In the early 80’s I started experimenting with better AM detectors in many receivers including Collins R-388, 51J3, TMC GPR-90, and Drake R4B and C line. In the tube receivers I used a 6C4 or half a 12AU7 low Mu triode as an infinite impedance cathode follower. Schematics are every where including the ARRL handbooks. The lower distortion with; weak or strong signals and modulation over 90% and or very dense processing was more than satisfying to the ear at the volume control. Yes modify from the volume control to the speaker adding at least 12 Db of negative feedback !
Next I was guided to improve the carrier filter. Usually I would see a L or Pi section filter following the half or full wave diode detector. Yuck, yes the carrier was gone but so are the syllabic brilliant audio highs. The cure was a simple series tuned notch carrier filter. I used a sub-miniature 470 uH choke (about the size of a 1 watt resistor) in series with a tiny silver mica cap from cathode to ground. In the Drakes I used a infinite impedance J-Fet source follower detector with carrier filter from source to ground. Select the cathode or source resistor to accommodate all signal levels and dense negative going modulation. I did not have to add negative bias to the cathode or source for reasonable results.
In the tube receivers I also used a Cathode follower AGC detector which easily makes any tube grid leaky RF and IF amps or leaky caps toe the new low impedance AGC line. I used an excellent circuit from a Collins modification article and improved it using a fast attack/slow release circuit. With no pumping on SSB apparent one could also easily copy an AM signal and hardly loose any words with a strong local thunderstorm with its fast recovery.
With the home brew solid state synthesized receiver project I used mostly all Phillips IC's and their gilbert cell mixers/demodulators (SA602A family) worked well but my favorite listening was with tube receivers and the 6C4 Detector!
73 John, K1DEU
