[Hallicrafters] S38 series and an audio discharge resistor

[Rich Oliver]

Kevin,

Perhaps you could bring the low end of the resistor out with a clip lead 
so you could easily connect or disconnect it for comparison.

-Rich, KC9GQ

klcroxen at fas.harvard.edu wrote:
> 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
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