[AMPS] Shunt regulated high voltage psu

[rich]

>
>> Using a string of zeners to bias a shunt transistor is not necessarily
>> a good way to improve voltage regulation. Typically the diode current
>> is no more than required to supply base current to the transistor (a
>> few milliamps) but at low currents the dynamic resistance of the
>> zeners increases dramatically. The improvement in regulation from
>> adding the transistor may be largely lost by having reduced the diode
>> current.
>
>Why would that be true, if you run nearly constant current through 
>the diodes???
> 
>> Also, a voltage regulator such as a string of zeners ("amplified" or
>> not) makes it very difficult to protect the amplifier against screen
>> current going outside the normal range, due to overdrive, incorrect
>> loading or a range of other causes. This may or may not damage the
>> tube, but it will damage signal quality and it needs to be detected.
>> But any kind of current sensor that you put between the zener chain
>> and the screen grid will add to the dynamic resistance of the screen
>> supply, and spoil the voltage regulation. The main reason why I'm so
>> keen on active regulators is that you can put the current sensor
>> *inside* the feedback loop, so it has no effect on voltage regulation.
>
>If the source voltage is at least twice the screen operating voltage, 
>the maximum possible screen dissipation is equal to the standing 
>dissipation in the shunt regulator.
>
>What could be more simple?

Bingo

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
end


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