Re: Quad 11 test run
Not true about push-pull generally being safe with only one valve in place I'm afraid. The Quad II uses a shared cathode resistor for both KT66s and it's this which largely controls the current through the valves. If you take one valve out the immediate effect will be to halve the current. The voltage will then drop across the resistor and this reduced bias will cause the remaining valve to turn on harder i.e. pass more current. A new balance point will be reached but the valve will be dissipating a lot more power inside it than before, and its half of the (precious) output transformer primary will be passing the same increased current. The valve and the transformer may well last, but it would be sad indeed if either of them didn't.
As far as the hum goes, that will be a great deal worse with only one output valve in place. The high voltage supply to the anodes of these comes from a point before the smoothing choke. So there is a good deal of 100Hz ripple on the HT there. The amp's design relies on the currents through the two KT66s being well enough matched that the current ripple, which results from the voltage ripple, is cancelled in the output transformer given that the two halves of the primary are counter-wound. If you have excessive 100Hz hum (check the frequency on your scope - you can just look at the signal across the speaker or, better, a resistive dummy load) then it will be coming from somewhere in the power supply. Mismatched KT66s could be one source.
If your hum is 50Hz rather than 100Hz then that will probably be coming from the mains wiring, or a grounding issue, or the heater circuitry including the possibility of heater-cathode leakage in one of the EF86s.
But the first thing I'd do is to measure how much hum there is. With the amp's input shorted to ground there should be around 1mV RMS (say 3mV peak-to-peak) or less across an 8ohm load, assuming the output transformer secondary is jumpered for 8ohms. Much more than that and the amp is running out of spec.
Cheers,
GJ
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