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In Reply to: RE: Touche', sort of... posted by cpotl on February 13, 2015 at 08:15:26
Max plate dissipation is, I think, 60W.
If you bias at 300mA and plate voltage is around 130V (in my Atma-sphere output stage), then you're running around 40W, which should not overtax the tube. (It's close to the Golden Mean espoused by you know who.) FWIW, I never had a tube failure in 8-10 years using these parameters.
Follow Ups:
"Max plate dissipation is, I think, 60W.
If you bias at 300mA and plate voltage is around 130V (in my Atma-sphere output stage), then you're running around 40W, which should not overtax the tube. (It's close to the Golden Mean espoused by you know who.)"Ah yes, the golden mean and "you know who"! Far be it from me to defend anything that YKW says, but there is perhaps a difference in that in true class A, like an SET, the mean power dissipation in the tube increases by a much small factor when the music gets loud, in comparison to the increase in a class AB amplifier. And I would still maintain that practically-speaking, a circlotron or totem-pole OTL is operating in class AB when the music is loud. So one probably ought not to be taxing the tubes too much in their quiescent state, in order to leave plenty of headroom for the loud passages.
However, having said that, real-life experience such as yours with the 6C33C tubes run at 300mA is probably a much more reliable indicator of what one can get away with than mere theorising!
"FWIW, I never had a tube failure in 8-10 years using these parameters."
Ironically enough, just this morning I have been trying to track down a problem in one of my OTLs, which uses 6C33C tubes. One of the output tubes seems to have developed a problem; it goes into a run-away mode after a few minutes and starts conducting more and more. This has happened after quite a few years use, and I was only biasing them at something like 180 mA quiescent. So my low bias current prescription is certainly not a panacea! I did have one other 6C33C failure a couple of years ago, but in that case it was the opposite kind of problem; it stopped conducting completely.
Chris
Edits: 02/14/15
I guess the speaker is a big factor in whether the tube will be overtaxed on transients, when it surely does go into AB mode. (So far as I know, the 6C33C cannot be driven into A2. Why that is so, I don't know.)
You can't push it into A2 very easily due to the immense grid currents involved. If we went to a semiconductor driver we might be able to get a few extra watts.
The 6AS7 goes into grid current while the grid is still about 15 volts negative with respect to the cathode- its onset of grid current is much more gradual by comparison. We can drive the tube to about +15 volts on the grid before the saturation currents become so immense that they saturate the driver (which in turn saturates the voltage amplifier).
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