[Amps] RE: 4CX250B Screen Supplies

[R.Measures]

On Jul 21, 2004, at 10:44 AM, Will Matney wrote:

> Yes, true the regulation is not near as good as some newer developed 
> methods but, that was the way it was done for years at least shown 
> from 1980 and back. Using 1/3 the rating on a zener is correct as they 
> tend to fail easily. The potentiometer was mentioned because most 
> customers I've seen wanted a linear variable voltage control 
> (especially bias circuits), but a stepped control with a rotary switch 
> will do a similar function, if using a long zener string or multiple 
> regulator tubes. The potentiometer method keeps down the number of 
> parts and the cost of the circuit.

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

> Actually the potentiometer can be placed on either side of the tube or 
> zener with its wiper connected to the screen lead. This way if the 
> wiper fails, the screen voltage most likely will be lower instead of 
> the full voltage being applied. Then if it does go high, the varistor 
> will take over if sized correctly. Another zener could be used for 
> this as a protective device. We used to use them across the C-E 
> junction on 12 volt HV power supply transistors to snub any spike from 
> the oscillating transformer primary. They were set to 80% of the VCE. 
> No matter what a quick acting fuse should be placed in the screen line 
> too. The rule being, the less amount of components the better as one 
> will sure fail. Heck, I've even seen some that got by on using a 
> string of neon bulbs, but I don't recommend it! For some home brewers 
> this is a must who have limited funds to achieve the same result, or 
> when rebuilding tube type equipment.
>
> One last way to be looked at would be a series pass transistor with 
> zeners and pot on its base to ground, or simply a couple of zeners. 
> This was a basic circuit for control voltage but can be upped in size 
> for screen control also. The transistor should be able to handle the 
> unregulated voltage and at least twice the current. At 300-350 Vdc, 
> these transistors are available everywhere. Some televisions used 
> these as the horizontal output transistors, etc.. The screen circuit 
> still needs the protective devices between the supply and the screen 
> itself with any regulation circuit.
>
> All in all, there are a multitude of ways to accomplish similar tasks 
> and all should be investigated. I'm a firm believer that in life, we 
> get to picky and complicated in our designs.
>
> Will Matney
>
>
> This is guaranteed to spoil dynamic regulation, both with zeners and 
> with gas regulators.  The caveat with zeners is, except in Antartica, 
> don't run them at more than 1/3 of their dissipation rating.  //  The 
> easiest way to make a zener string adjustable is to make the tap 
> movable.
>
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org