Brett:
I ran your circuit through Spice and it is quite nicely
behaved.
As you submitted it, there is a huge amount of carrier
leaking through. (I used a carrier of 500 kc for this
test.) But that is easy to get rid of, and once it is
down to a low percentage it can be considered
harmless.
I have never seen that topology before. It is rather
like a "tube" version of the Belar AMM- series of
modulation monitors. A diode detector direct-coupled
to an amplifier. After that you can break out the signal
and do as you see fit. Although the Belar uses an
L-C lowpass filter prior to that amplifier.
I used a 12AU7 tube, one half as a diode-connected
demodulator and one half as a triode connected as a
cathode follower per your writeup. The distortion was
in the vicinity of a percent at all frequencies. As you
lowered the input level it got a bit worse. But I would
expect that demodulator to sound very nice indeed. It
works acceptably well down to just a couple of volts
of input RF.
- Jim Tonne WB6BLD TonneSoftware.com
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