wrote:
>
>On Jul 11, 2005, at 11:21 AM, Ian White GM3SEK wrote:
>
>> R wrote:
>>
>>>
>>> I have never had a case where the 250v-rated 3AG fuse in the primary
>>> of the screen supply xfmr failed to open when needed to protect the
>>> screen
>>
>> And the mains fuse protected the screen from what..
>
>... exceeding the 275w max diss. rating during a glitch.
>
>> . and how?
>
>... by stopping the flow of current into the screen PS.
>
If the "glitch" is an arc from anode to screen, the mains fuse won't
protect the screen at all.
>- note - Reverse screen-I is handled by a screen-cathode bleeder R
>which draws c. 30mA.
Both series and shunt screen regulators have good and bad points, but
it's important to think exactly what would happen if a range of
different things go wrong.
If the reverse screen current exceeds 30mA, the series regulator cuts
off and then the screen voltage starts to rise... which increases the
reverse screen current... which incr - BANG! In this fault scenario the
mains current for the screen supply goes down, not up, so a mains fuse
would be no help at all.
There is a copy of a QEX article on my website which includes a range of
published screen circuits that either don't ever work properly, or could
damage the tube if anything goes wrong:
http://www.ifwtech.co.uk/g3sek/boards/tetrode/qex-article.pdf
--
73 from Ian G/GM3SEK
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