-----Original Message-----
From: measures <2@vc.net>
To: Ian White, G3SEK <g3sek@ifwtech.com>; AMPS <amps@contesting.com>
To: <amps@contesting.com>
Date: 14 April 2001 03:39
Subject: Re: [AMPS] Zener screen supplies
big snip
>>>
>>There's nothing inherently wrong with current limiting, or any other
>>design features. They can all be made to work.
>
>With a zener-string screen regulator, operated through a high resistance
>from the anode supply current limiting is automatic.
A clarification - my original comment about current limiting referred to the
sinking capacity of a shunt regulator, so that the regulator didn't have to
handle arc current, and the screen meter got protected.
Steve
>
>> The foolishness is in
>>*assuming* anything, when we should be *designing* everything!
>>Everything needs to be designed twice over, once for normal operation
>>and again for a range of fault conditions.
>>
>>Good flashover protection for the screen requires at least four separate
>>design features:
>>
>>1. B+ surge resistor to limit the maximum possible current. This gives
>>you a basis to design the rest of the protection features.
>>
>>2. Something to protect the screen itself (and the bypass capacitor)
>
>Why protect the screen in a bad tube?
>
>>against overvoltages when the flashover connects the screen to the
>>anode.
>>
>>3. Protection for the screen supply against (a) the overvoltage coming
>>back from the screen, and (b) any actions that screen protection circuit
>>takes (eg applying a short to ground).
>>
>>4. Shutdown circuits (fuses, breakers etc) to impose a cutoff on the
>>whole scenario.
>>
>>Note: this is AND logic! To prevent a microsecond arc from turning into
>>a soldering iron and checkbook job, the amplifier needs ALL of the
>>above.
>>
>>So maybe it isn't so simple, after all...
>
>Why would an arc stop after one microsecond? If the tube is gassy,the
>party is over.
>>
>cheers, Ian
>
>- Rich..., 805.386.3734, www.vcnet.com/measures.
>end
>
>
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