One Valve Red Plating - BIAS

[Littlewyan]

Hi Guys

I thought all was well until I noticed that the valve closest to the Can Capacitors was red plating sometimes when playing the amp. Now the bias is fine on both valves, doesn't matter what valve I put in that socket the same thing happens. Disconnected the NFB in case it was feedback, same issue, swapped the grids, issue started happening on the other valve. Checked the Bias whilst playing and found on the side thats fine the BIAS Voltage increases from -31V to -54V. On the side with the issue it only goes from -31V to -40V.

Changed Coupling Caps from PI, same issue. I'm really at a loss here :S I mean I checked the bias on the valves whilst playing and the red plating side can go from 20Ma to 100Ma, whereas the side thats ok goes from 20Ma to 60Ma at the most.

Really at a loss here :S

Edit: Also if I stick my multimeter probe on the anode of the valve thats having the issue the amp squeals. On the valve thats ok I get no squealing. Its definitely something to do with the side with the 82K Anode Resistor. As I swapped the grid wires at the Coupling Capacitor and the issue moved to the other valve. Also its not Oscillation from the NFB as I tried disconnecting that.

[Littlewyan]

Ok turns out it was my PI Valve giving me funny bias increases as it wasn't balanced, swapped it and now thats all good. Still get a bit of red plating though, very weird. Everything checks out ok....... My B+ is slightly higher at 425-430V so maybe its the 6V6s I've got not taking too well to the higher B+. I put my EL34s in there for a short while and didn't see them red plate but didn't really test them. Panic over! Can enjoy the amp now.