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RE: Touche', sort of...

Posted by cpotl (A) on February 13, 2015 at 15:18:19

In Reply to: RE: Touche', sort of... posted by Ralph on February 13, 2015 at 14:18:40:

"The S-30 does considerably more than just 640mW of class A power (its idle current is around 400mA)."

I thought the S-30 has 5 6AS7 tubes per channel, and hence 5 triode sections for each "half" of the output stage of a single channel? Surely the quiescent current for each triode section can't be as high as 80mA? That would push the plate dissipation uncomfortably high, I would think. I was assuming 40mA quiescent for each triode section, and hence 200mA for each of the two banks of tubes.

The maximum power that keeps within class A will be when the current in one bank drops (essentially) to zero at the peak of the waveform, with the current in the other bank therefore reaching twice the quiescent current; i.e. 400mA under my assumption of 40mA quiescent per triode section. A peak current of 400mA means an rms current of about I=280mA, and hence a power into R=8 ohms of P=I^2 Z, hence giving the 640 mW I estimated. Did I underestimate the quiescent current per triode section?

"In the case of the M-60 (with 1 amp of idle current) it remains (barely) in class A2 operation up to full output into an eight ohm non-inductive load (insofar as a 'scope on the cathode resistors is able to show anyway; cutoff does not occur until the amp is into clipping)."

Again, I'm puzzled by your figures. The M-60 monoblock has, I think, a total of eight 6AS7 output tubes, and hence 8 triode sections for each "half" of the output stage? An idle current of 1 amp would mean 125 mA per triode section, which with, say, 140V B+ supplies would mean 17.5W plate dissipation per triode section, which is far too large. (13W maximum is quoted in the datasheet I looked at.) So I am suspecting perhaps the idle current you are quoting is the *sum* of the idle currents in the two halves of the output stage? If we assume that, then my calculation for the M-60 would be that the peak current through the 8 ohm load, while staying within class A, would be 1 amp (i.e. twice the idle current in one half of the output stage). Thus the rms current would be 1/(sqrt2) amp, implying a maximum of 4W into 8 ohms while staying in class A. That compares with the M-60's 45W achievable power into 8 ohms.

I'm not quite sure how to account for the discrepancy here, since you are saying it stays in class A, for an 8 ohm load, right up until the 45W maximum output power is reached. That requires about 3.3 amps peak current into 8 ohms. If the quiescent current per half of the output stage is 0.5 amps, then by the time one bank of tubes has reached 3.3 amps in its conducting part of the audio cycle, the other bank of tubes will have well and truly reached zero. Or are you saying that the "non-conducting" bank never truly turns off, because of a stray microamp or two (or even a milliamp or two) of current, and that therefore technically it could still be called class A because neither bank of tubes ever completely literally stops conducting?

Chris

Edits: 02/13/15 02/14/15

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