Sloppy sine wave

Frequency?

Y'know, if you're going to build a circuit out of toobs, you really ought to build the WHOLE circuit out of toobs.

How about using a triode as the plate load, with the grid held at about half of the plate potential with big-ass resistors (1M-ohm?) and a big- ass cap from grid to cathode. I'm pretty sure that the impedance looking into the cathode will be equal to 1/transconductance at DC, but be equal to the plate resistance at AC.

Or, just resistively load the thing and be done with it.

The sine frequency is about 1 kHz. The noise from the switcher is around

I'm not a tube purist - I'm content with a single transistor in the circuit. In this design the triode will hopefully be performing the dual function of RF amplifier (with the signal injected into the cathode) and demodulated audio amplifier (with the signal injected into the grid.)

The cathode is bypassed for audio gain (but not RF as the signal is being injected there) with a small electrolytic capacitor in series with an RF choke.

The single transistor gyrator load is nice because it presents a high impedance to RF, but just looks like a resistive load at low frequencies, and you can also take the audio output from the emitter of the PNP and get an emitter follower output for the demodulated audio from that. Not bad for a PNP, a 10nF capacitor and a couple of resistors. I'll try to post a schematic at some point...

The main problem will be keeping the switcher noise out of the RF stage.

If the switcher is at hundreds of kHz and the sine is at 1 kHz, I'm thinking oscillation.

Those smell like (sub)harmonics. IM?

Are you getting IM from the switcher or what? I would think a nice RC or pi filter at the switcher would work as an LPF.

I have a cheap condenser mic than can whistle when loaded by certain preamps. It uses an RF stepup switcher for the FET Vcc in the mic. That couples with the phantom power somehow.

It looks just like the standard switcher ripple that I can see on the switcher's output, but superimposed on the audio. An LC between the switcher's output and the terminal block that feeds the B+ and filament voltage is what I'm going to try.... ; )