Paul Christensen wrote (regarding the Alpha PA-77):
>
>> What happens to Q2 in a major surge depends on the other things
>> connected to R20. If the circuit is at all conventional, R20 is the grid
>> current metering resistor, so there must be a meter connected across it,
>
>Yes. I failed to mention that the R20 sample point also routes over to
>the multi-meter circuit for grid I monitoring.
>
>> and hopefully also a protection diode (anode to ground). Then the
>> B-minus rail and anode current meter connect to the top of R3.
>
>Could be...I didn't look this far.
>
After downloading the PA-77 manual and schematic (from the excellent
"Boat Anchor Manuals" site, http://bama.sbc.edu) it seems that the
circuit is different from usual meter/bias circuit for a GG triode. In
the PA-77 the B-minus is connected to the *cathode* end of the bias
zener, so the bias zener and EBS handle only grid current, not total
cathode current.
I've no doubt that the Alpha arrangement is competent (yes, there is a
surge protection diode in the appropriate place) but its detailed
behavior under surge/arc and other fault conditions will be a little
different from the more common circuit.
>> Q2 *will* survive OK if the base
>> resistor is increased from 100 ohms to say 10k (and maybe change Q2 for
>> a device with higher current gain... I don't know the 2N5184).
>
>I agree...perhaps going from 100-ohms to 10K in order to increase input
>isolation and changing Q2 to a darlington-pair may be added
>insurance.
Don't use a Darlington - that would have twice the Vbe drop of a normal
bipolar, so the Ig trip point would be about twice as high. The 2N5184
only has a current gain of 10-50 (tnx Marv!) so there are many hotter
devices that could be tried. This may slightly lower the Ig trip point,
but the increased base resistance will tend to raise it, so you may not
see much change.
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
--
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