>
>Tom Rauch wrote:
>>> There are ways to design around this problem, but the only reliable
>>> way is to use a higher-rated device and/or a lower rail voltage.
>>
>>.......or to clamp the shunt side of the series resistor with a large
>>zener with a capacitor across. You can use a zener clamp rated
>>below the maximum voltage breakdown of the shunt device, and
>>above the highest desired regulated voltage.
>>
>It doesn't make sense to spend money on a high-power, high-voltage zener
>(don't forget the heatsink) when you can uprate the shunt MOSFET for
>much less cost.
>
// Agreed. But does it make sense to use inexpensive 20v - 30 V. 5W
axial lead zeners which require no heatsink?
>The only exception would be if you insist on running the screen from the
>B+ supply, and wasting a large amount of power in a 100W high-voltage
>resistor.
// Powering the 400V screen regulator from the +1200V mid-point of the
+2400V FWD anode supply in my 2x 4cx250R linear amplifier consumed a bit
under 10w total. This scheme provided failsafe screen protection in case
of unpleasant surprises such as:
1. Loss of anode V.
2. Loss of load.
-- Without a failsafe means of screen protection, either condition can
be fatal.
>
>>This actually is a good idea in ANY case, because the shunt diode
>>with protect the screen system if the main regulator goes on
>>vacation.
>
>The protection isn't as good as it seems at first sight. Because of
>component tolerances, the zener voltage has to be several tens of volts
>above the correct screen voltage, so the screen dissipation can easily
>be exceeded if the shunt regulator goes open-circuit. It's also a fault
>that could quite easily be missed in the excitement of a contest or a
>pileup.
// So why not use the simple, foolproof, shunt screen-regulator that
Eimac recommended for 4cx250s and 4cx1000s back in 1955?
> Unfortunately an overheated screen grid often leads to a
>permanent increase in negative screen current, which may then make the
>tube impossible to operate at full power.
>
// Which is why I use a simple shunt zener regulator for low screen V
tubes. When and if a zener fails, it shorts - thereby safely decreasing
screen V. However for >800 screen V tubes, a pass-type regulator (with a
25mA shunt bleeder and screen PS transformer primary fuse) is clearly a
better engineering choice. (see Figure 8 on my Web site).
- 1N5357 5W, 20V, DO-5 case zeners currently cost 80-cents US each at
Digi-Key. 20 would be needed for a 4cx250R supply - 18 for a 4cx250B
supply. The safe current carrying ability is c. 85mA - which enough for
8, 4cx250s in linear service.
>After over 10 years experience of being forced to run small tetrodes on
>the razor's edge of screen dissipation (as the only cost-effective way
>to get UHF power in Europe, before the Russian tubes came in) I've found
>that the only truly effective form of screen protection is a current
>monitor, completely independent of the voltage regulator, that
>switches-off the screen in milliseconds if there's a fault. It means you
>can forget the amp and concentrate on the operating.
>
cheers, Ian
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
end
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