A resistor from screen pin to ground will take care of the negative current
by supplying a constant bleed. Calculate what is needed based upon the
actual negative current without the regulator in place. If you have
sufficient PS overhead you dont have to be exact.
Carl
KM1H
----- Original Message -----
From: "Vic K2VCO" <k2vco.vic@gmail.com>
To: <amps@contesting.com>
Sent: Saturday, September 01, 2012 11:39 PM
Subject: Re: [Amps] Small transformer evaluation
Interesting idea. Nominal screen current is 35 mA and voltage is 325.
Current goes negative in zero-signal state so the regulator has to deal
with that. I want to be able to vary screen voltage for experimentation.
Lower voltage gives better IMD if someone wants to operate SSB, higher
will give more output on CW.
On 9/1/2012 7:11 PM, Bill, W6WRT wrote:
ORIGINAL MESSAGE:
On Sat, 1 Sep 2012 17:55:48 -0700, Jim wrote:
Why not just lift the negative end of the HV bleeder with zenner diodes?
If
the plate voltage goes away, so does the screen voltage. I hear that
screens
don't make very good plates?
73
Jim W7RY
REPLY:
That will work, but it requires some very high power bleeder resistors.
For
example, if the HV is 3 kV and your screen requirement is 400 V at 80 mA,
the
total power dissipated is 208 watts in the resistors plus the 32 watts
for the
zener. It's just that much more load on the plate transformer.
It's certainly an alternative but that's a lot of HV to throw away. As
for
protecting the screen when HV is lost, I'd include a simple HV detector
that
shuts down the screen supply when HV is lost.
73, Bill W6WRT
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Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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