Quote:
Originally Posted by Lucien Nunes
Obviously this does not apply to a valve amp unless some permanent load is applied to the secondary, as open-circuiting the output with signal present can seriously stress the output transformer primary, valves and holders with overvoltage.
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I've been playing with a small push-pull valve amp (<0.5W) using Russian rod pentodes which a friend made and for which I've recently made a custom output transformer - the amplifier is seriously good BTW - and as one of the tests I wound up the input with no load connected while 'scoping the output anodes.
The amplifier runs from 100V, and I didn't see any voltage excursions beyond 220V.
With negative feedback applied, the circuit was very tame - disconnecting the load made under 5% difference in anode AC voltages.
Though, I'd concede that with a single-ended amplifier, high voltages could indeed be produced, the output transformer would act like a car ignition coil, or a line output transformer, as the valve goes rapidly into cutoff with a strong signal to its grid.