[Amps] RE: 4CX250B Screen Supplies

[Will Matney <craxd1@ezwv.com>]

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. 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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