[Hallicrafters] HT-37 Bias

[David C. Hallam]

I assume that the operation of the final amplifier in the HT-37 is the same
as the HT-32B, although I don't know that for sure.  The HT-32B should draw
about 50/60 mA at idle with proper setting of the bias.  My HT-32B has a
very simple modification, a capacitor and 2 resistors, which was installed
years ago by Robert Orwin W9YKA.  He called it a "Tube Saver".  He claimed
that it reduced the key up current to about 10 mA.  I have never measured
the current to confirm the effect of the mod, but I would believe he knew
what he was doing.

David C. Hallam KC2JD

-----Original Message-----
From: hallicrafters-bounces at mailman.qth.net
[mailto:hallicrafters-bounces at mailman.qth.net]On Behalf Of Bill
Sent: Wednesday, August 18, 2004 9:43 AM
To: hallicrafters at mailman.qth.net
Subject: [Hallicrafters] HT-37 Bias

Can someone tell me why the finals in the HT-37 are apparently run
100% of the time at full plate dissipation?  This condition occurs even
during
receive and standby periods.  The manual for the HT-37 under the BIAS
section,
states "the final amplifier tubes are operated close to rated plate
dissipation for optimum performance".

Does this mean all the time or just during transmit?  It would seem to
me
that a simple circuit mod to increase the bias to a higher level during
receive
or standby periods would allow these tubes to cool down.  Then again
Hallicrafters may want to tubes hot to allow the rig to stay warm and
stabilize  and also to stop heat cycling of the finals limiting
mechanical
stresses on the tubes.

Anyone have any ideas on why this is the way it is?  I'm inclined to add
a simple circuit to increase the BIAS on these tubes during receive or
standby to cut down on the heat.

K2WH

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