Different B+ voltage for each power tube?

[Jab]

Hi

My '72 Super bass hasn't been sounding right for some time. I've previously biased it on one tube with an ohm resistor to ground. Today I bought a bias meter that gives me all the readings without having to do any maths. I was surprised to notice that V4 and V7 are getting much less anode voltage than V5&6. V7 is getting the least, meaning that if I bias V7 to 50%, V6 goes over 70%. Same thing if I swap tubes into different spots. Tubes are pretty well balanced according to my Orange valve tester.

What could be causing this, is it normal?

Not sure if this is the only reason why the amp isn't sounding right. The overdrive gets a bit ugly and messy with some irritating high end frequencies that are giving me tinnitus. I use either the Rich mod pec pot PPIMV or an SPL Reducer to tame the volume. Right now the PPIMV sounds pretty bad, the attenuator works ok for 3-5db (or really really low, when it sounds fine). Best with no PPIMV, volume on 2 and a good OD.

thanks

[Roe]

check for carbon traces at the tube sockets and look for oscillation

[Jab]

Thanks Roe, I don't think I know how. Can't see any obvious burn marks and don't have an oscilloscope. Do you mean just poking at the wires while playing?

[axeman]

Make sure that all power tubes make good contact on pin5.

[neikeel]

Is your PPIMV pot un-balanced, worn or dirty?

[Jab]

Ok, so the tube sockets might be making bad contact, especially at pins 5. Just scrape with a small screwdriver? Perhaps I should be considering new (or Nos) sockets?

Pot is a dual 500k PEC. Pretty new, closed design so probably not worn or dirty. Can't say if it's balanced, didn't test before installing.

I took the amp to a tech a few weeks ago but all they did was change the PI tube. My valve tester says the original is ok.
thanks

[neikeel]

No point in cleaning the sockets once they have arc'd.

I suggest good quality ceramics as the US amps with present day wall voltages come out well over 500v.