Improving the fidelity of AM reception audio

[wa2ise]

This is a set of mods to improve the fidelity of AM detectors commonly used in tube AM radios and the AM section of tube tuners.

We split off the AVC circuit from the audio detector. Because the filter cap in the AVC circuit will reverse bias the detector diode some. This will cause the high amplitude negative peaks of the audio (which causes very low RF valleys) to be clipped off. This rarely happened back in the old days, but with today's heavy audio processing by radio stations, it can be a significant source of distortion. Most AM radios use tubes like the 12AV6, which provide a pair of detector diodes. The second is usually not used and is tied to ground in most radios. Here we unground it and use it for the AVC circuit. A 100pF cap coupled to the plate of the last IF tube feeds the new diode. This also gives a benefit of keeping the gain down as we tune across a station, so we don't get blasted as much when slightly off tuned.

A second mod is to reduce the usually 200pF audio RF filter cap. Here we used 50pF. This avoids "tangent" distortion; the clipping of negative going peaks of higher frequency audio. Contrary to popular belief, most AM stations broadcast a full 10KHz of audio, for an RF bandwidth of 20KHz. Sure, station frequencies are spaced every 10KHz, but not in the same town.

Another mod is to reduce the heater voltage applied to the detector diode tube. Here in a series string AA5, we can do this by paralleling a 330 ohm resistor with the 12AV6 heater. This will cause the 12AV6 to see a heater voltage of 10V. This lower heat will produce less "contact potential" seen by the diodes. Less stray electrons boiled off the cathode. Less bias for the RF signal to overcome. Sure this also impacts the triode, but its self bias only drops from 1V to about 0.6V. Which is not serious. If this becomes a problem, one can "borrow" some negative bias off the 12BE6 local oscillator circuit by connecting a set of resistors totaling around 30 megs from grid 1 of the 12BE6 to the grid of the 12AV6. You probably won't need this.

If, in a tuner, the detector is a solid state diode, replace it with a 6AL5 or equivalent sub-mini tube. And use both diodes, one for audio and the other for the AVC. And run the heater on reduced voltage of around 4 to 5 volts.