Minimalist OTL speaker amp.
Fooling with 6c33 curves, I realized there was little audio advantage to running into Class B areas. With just one pair of 6c33, good linearity demands fairly high idle current, and the mighty 6c33 can stand the strain. Working Class A, there is an alternative to complex Futtermans: the White Cathode Follower.
The WCF is "flawed" in that it can not get out of Class A. But it is dead simple to drive. It is not true Push-Pull, because the drive for the bottom comes from the top, but linearity is excellent up to 80% of what you can get with a true push-pull stage stuck in Class A. It is in fact two voltage amplifiers bound-up in a unity gain feedback loop. Although the 6c33 is a poor voltage amp, if you get the two halves balanced (not an easy thing) then 2nd harmonic vanishes and 3rd harmonic tends to be low up to a dB or so below gross clipping.
This seems to be good for about 2 very clean watts into 8 ohms, 3 watts at a couple % THD. Power is higher in 16Ω.
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It is not easy to get biased-up for best power in very-very low load impedance. I suspect my 6c33 model is poor, and that real 6c33s vary a lot. So it has to be hand-biased.
Start without the 6c33 tubes in socket. Trim the 12AX7 cathode resistor so that the plate voltage is 40%-42% of the supply voltage.
R3 can be a series string of say fourteen 10Ω 2-Watt resistors: we want to start around 120Ω to be sure the tubes don't burn. Put a voltmeter on the bottom 10Ω resistor. We want to see 2.9 to 3.2V here (290mA-320mA cathode current). With 140Ω it will probably be low. Let it cook a few hours to stabilize the tubes. Then start shorting-out the top 10Ω resistors for total resistance of 130Ω, 120Ω, 110Ω, etc, until you get to about 3V on the bottom resistor (300mA cathode current).
R1 should be 60% of R3, whatever R3 ends up being.
Working this way, you should get a couple watts at 1%, a mix of 2nd and 3rd, higher harmonics well under 0.1%. Damping factor will be about 2. Frequency response will be below 20Hz to above 100KHz. If you use it for tweeter-only work, you could make all the amplifier caps 10V smaller.
BTW: you may notice that the claimed power is higher than the Class A bias implies. Tube nonlinearity raises the peaks more than the troughs, so we get over 700mA peak with 300mA bias current.
One fault of the WCF is poor power supply rejection. You will need a VERY clean power supply. I have sketched a 3-stage R-C filter to get you started: it should not be a buzz-box, but it won't be dead-clean on high efficiency speakers. Replacing one or two of the 50Ω resistors with something like Hammond 159V (1.5H, 500mA, 27Ω ) should clean it up good. Yes, the last capacitor MUST be big, like 470uFd, because the top of R1 has to be held solid to develop the correct drive for the lower tube.
The "320V" raw supply is meant to be a standard 220V-240V winding plus rectifier and capacitor. Normal winding and utility variations don't matter.
If you bias a heater winding up to about 70V DC, you can heat both 6c33s from one winding without exceeding heater insulation breakdown limits. If you wire the 12AX7 for 6V, twist and route the heater leads well, it can be fed from the same winding.
Heat output per channel is about 72 Watts heaters, 110 Watts in the plate circuit. For stereo, you want a 200VA heater transformer, at least 400VA plate transformer.
