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In Reply to: RE: Drd monkey or direct coupled posted by hennfarm on April 24, 2016 at 16:55:14
The problem with the 6DJ8 as a driver is that they do not like to be run at high voltages - so you need a moderately high gain (mu) for the output triode. The max plate voltage spec is 130v, and there are widespread reports of difficulties if the voltage exceeds 90v.
The thing about power tube operating points is that the plate-to-cathode voltage, the plate current, and the load impedance are connected to each other. I post this every couple years, but it's still not widely known :^) ---:
My target is usually RL = RB - 2.38*rp
where RL is the load impedance, RB is the "beam resistance," i.e. Vp-k/Ip, and rp is the plate resistance at the chosen operating point. Vp-k is the plate to cathode voltage, and Ip is the plate current. You can squeeze out a little more power by reducing the RL, at the price of slightly increased distortion and decreased damping factor. You can usually go as low as 70% of the calculated RL without severe penalties.
Follow Ups:
When you posted this info a few years ago i printed out a paper copy to stop me forgetting all about it. I have used it to good effect in the last two amps I built, a Siemens F2a and a 6CB5A, both in triode mode. However, when I tried this method with a +1KOhm rp tube, the results were very weird and totally unusable. Might have been my deficient math skills of course. There is another thing I don't understand (one of many), where does the "2.38" number come from?
2.38 is 3 times the cube root of 1/2 - probably not what you were asking? :^)
It comes from using the 3/2 power law for triodes, and drawing the load line from twice the quiescent current at zero bias, through the quiescent operating point.
Thanks, that's what I was looking for.
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