If the 6EJ7 triode wired miller effect capacitance causes -3dB @ 36kHz; that is -1dB @ 18kHz.
What is the distributed capacitance of the PRC-2?
What is the plate impedance, rp, of the triode wired 6EJ7?
What is the resulting -1 dB high frequency from those?
I am not sure which comes first . . .
Microphone -1dB @ high frequency
Tweeter - 1dB @ high frequency
My ears -1dB @ high frequency (no question, this IS the limiting high frequency factor of my system from microphone to ear).
I never saw an air gapped signal choke that did not introduce hum in the presence of a magnetic hum field.
What is the distributed capacitance of the PRC-2?
What is the plate impedance, rp, of the triode wired 6EJ7?
What is the resulting -1 dB high frequency from those?
I am not sure which comes first . . .
Microphone -1dB @ high frequency
Tweeter - 1dB @ high frequency
My ears -1dB @ high frequency (no question, this IS the limiting high frequency factor of my system from microphone to ear).
I never saw an air gapped signal choke that did not introduce hum in the presence of a magnetic hum field.
Professional mic preamps and other pro preamps tend to strive for bandwidth far beyond the 20 khz (and I at 45 have enough trouble hearing 18 khz), why is that?
Wasn't I taught it is to preserve transients and phase relationship? Isn't that part of the reason why we check for 10 khz square wave response of an amplifier - no only to test NFB loop stability, but an ideal amp will draw those square waves nice and square into its nominal load? At least, with no nasty ringing around the leading edges.
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The only thing it does is provide low Z-in into the triode-strapped 6EJ7. Which has a significant Miller capacitance.
L5 could very well be a hum magnet (pun intended). Maybe a DN2540 CCS plate load instead?
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Alas, the manufacturer isnt likely to share the detailed specs on the PRC-2. Most likely they don't even know/measure themselves.
The rp of a trioded 6EJ7 is on the order of 5K. Mu is 55.
But the plate load choke picking up hum is a very real concern. I will admit I haven't used plate chokes in real life. If I did, I would probably pot every one in a ferrous enclosure (magnetic shield) as well as observe the minimum distances and orientation to minimize coupling.
Looks like my 1st schematic will be the one I'll try. The second idea maybe for later.
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But there's nothing special about pentode 6EJ7, in that case just go for a traditional pentode EF86 line stage, as it is a low noise (by 1960's standards) audio tube.
I actually have one version I simulated. With 100K Rplate and 470K Rscreen and unbypassed 1.3K in the cathode the overall THD (coming out of WCF) is curiously low with no NFB. Must be some distortion cancellation going on there, but whether this will actually happen in real life, I don't know.