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In Reply to: RE: Touche', sort of... posted by cpotl on February 14, 2015 at 16:08:45
What load impedance did you use?
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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.
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