[Amps] RE: 4CX250B Screen Supplies

[R.Measures]

On Jul 22, 2004, at 2:10 PM, Will Matney wrote:

> 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.

Hello, Will --  As I see it, not all zener-strings are equal in 
performance.  A major problem with zeners is that the dissipation 
ratings are for a case temperature of 25ºC - which is unrealistic 
except outdoors in a blizzard.  Thus, unless the engineer reads All the 
specs and starts to actually think, the result is bound to stink.
end

>  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.

Since FETs are virtually immune to secondary breakdown, they are 
superior to Bi-polars in applications such as this.

> 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?

CRT monitors are being replaced with TFT and plasma panel-type monitors.

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