Well, it seems I was just about 100% wrong on that. I've just been put right on this on the SoundOnSound forum by my mate Dave, who's an ex-Blackstar amp designer.
What causes the hum in the PI valve position is leakage between the heater and cathode. Now that fact that I've bought 'balanced' valves for use in this spot and the hum has gone, is nothing to do with the 'balanced' bit. As Dave pointed out to me, it's unlikely that the gain control resistors for the two halves of the valve are any better than ±5% tolerance, and most likely ±10%, so that matching the triode gains to 99.9% isn't going to make any difference.
So it's most likely the fact that in the testing and selection process, any valves with high heater/cathode leakage are being weeded out, so that only those with little or no heater/cathode leakage make it through. Now, I have no idea whether the same criteria are being used for other valves that aren't being 'balance' tested, and that probably depends entirely on the mindset of the valve suppliers. They may have to reject a lot of their valve stock if they were too rigorous! But I'd still recommend paying a bit extra for a PI valve that has been tested for use in the PI slot.
And hum balance pots (if fitted) go in the heater line and are nothing to do with the phase inverter circuit.
Last edited by Simon Barden; 10-05-2017 at 04:13 PM.
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