EL84: Class A PP at 252V, right?


Wrong. At least with 253V on the screens at idle. I wanted to get rid of the crossover distortion I was seeing on my '67 Harmony H415 2 x EL84 amp. The scheme shows 299 net volts on the plates and mine was running 360-375 depending upon the idle. So I added a 500 ohm 10 watt dropping power resistor. It's dissipating 5 watts in the dropper.

Now it's 286-252 net plate volts, 35-43ma, and 9.9-10.9watts each at idle, depending upon (cathode) bias adjustment.

Class A with idle of 252V @ 43ma ?


Fairly close but no cigar. Cutoff is being reached on the center line whereas the peaks are not clipping.

Turn up a little and:

Definitely no crossover distortion any more. The big dropper goes from 50 to 52ma depending upon signal. Also showing that it's not quite class A. I guess a hotter idle around 12watts would get me there but class A wasn't the goal and 11watts should be more trustworthy than 12watts.
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your amp's class is first and foremost determined by your transformer.

Not at all. The class of an amp is purely determined by how long of the cycle the tubes are conducting, which is directly controlled with the bias. It has nothing to do with the transformer.

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