.

[nee]

Well, I fixed it but I feel I've subverted the laws of physics. I accidentally pushed the brass ground rod hard against the chassis and the buzz and howling stopped. I connected a wire between rod and chassis but that was no good - the rod had to be physically up against the chassis. I drilled a hole and connected a small tag to the chassis and ran the rod through it - the amp has never been quieter. The input wire from jack to V1 can be moved about any old place and it makes no difference to the noise, whereas before it had to be in a certain position. Very weird.

I

[nee]

Oops! Nearly fixed it. This really has got me stumped.

I have a switch which removes the cathode bypass cap from the 2nd stage (I call it Hi/Lo Gain). When I'm in the Lo Gain mode I get buzz if the Bright switch is not enabled. Once the bright switch is enabled everything is fine. The Hi Gain mode is fine in all positions of the Bright switch.

I can't seem to figure out where the grounding issue could be regarding the bright caps.

It definitely sounds like earth buzz rather than a hum.