[Hallicrafters] HT-37 Bias Adjustments

[wb8jkr at juno.com]

  Might I make a suggestion?  In looking at the
schematic the HV DC output is fed to a choke input
filter, the output capacitors have a total voltage rating
of 1200 volts. The HV secondary on the power 
transformer is 880 volts each side of the center tap,
in order for a choke input filter to "act" like a choke
input filter there must be a certain MIMIMUM current
load on the supply to prevent the capacitors from 
charging to the peak transformer output voltage which
in this case assuming about a 40 volt drop for the
rectifier tube, about 880 X 1.414= 1204 volts
across the filter capacitors. Needless to say this
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



On Sun, 22 Aug 2004 10:00:40 -0400 "Al Parker" <anchor at ec.rr.com> writes:
> Hi Bill,
>     Well, that's the first measured indication that the later ones 
> are
> doing what they should.  Yes, change R-64 to a lower value to allow 
> you to
> adj the proper bias at abt half on the pot.  I have the same low 
> voltage
> probem in my HT-32, as I replaced the selenium and changed a 
> dropping
> res., but now also can't get much better than -44v.  I've gotta get 
> into
> it & fix that, and probably will make the mod. to get better stdby 
> bias.
>     I would have hoped that there would be more than 3v change.  If 
> it was
> easy to ck actual plate current it'd give us a better handle on 
> plate
> disipation.  Your actual temp readings will also tell the story.  
> We'll be
> interested in hearing what you find.
> 73,
> Al, W8UT
> ----- Original Message ----- 
> From: "Bill" <k2wh at optonline.net>
> To: "'halliqth'" <hallicrafters at mailman.qth.net>
> Sent: Saturday, August 21, 2004 11:34 PM
> Subject: [Hallicrafters] HT-37 Bias Adjustments


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