|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
74.194.39.162
In Reply to: RE: Touche', sort of... posted by Ralph on February 13, 2015 at 14:18:40
"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)."Just for fun, I've been playing around with a Spice simulation of an M-60 this afternoon. Up to a point, I think I see now what you are saying. Namely, that the current in the bank of tubes that is heading towards non-conduction in a given half cycle of the audio signal goes to zero rather more slowly than one might perhaps have expected. ("Slowly" in the sense of requiring a more negative audio signal voltage to drive it into non-conduction than one might have expected.)
The quiescent current per tube bank was 533mA in my sim. I think this is roughly in line with what you were saying (about 66mA per triode section). At about 5W output into 8 ohms, I(max) through one bank of tubes is 1.27A, and I(min) is 0.125A (so not cut off yet). At about 10W output, I find I(max)= 1.65A and I(min)= 0.04A, so maybe one could still say it has not cut off. At about 17W output I get I(max)= 2.10A and I(min)= 1mA. Does one call this cut off or not? At an output of 20W, I get I(max)= 2.33A and I(min) really now shows as zero.
So, in summary, I suppose the question of what power level it transitions from class A to class AB really comes down to a matter of technical definition, and at what point one draws the line. If one were to say that it is class A if even one picoamp of current is flowing through the tube bank that is heading to cutoff, then I suppose one could claim that it *always* operates in class A. But this would certainly not be in the spirit of what class A is supposed to mean, I think!
It seems to me that the whole notion of class A for any kind of push-pull amplifier essentially assumes that the transfer functions for the two halves of the output stage are pretty much linear. If one is depending for calling it class A upon gross nonlinearities that mean the output tube banks are difficult to push into non-conduction, then it's not clear that there is any useful class A quality to the behaviour. It really seems to me that one would then be just claiming class A behaviour on a technicality of a definition pushed beyond its regime of sensible applicability.
Of course, the Spice simulation I did is principally limited by the accuracy of the tube models, but I think it has probably captured the essence of what one would find in an actual version of the amplifier.
Chris
Edits: 02/15/15 02/15/15Follow Ups:
What load impedance did you use?
I used an 8 ohm load impedance in my Spice simulation of the M-60.
Just to be absolutely sure I'm using the right magnitude for the bias currents for the output tubes, could you confirm that something like 60 mA or so per triode section is what you are using?
By the way, calculating the output impedance for the M-60 simulation in what I would regard as the "standard" way (putting an AC current source across the output terminals and calculating Z as voltage over current), I get results that vary between about 4 ohms and 9 ohms, depending upon what the output impedance of the signal source is. (This is reasonable, because the degree of feedback provided by the 2M ohm resistors from the output to the input stages depends on whether they just "see" the 100K ohm grid-to-ground resistors at the input, or whether they see lower impedances in the case that there is a low output impedance signal source connected to the input.) Anyway, the 4 ohm figure I get for the output impedance, corresponding to having a fairly high impedance signal source, seems to accord with the figure you quote for the M-60 on your website. I guess that gives some confidence the simulation is reasonably trustworthy.
Chris
60ma is about right. Actually in the M-60 it often tends to be slightly north of that number, but not by much.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: