>Hal wrote:
>>
>>Some time ago, mention was made about using an MOV near the power
>>amplifier tube as protection against transient surges, perhaps in the
>>filament area.
>>
>>Looking at Mouser's MOV selection, the reader is presented with a
>>plethora of selections, each one having a 1MHz. capacitance listing.
>>
>>My question now for the reflector is would the inherent capacitance in
>>an MOV mounted near the final and connected to the wiring going into
>>the tube have an effect on the circuit because of the device's inherent
>>capacitance, or should a Metal Oxide Varistor not be used at all, with
>>a better choice being perhaps a gas-filled device?
>>
>At voltages below breakdown, MOVs look like capacitors of a few thousand
>pF so they can be used wherever a real capacitor would be OK. In
>practical terms, you can connect an MOV across almost any existing
>bypass capacitor.
>
>A good example is an MOV connected across the screen bypass capacitor of
>a tetrode, to protect that capacitor from voltage breakdown (which would
>probably cost you a whole new socket).
>
>Spark gaps are only a few pF, so they can be connected directly into
>most RF circuits, for example across the transmission line.
** For the grounded-screen AB1 Ugly amplifiers I built, MOVs popped
like popcorn during a HV to ground arc. The solution was a spark-gap
(not shown in diagram below) calibrated to arc at c. 500v more than the
screen potential (fil CT to ground potential) , paralled by some
ordinary, seriesed, 3A Si rectifiers with the total PIV selected to be
just above the normal screen-V. (Ds at http://www.somis.org/D.a.05.GIF)
In the event that something vile gets past the spark gap, the Ds diodes
short and limit the damage to only themselves. // note -- steering
diodes should probably also be used to keep reverse polarity current out
of the screen supply. /see diagram/
>
>Different devices, different uses. MOVs start to protect at only a
>little more than their maximum rated operating voltage. Spark gaps may
>protect better, but only when they've fired - and that requires a much
>larger impulse voltage than the rated operating voltage.
>
>
>--
>73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
> Editor, 'The VHF/UHF DX Book'
>http://www.ifwtech.co.uk/g3sek
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