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my philosophy ... and the way DL explained it: that the triode ahead of the cathodyne should be considered part of the inverter"PI" is not insightful.
Think "DRIVER".
TO drive a push-pull output stage, we need two (and a half) things:
1) GAIN from the 1V-3V max out of preamp to the 10V-30V needed for power stage grids
1a) Gain to cover NFB
2) TWO OUTPUTS of opposite polarity
There's no way to do all this in a single triode.
You can do a triode and a driver transformer, but that's expensive (and not NFB-friendly).
You can use a gain-stage to drive one power tube, and another gain-stage diddled-down to gain-of-one inverting to drive the other power tube; the ParaPhase family, simple and self-correcting.
The Cathodyne was a killer invention, coupled with 'new' (late 1930s) more-sensitive power tubes like 6V6 and 6L6. However it lacks gain, so it "always" used with another tube in front. This can even be a lower-current higher-gain tube, since the cathodyne has huge input impedance.
There are some strange-tricks possible with screen-grid tubes which give antiphase outputs with gain. Symmetry and max output are usually poor. Never seen it done as a power-amp driver.
The long-tail has lower max output and lower gain than the same twin-tube, but very symmetric *clipping* action. (The Cathodyne is symmetric un-loaded, but power tube grid current makes it go wacky.)
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biasing with regard to current draw seems to originate in the 90'sIt goes with OVER-volting. In self-bias, plate voltage is limited by idle dissipation and reasonable load impedances. Fix-bias allows higher plate voltage while setting a low idle dissipation. Most medium fix-bias amps didn't take plate voltage a lot higher than a self-bias amp. Idle current was not real critical. 25,000 Watt audio amps in radio stations did set idle current very carefully.
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biasing with regard to current drawFix-bias P.A. amps, also some Fisher and Dyna hi-fi amps, had points to measure power tube cathode current. This often involved 10 ohm resistors with straps to bypass the resistors in normal operation. 10 ohms allowed reading ~~100mA current zone with the usual 1.5V lowest scale on common meters (no 199mV DVMs). Many Williamsons had shorting phone-jacks where you would plug in a 100mA current meter. The biggest Dynaco had a VU meter which could be switched to the cathode return so you could trim bias voltage.
http://www.dvq.com/hifi/images/dyna_mk6.pdf (3MB PDF file, page 12; 7.8r and 2K9 + VU meter)
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the 70% rule, which I've been using but never liked or fully acceptedSuck it up and accept it.
The effect on max-power is insignificant. (A way-high idle current hurts max-power, but in most commercial-like amps the tubes would be red-plating before that; it can be an issue in flea-watt toy amps.)
Best idle economy and life is zero idle current.
But that sounds awful on small sounds.
Best overall sound quality is very high idle current. In most commercial-like amps the tubes would be red-plating. Indeed I could hear some small improvement when my Fisher was biased to a dull glow. Didn't dare to leave it there.
SO: way-too-low sounds bad. Way-too-high sounds best. There is a w_i_d_e range between where sound quality (and max-power) hardly changes. In many amps, 50% Pdiss to 99% Pdiss is nearly all the same. But bias drifts with wall-voltage. Aiming at 50% and drifting low may get rough. Aiming at 99% and drifting high cooks tubes. So split the difference, 74%, BUT lean low because cooked-tubes is worse than rough soft sounds. Thus 70%.
FWIW: the well-designed Dyna VI seems to aim at 77% safe Pdiss, BUT there's a front-panel meter so you can keep an eye on it. And note that it has no way to measure each of the four tubes individually.
Topic: Princeton/Deluxe (Read 11451 times)