[Amps] Small transformer evaluation

[Jim W7RY]

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



-----Original Message----- 
From: Bill, W6WRT
Sent: Saturday, September 01, 2012 1:55 PM
To: AMPS
Subject: Re: [Amps] Small transformer evaluation

ORIGINAL MESSAGE:
On Sat, 01 Sep 2012 13:37:10 -0700, Vic wrote:

>Some of you will remember that I was wondering about utilizing a small 
>Philco power
>transformer (model 45-7011-7) for a screen supply. It puts out 380-0-380 
>vac plus 5 and 6
>v for filaments. I have hooked it up jn a full-wave ct circuit feeding a 
>choke-input
>filter and 20 uf capacitor. It is feeding a 4300 ohm 50w resistor for a 
>load.

REPLY:
You should not use a choke input filter for a screen supply except for 
constant
carrier modes such as FM or RTTY. When the screen current varies as with SSB 
and
CW, the varying current through the choke will cause voltage spikes and 
slumps.
You can easily see this with an oscilloscope. The screen voltage should 
always
be constant.

A simple capacitor-only filter is best. And it will solve your low-voltage
issue.

73, Bill W6WRT
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