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RE: Class A Operation/Bias Points with PP 6B4G

Those are great questions.

Jeff was, IMO, saying that 45ma. at the normal 250 volts is/was the way to go. I disagree.

In a push pull amplifier using the RCA 2a3;

At 250 volts plate and 60ma. plate current into a P to P load of 4700 ohms, there is 8.5 clean Class A watts.

At 250 volts plate and 45ma. plate current into a P to P load of 4700 ohms, there is only 4.7 watts of clean Class A power.

If there is a need for power over the 4.7 watt level, the amp with 8.5 watts will sound more "relaxed and less strained".

Yes, this hobby is very subjective. But lower distortion always sounds better to me. I think that some people like distortion, I don't. There in lies the rub.

If one wants only 45ma. of current, the plate voltage needs to be increased and the plate load needs to be increased if the max available Class A power is desired.

Same 45ma. of current with 300 volts across the tube and you can get 9 watts Class A with distortion being very much the same but this is into a 9100 ohm P to P load.

So there is nothing wrong with having only 45ma. through your 2a3 (6b4g) but the other parameters need to be adjusted to suit.

Simply stated, Class A operation is when the plate current changes with the changing grid voltage in a linear, symmetric way never reaching the cutoff region (non-linear part of the plate curve just above cutoff) or the plate saturation region (non-linear part of the plate curve just below plate saturation).

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

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