[Hallicrafters] Re: HT-3X Bias Adjustments

[wb8jkr at juno.com]

 If one wanted to modify the bias circuit to allow
normal AB1 operation in transmit with about -49 volts
of operating bias and have the tubes placed in
cutoff during receive or standby, I would replace 
HV bleeders R67 & R68 with 10K ohm 25 watt
units, that way the bleeders should provide enough
minimum current draw for the filter choke to do its job.
 The disadvantage of course is then you'll have about
50 watts of heat dissipated under the chassis.

Mark  WB8JKR


On Sun, 22 Aug 2004 10:36:31 -0500 "Mark Shaum" <k9tr at dtnspeed.net>
writes:
> 
> ----- Original Message -----
> From: <wb8jkr at juno.com>
> Subject Bias Adjustments
> 
> 
> > exceeds the rating of the capacitors. I suggest
> > that Hallicrafters designed the 6146's to draw a
> > certain amount of minimum current to provide the
> > required minimum current load for the choke input
> > supply. Getting rid of this minimum current may cool
> > off the tubes a bit, but may cause you a problem in the
> > HV supply filter.
> >
> > Mark  WB8JKR
> >
> 
> Exactly!  In their infinite wisdom Hallicrafters chose to allow the 
> pair
> of 6146's to act as B+ bleeder resistors.  With my line voltage 
> here, my
> stock HT-32B had over 900 volts on the plates of the tubes when in
> standby mode.  I have a surplus solid state HV relay in mine which
> places a 20ma load on the B+ when the rig is NOT in transmit by
> switching a resistor in parallel with the existing B+ bleeder.  It 
> is
> switched out during transmit so as not to add 20ma more to the draw 
> on
> the B+ while transmitting.  Resting B+ at idle is now about 800 
> volts.
> 
> Pros are less B+ on the filter caps and tubes most of the time.  
> Both
> may last longer.
> 
> Cons are two, a bit more heat is generated and you have to modify 
> the
> factory designed circuitry in  a classic rig.
> 
> Mark, K9TR


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