> 4. Re: Small transformer evaluation (Jerry Kaidor)
> 5. Re: Small transformer evaluation (Bill, W6WRT)
> 8. Re: Small transformer evaluation (Vic K2VCO)
>
> -----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.
+++++++++++++++
ANOTHER REPLY: I RESPECTFULLY DISAGREE WITH SOME OF VIC'S REMARKS. I use a
choke input filter in a screen grid supply, and like it. Vic is correct that
such filters can cause voltage swings with varying loads, but if the power
supply is sufficiently bled and is shunt-regulated, the choke won't see the
load variations and the voltage will stay constant. I like choke input filters
because if the choke is big enough (has "critical inductance"), it acts as a
passive regulator by itself. Once the current through the choke is high enough
to cause current flow through it for a full 360 degrees of conduction, the
output voltage does not change (except for the IR drop). Also, since the choke
in effect smoothes out the current flow pulses, it makes the transformer run
cooler. I admit that the choke will have to be fairly large for low values of
current, since the size of the choke that acts this way increases with
decreasing current through it.
>
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>
> I hear that screens don't make very good plates? Jerry Kaidor, KF6VB
>
REPLY: NOT FOR LONG, PARTICULARLY SCREEN GRIDS IN CERAMIC TUBES! Good screen
supplies, like Ian GM3SEK's, have protection circuitry to shut off the screen
power (should be FAST) if (when?) plate voltage fails or screen current, in
either direction, goes too high.
Gene May
WB8WKU
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