rich Wrote:
>// Agreed. But does it make sense to use inexpensive 20v - 30 V. 5W
>axial lead zeners which require no heatsink?
>
Depends what you want the zeners to achieve - spike protection or
something longer-term.
>>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
That implies you were drawing less than 10mA total, to supply the
screens for two tubes *and* bleed a keep-alive current down the zener
chain. With the voltage dropping out of regulation at every peak of
screen current, it sounds like a failsafe splatter generator.
>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?
>
BECAUSE IT DOESN'T HAVE *ANY* VOLTAGE REGULATION!!!
Half a century has gone by since that book was written. A lot of things
have been learned since then, but that dumb book hasn't learned a single
one of them.
Most of us try to learn a little more than a dead tree can.
>// 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.
>
Unfortunately those calculations are only valid for small values of
"linear".
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
--
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