[morzh]

I just defeated a major part of that distortion.

Funny that I tried thaty before but did not think it worked.
Probably because there were two reasons - the tuned hum and the detector distortion.

So, tuned hum right now is not present.

What I did, I removed the 0.5uF capacitor #25 across the cathode resistor #21 in the detector. The large capacitor is there to increase gain in the sound frequencies. This indeed increases the gain (you can hear it immediately) but also increases distortion.
The capacitor there should be on the order of a few hundred pF (uuF) to only bypass RF. BTW it works almost the same without that cap, the difference is little.

But the difference in sound quality is very noticeable.


As a post-note, the voltage across the plate-cathode of the detector in this and other schematics is shown (I think ) wrong. They show it in the renge of 40-60V in plate detectors of 20, 70 or 90, and the voltage applied to the load is the 250V or so. The problem with all this is the very principle of the plate detector which should be at the border of the tube not conducting, which means the plate voltage should be almost the same as the B+ and the whole voltage drops across the tube. If the voltage is 40V it means the tube is conducting close to 0.5mA.
Unless they mean RMS voltage and the volume is very high (still don't quite get how it can go that low).

And lastly this voltage is very tube-dependent. One tube goves you 209V where another will be 160V.