[Amps] RE: 4CX250B Screen Supplies

[R.Measures]

On Jul 26, 2004, at 12:18 AM, Will Matney wrote:

> 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."
>
To me it is surprising since a 2-tone, 1000Hz-spaced test should 
consume constant current from a C-filtered PS whether it is regulated 
or not..

> -=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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>
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org