Can a failing 7199 sound like this?

[Retrovert]

Be careful with the spec. It's +/- 200 v MAX. Peak or 100v Max, steady state.

Yes, there are two values, depending upon whether the heater is positive or negative. I've explained why the disparity exists.

You are correct that it specifies 100 VDC maximum, which is unrealistic as it exceeds the insulation's limits.

 

[Retrovert]

You are not reading me correctly (as usual). I am not saying there is an elevated voltage added there, I am simply stating that what I think is merely a floating voltage of -21Vdc measured (with a Fluke 87) from Pin 5 (heater) to pin 8 (cathode. I bet if I grounded the heater terminal with about 10k ohms, the voltage would dissipate to near zero. Therefore, I don't see any concern for the ST-70 or ANY amp that doesn't ground the heater transformer winding C.T.

The ST-70 does not have elevated heaters so the heater-to-cathode limits (6.3 VAC and approximately 130 VDC) are exceeded by design. This is fact.

I think you are possibly seeing the heater floating up from cathode-current leakage, which itself proves that leakage is occurring.

Again, the leakage problem is well documented and it severely stresses the heater insulation.

 

[triode17]

I'm still curious as to what your triode side cathodes are reading to ground. My 2 are 85v and 95v.

One is 100v, I don't know what the other is but should be very close to 100v.