[Amps] RE: 4CX250B Screen Supplies

[Will Matney <craxd1@ezwv.com>]

Rich,
Yes, you are correct. I myself would build it the same as you. The main 
reason for showing these other ways was because that's how all the 
manufacturers used to do it and some still do, (diode strings)! I was 
looking over some schemes used by Alpha, QRO, and a few other 
manufacturers who are using tetrodes and seen they are still using zener 
strings. I agree that this does nothing for the IMD and all would like 
this cleaner (no splatter) but that's what the customer is getting. 
However, if someone wants to homebrew an amp with limited resources, 
they're most likely going to copy one of these circuits that's being 
used or was used in the past. Myself, I would use a shunt regulator with 
semiconductors, either transistors or FET's. If I recall, the tetrode 
board that Ian White builds uses a FET for the regulation which is as 
close to a tube as you can get using a semiconductor. Ian, correct me if 
I'm wrong on this. Actually, some amps screens are running off of 
unregulated supplies which cause problems. I myself was brought up 
around tubes which I count a blessing as most don't even teach the 
subject anymore. This is a shame because anything in broadcasting with 
any power uses vacuum tubes. Even industrial machinery uses diodes, 
ignitions, etc yet to this day. I was told that the military, even 
though they use tubes in their transmitters, do not tech tube theory! 
So, whats the new technician going to do when he is called upon to 
repair a final PA? What about the radar scopes and the CRT's all over 
the place? Anyhow, I guess the old saying still goes, "There is more 
than one way to skin a cat"........

Will Matney


Hello, Will -- The potentiometer method works well AB1 grid bias 
circuits because there is virtually zero grid current.  However, since 
there can easily be over 100mA of screen current in AB1, the 
potentiometer method causes screen unregulation which =s IMD.  One 
work-around is to use different value of zeners in the zener-string so 
that a finer adjustment of screen V results.  For example, the 4cx250R 
requires c. 400V for the screen  12, seriesed 30V zeners would produce 
360V.  By putting 6, switch selected 10V zeners in series with the 12, 
30V zeners, the screen-V could be set to 360, 370, 380, 390, 400, 410, 
and 420V.
end


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