Several months ago, Tom Rauch observed that a protection circuit mod
for the older vintage amps like this one that use 3CX800A7 tubes
would be a very good idea, since a primary failure mode is exceeding
the 60 mA (per tube) max grid current rating. Has anyone designed
one? I have three of these amps, and would like to add such a mod
if I had a good design for it. The way I currently run them is to
never drive them higher than 30 mA for the two tubes combined, and
whatever the power output is is what it is.
Several concerns. First, the reset MUST be quick -- a pushbutton
that resets instantaneously, so that you don't lose Q's in a contest
or pileup. Second, I note that on SSB (altho I'm a CW man, I do work
SSB for contests), I often see flashes of overdrive LED when the
indicated grid current is only 20 mA for two tubes. This is with
heavily compressed audio (peak and average should not be widely
separated) and both meter and light calibrated per Ten Tec
instructions (DC voltage at a suitable point in the circuit).
73,
Jim K9YC
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