[Hallicrafters] HT-37 Bias

[David C. Hallam]

Hi,

I went to the garage, dug around in an old file cabinet, and found the
documentation that W9YKA sent with the change to the HT32B bias circuit.

He changed R168 from a 3.9k to a 4.7K, changed C194 from a 40uf to a 10uf,
and added a 8.2K resistor from the junction of R167/R168 to contact K on
RY101.

That's not bad.  After 20+ years and 6 or 8 moves, I could still find the
paper work.  Or maybe it is bad, a sign that I never throw anything way and
have too much junk.

73

David  KC2JD

-----Original Message-----
From: Al Parker [mailto:anchor at ec.rr.com]
Sent: Thursday, August 19, 2004 1:23 PM
To: dhallam at rapidsys.com; Hallicrafters List; k2wh at optonline.net
Cc: ab0zz at arrl.net
Subject: Re: [Hallicrafters] HT-37 Bias

Hi folks,
    Well I finally ckd the schematics, and did a measurement in my HT-32.
There is a difference in ckts in the bias supply from the HT-32 to
the -37.
    In the HT-37, a relay contact changes the bottom end of the voltage
divider in the bias supply to put 47k across the 18k resistor during
transmit, thus lowering the bias voltage then.  Conversely stated, the
HT-37 the switches the bias voltage to a higher value when in standby,
thus reducing, maybe to cut-off,  plate current.
    The HT-32 mk I  and HT-32A skems do not have this feature.  I ckd my
HT-32, which gets used weekly, and the bias does rise about a volt when
not in transmit, but that's just a function of current draw I expect, it's
not enough to change the plate current very much.
    It wouldn't be hard to use a set of the relay contacts in the -32's to
provide the -37's feature, maybe that's what W9YKA's  "Tube Saver" ckt
does.  David, next time you are into your HT-32B under the chassis, maybe
you could let us know what you find.
    Bill, K2WH, and Bill, AB0ZZ, how about measuring your HT-37 bias
voltage in both situations, MOX and standby?
    "inquiring minds need to know"  ;-)
73 all,
Al, W8UT