[Hallicrafters] S38 series and an audio discharge resistor

[klcroxen at fas.harvard.edu]

In issue 168 (May, 2003) of _Electric Radio_, there is a brief article 
by Paul Thomson (W0OD) about the 1930's technique of using an 
audio discharge resistor to allow for full audio recovery and 
reduction of up to 50% in total harmonic distortion, a technique he 
says was neglected in the design of post-war radios, inter alia the 
S38 series (and the S85).

For the S38, he recommends 91K as the correct value, connected 
across C24c (i.e. the 220pf "c" section of the multi-section mica cap 
that runs from the black wire on the output side if the final IF 
transformer to the chassis ground and cathode of the 12SQ7). This 
resistance (91K) value he also lists as correct for the S85 in the 
same position (at the cathode of the 6SQ7). Other receivers need 
different values, and the editor of _ER_ also mentions that the idea 
can't work at all for other designs where the volume control forms 
part of the detector load.

Now as I had just recently finished recapping an S38A and it was 
still sitting on my workbench, I thought I'd give the idea a try, and 
put in a 100K resistor I happened to have on hand. It seems to work 
OK, with improved audio quality. But I'm not certain, since as part 
of the restoration process I also replaced the '60's vintage 3" 
Jensen speaker that had been stuck in there at some time with a 
new 4" Radio Shack 40-1421. So I might not be remembering quite 
accurately what the set sounded like pre- discharge resistor. 

Is this technique of adding/having  an audio discharge resistor for 
improved audio quality a well-known one? Or are my unscientific 
ears playing tricks on me, since I'm not really used to what an S38A 
"ought" to sound like?

Regards,

--Kevin Croxen