On Jul 11, 2005, at 3:16 PM, Ian White GM3SEK wrote:
> 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.
A tube that arcs from the anode to the screen is already a goner, Ian,
so there is nothing of value to protect. Common reasons for high
dissipation in the screen are loss of load, loss of anode supply, and
mistuning. In all such cases a fast-acting fuse suffices.
>
>> - 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.
Which is why the amount of screen bleeder current is chosen to be >>>
than the maximal reverse screen current according to the characteristic
curves.
>
>
Rich Measures, 805.386.3734, AG6K, www.somis.org
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