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And it may not really even BE a true mu stage driver, but anyway....My question is about the cathode of the lower triode in the so-called mu stage. Should that bypass capacitor be ever so small(.01ufd)?
Could replace both the resistor and the capacitor with a AA NiCad as I doubt the input will ever exceed 2V PP, but I'd like to try this schematic as I already have the Grid Chokes and most of the film capacitors required.
Edits: 04/03/21Follow Ups:
I would think the purpose of the .01uf cap is to boost the high frequencies by eliminating the local current feedback at those frequencies. The filter would be the .01uf cap and the 1k resistor in parallel with the cathode impedance. So approx 14085//1000=933 so the knee of the filter is somewhere around 17kHz unless I got the math wrong.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
However, it is too small to work other than to improve the measurements to 20khz. I would leave it out.
a cap bypassing its cathode resistor or replace it with a 100uf as was in the original design (SRPP?).
The original circuit is SRPP in name only. It does not produce the desired anti-phase mechanism, because the load impedance is too high. Both sections operate in phase, with the upper section consuming supply voltage and power for no benefit. Gain of the lower stage is substantially reduced in this arrangement, as is the maximum output swing. The 300B would be better served if it was driven by a conventional grounded cathode stage.
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Buy Chinese. Bury freedom.
A mu stage? Who knows?It differs somewhat from the faux SRPP but I am unable to tell exactly what it is doing.
Edits: 04/05/21
The upper section is a constant current source for the gain stage, and its cathode serves as a low impedance driver for the 300B. It's a reasonable topology, but the issue I see is low B+. That limits output swing. OTOH, if the driver produces sufficient output to drive the 300B to full output, perhaps this is satisfactory. The only question might be whether the 6SL7 swings too close to the rails under these conditions, as might be evidenced by an increase in distortion.
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Buy Chinese. Bury freedom.
And the voltage to the driver stage is easily changed with the 62K resistor in the power supply.
Still very curious as to what to do with that lower cathode small-value bypass cap. Leave it out or go 50-100uF?
Without the cap, -1dB response (relative to 1kHz) occurs at about 100kHz. With the cap, it occurs somewhere north of 200kHz, but the cap introduces a rise of about 0.5dB at roughly 45kHz.
With 100uF at the cathode, gain at 1kHz increases about 1dB. The -1dB response occurs at about 150kHz. I wouldn't do this, just because it represents one more unnecessary cap in the signal path. In fact, I wouldn't use the .01uF either. It simply isn't needed.
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Buy Chinese. Bury freedom.
Introducing an unnecessary cap is generally something I wish to avoid.
With an active pre-amp and efficient horns, gain should not be a problem.
Page 115.
Omnes feriunt, ultima necat.
Original version is above, without alleged 'mu stage' modification.
Note the 100uf cap bypassing the cathode resistor on the lower driver tube.
This was originally a Black Gates cap and a Kwiami resistor but I changed it out for a NiCad which, to my ears, sounded better. But this was 15 years ago and I have since lost the resistor and now unobtainium cap. :-(
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