11-05-2021, 01:25 PM
The sensitivity on my restored 20 varies dramatically across the dial, being least sensitive at the low end. Since I had to rewind the primaries on both coils, I thought maybe it was a problem there. However, that rewound portion of the coils is in the plate circuit and not really tuned. Hooking up the spectrums analyzer to check the antenna coil and the 2 intermediate coils yielded a flat response across the dial. This was with the radio off, the sweep hooked to the primary and the analyzer input hooked to the grid wire with isolation resistance.
Turning the radio on yields a very different result. With the volume on full, sweep hooked to the antenna, and the analyzer input hooked to the grid of the plate detector, the signal level drops about 5db from 1500 kc to 550 kc. With the volume turned quite a way down, signal drops 14db from 1500 to 550. Quite dramatic.
Much of the volume control on these radios is via a change in bias on the 2 24 tubes (the volume control changes the cathode voltage while the grid is at 0 volts.) Grid bias is at -2.9 volts at full volume and -11 volts at minimum volume.
So the question to you tube gurus is: Are the #24 tubes less sensitive to lower frequencies when approaching cut off bias? If so, then there is potentially no solution to the lower sensitivity without restructuring the entire volume control system. I would rather keep it original. I may try changing the 2 tubes to see if there is any difference.
Turning the radio on yields a very different result. With the volume on full, sweep hooked to the antenna, and the analyzer input hooked to the grid of the plate detector, the signal level drops about 5db from 1500 kc to 550 kc. With the volume turned quite a way down, signal drops 14db from 1500 to 550. Quite dramatic.
Much of the volume control on these radios is via a change in bias on the 2 24 tubes (the volume control changes the cathode voltage while the grid is at 0 volts.) Grid bias is at -2.9 volts at full volume and -11 volts at minimum volume.
So the question to you tube gurus is: Are the #24 tubes less sensitive to lower frequencies when approaching cut off bias? If so, then there is potentially no solution to the lower sensitivity without restructuring the entire volume control system. I would rather keep it original. I may try changing the 2 tubes to see if there is any difference.
