The grid-trip circuits I have used for years on 8877's
and others look at the voltage drop across a resistor
hung from the cathode to ground. This voltage goes through
a pot to set the desired trip current. The pot gives another
C. 1000 ohms or so "glitch" protection for the base of the
transistor that opens the PTT line by removing coil voltage
from the VOX/QSK relay. I have had many "big bangs" over
the years, but have never lost a grid-trip transistor. (knock on wood)
Perhaps just good luck, or over-engineering.
(((73)))
Phil, K5PC
> >For instance, typical grid-current
> >"protection" circuits for 8877s use a 1A to 2A transistor to interrupt
> >the flow of grid-current. However, during a parasite, grid-current goes
> >out of sight. An 8877 cathode is capable of >50A-peak emission. This
> >amount of current will C-E short a 2A transistor in short order. Thus,
> >there is essentially no grid protection during a major parasitic
> >oscillation.
>
> Please can you give an example or reference to such a deficient circuit?
> I've never seen one like you describe (but maybe I've led a sheltered
> life). Where exactly is this 2A transistor?
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