[Amps] small transformer evaluation AND POWER SUPPLY FOR SCREEN GRIDS

Gene May gene-may at hotmail.com
Tue Sep 4 20:45:18 EDT 2012




 


> 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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