Like many amplifiers, the Alpha 76A (and variants), does not have a grid
trip circuit. Years ago I installed build a board that did a number of
tasks in addition to a grid trip, including SWR trip, peak reading and a few
other things. However, it was plagued with rf so I ripped it out. One of
the difficulties with the normal simple grid trip circuit is that the grid
metering is above ground (by something like 5.6 volts).
The real reason I wanted grid trip was because I might have the multimeter
switched to rf power output and because both the rf output meter and the
grid trip meter have the normal analog meter delay.
There is a fine grid trip circuit on the internet for the 76A, however I
took a different path and it works fine.
FIrst, I installed a peak reading wattmeter circuit in the rf output
circuit. It runs on 12 volts, but I installed a small 12 volt regulator
running off the 28 volt supply.
Secondly, I installed an led array between the two panel meters. No holes
need be drilled. This array is driven by an LM 3914 led driver, which is
configured as a peak reading meter, which monitors the grid current. This
is a simple circuit. Because the circuit is above ground, it requires a
separate supply. That was easy. I found a small wallwart and opened it up,
then mounted it to an aluminum bracked attached to the back panel. So, no
matter what position the multimeter is switched to, I can monitor peak grid
current visually. Both will insure that I am not overdriving the amp.
Both circuits work fine.
73, Colin K7FM
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