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Old 30th Nov 2017, 2:56 pm   #1
Argus25's Avatar
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 1,723
Default AM Detectors again

I have seen some impressive and clever AM detector and AGC circuits in the past, but I think this one takes the cake for simplicity and genius.

I have attached a circuit of one of the most remarkable radios ever made, the Philco model 38-7 from 1937.

There are a number of things that make this radio special. It has an astonishing dial configuration where the tuning knob rotates around the outside of the retro-illuminated dial. A push in outer knob for fine tuning and the central tuning knob can lock to miniature cones, this mechanism was called "cone centric" tuning. Photo attached of one of these radios.

The way the cabinet wraps together is very artistic & clever.

This radio contained very interesting circuit innovations. One was the single turn loop around the 1st IF transformer (but not the topic of this thread I mentioned this on another thread) the other though was the ingenious AM detector & AGC using a 6J5 triode.

I had a very long discussion about this circuit with John Stokes many years ago. It wasn't a common idea, but we both agreed it was very clever.

So since there has been a lot of talk about AM detector circuits on the forum lately I though I'd post the circuit again to draw attention to it.

As you can see the IF signal is directly coupled to the grid and capacitively coupled to the plate of the 6J5 which is tied via a 1M resistor to a small negative voltage taken from the resistor that derives the negative fixed bias voltage for the audio output stage. The AGC voltage is taken from the plate of the 6J5.

For small signals, the grid-cathode of the 6J5 conducts as a diode and normal detector function is assured with no initial AGC action. When the peak level of the IF signal increases beyond the plate's negative bias, plate current flows on positive peaks charging the 110pF capacitor pushing the average plate voltage negative and thereby increasing the magnitude of the negative AGC voltage. Very good use of the 6J5.

It may well be one of the world's first examples of a delayed AGC in a radio based on one triode.
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