David Smith wrote:
>Rich,
>
>My data sheet shows 25 watts which works out to be a grid current of around
>380mA. This seems too high. I fear that it may not be that simple (mixing
>AC and DC characteristics) although my tube theory is fluffy at the best of
>times.
>
>Popular opinion from all who have responded to me (and thanks to you all) is
>a maximum of between 80 and 100mA.
>
Remember there's a difference between the maximum grid current you
would normally expect to run, and the maximum level you *could* run
without damaging the tube.
Depending on the speed of the trip circuit, it might make sense to set
the trip level about 50% above the maximum you'd normally expect to run.
That leaves the tube well protected, without causing too many nuisance
trips during tune-up.
The same question could also be asked about a suitable full-scale value
for the grid current meter. I generally make it so the normal reading is
about 2/3 of full scale, and then set the current trip at full scale.
BTW, all this can be done by pulling out the tube and using an external
power supply to force current through that part of the circuit. The idea
is to get all the metering and protection circuits calibrated and
working *before* the first time you light the tube,
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
New e-mail: g3sek@ifwtech.co.uk
New website: http://www.ifwtech.co.uk/g3sek
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