[Hallicrafters] S38 series and an audio discharge resistor

[Kevin Croxen]

Rich:

Yes, I suppose that would be the easiest way to give the whole thing a good 
comparative test. Next time I get an hour to myself over the next couple of 
days, I'll pop the bottom back off the receiver and give it a try.

--Kevin

On Monday 14 July 2003 4:33 pm, Rich Oliver wrote:
> 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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