[Amps] RE: 4CX250B Screen Supplies

[Will Matney <craxd1@ezwv.com>]

From Ian G3SEK,

"I'm jumping in here after a few days away, but all previous comments
have been read and noted too."

'Several people have pointed out that a two-tone test can give better IMD
results than you'd observe in a practical voice test. This has been
agreed many times over in the list archives."

-=True=-

"The real point is that even under these favorable conditions, a
zener-regulated supply (with a capacitor) *still* produces notably more
IMD than an electronically regulated constant voltage supply. This is
not surprising, because a well designed electronically regulated supply
will have a much lower output impedance than a string of zeners."

-=Fully understood and is the very reason I mentioned earlier that I 
would use a regulated supply myself=-

"The output impedance of a string of zeners is the Zzt value of the
individual diode (from the data sheet) multiplied by the number of
diodes in the string. For a typical 350V screen supply, many designers
use something like the 1N5369B (51V) in a string of 7 - but each diode
has a Zzt of 27 ohms, so that's 189 ohms for the string. You can get
this down to about 50 ohms using a string of 20 diodes at 15V each...
but that's about the lower limit for bare zeners. VR tubes fall
somewhere in between those extremes: better than a bad zener string, but
not as good as the best."

-=Correct, and was the reasoning behind me mentioning regulator tubes=-

"If you make the step to electronic regulation, the output impedance
drops straight down to a few ohms or even less. This is probably much
lower than most tubes really need, but there seems to be no middle
ground between "very low" with electronically regulated supplies and
"too high" with zeners or VR tubes."

-= Also true but the zener strings is what's being offered by most all 
the manufacturers. Thus the comment about most copying what they are 
doing. "Very low" is exactly what is needed and filtering will help at 
the get go.=-

"However, voltage stability is not the only reason for using electronic
regulation. The other two reasons - equally important - are that you can
include fast-acting trip protection for both the screen *and* the power
supply itself, without spoiling the voltage regulation."

"It's obviously true that electronic regulation with full protection is
more complex than a string of zeners. I'd love to find ways to do all
those things more simply - but I won't compromise on either performance
or protection."

-- 73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB) 
http://www.ifwtech.co.uk/g3sek

-=One thing that I should have explained earlier, and or is the way I 
feel about it, is that the problem should be nipped in the bud at the 
source. This being the filtering at the rectifiers. If the 120 Hz is 
filtered there effectively before the regulator, it should cut down on 
what the regulator sees and has to deal with. Filtering after the 
regulator should also better the 3rd order products. If the 120 Hz can't 
make it to the screen, it can't be mixed or "hetrodyned" by the tube. 
The incoming RF has enough frequencies than adding more by mixing in 120 
Hz. The regulator can do a lot but if it's fed a nasty supply voltage, 
some will get through, so I always look at the supply voltage first. 
Richard Measures mentioned using a resonant choke filter for the screen 
which I think will be a big plus and probably work. If not, it'll be 
darned close! I know it does well in cleaning up plate voltage.=-

Will Matney
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