The AM Forum

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  :)

Hi John ... I missed the previous discussion on this thread ... a R4B followed me home from Ten Tec hamfest and I was looking for discussions ... how do you think about this posting ?  ... is it still your preferred detector for this rx ? ... any other mods to look at ...thanks ... John WB4BFS beefus 

Good stuff John,

I have done quite a few hot-rod mods on the SP-400 like a new HFO design, New RF Amplifier and a Pullen Mixer and a product detector and regulation on the oscillators.

But the AGC, AM Detector and Noise limiter sections are still stock, weak or broke.

I do know that the AGC needs help because it has a leak of some kind and the gain is too low at equalibrium with no signal! She will not open up so I have been using it in MGC in CW. The 6N7 Noise limiter is just plain broke... The true half wave diode 6H6 AM detector seems to work fairly nicely with good sounding demodulation.

Mike WU2D

well how about that, Mike ! ... a BC779 made by Howard also followed me home...I am interested in what you have done to your Sp400 ... I looked over the schizmatic and they sound very similar ...its going to be a while before I can start on this one but it looks like a good candidate for a hi-fi am rx...73...John

Definately. I would say it's not a candidate, but a well proven performer and the best hi-fi RX for AM, in my opinion. Have fun with it.

The K2CU detector is much cleaner.

K2cu det ... not familiar with this one ...can you please point out a source ?  thanks .. 73 ... John

http://www.amwindow.org/tech/htm/alowdisdet.htm

The superpro has a 6J5 triode voltage amp into a 6F6 wired as a triode driver with a huge interstage transformer into a pair of 6F6's also hooked up as triodes in Class A Push Pull into a huge output transformer. That is a seriously oversized and inefficient behemoth of an amplifier - 25 Watts in just to get a few Watts out. That is what the audiophiles call a sweet audio system!

Only pure competitive specmanship or simple vanity could produce such a circuit for a communications receiver. The Hallicrafters SX-28 was sporting a push pull 6V6 output so...

Mike WU2D

thanks Mike, Steve and others .... looks like an up and coming project ...what fun !  73  John