On Mon, 31 Dec 2007 11:26:02 -0500, jsternmd wrote:
>What would typical grid current be for an Alpha 87A running 1500w
Excessive grid current is the major failure mode of 3CX800A7 tubes.
60 mA per tube is the absolute maximum. I religiously tune my Titan
425's (two 3CX800A7's) so that the grid current for two tubes
combined doesn't exceed 30 mA. I can nearly always get at least
1200W out this way. Occasionally as I QSY during a contest, or kick
it up to legal power (only one more dB), I may hit 60 mA. When you
run them this way, tubes last a LONG time. And even then, if
something breaks, you'll hit a LOT of grid current.
Occasionally I join a multi-op at N6RO. He mostly runs Alphas and
Commanders, both of which have grid protection circuits that kick
out when the limit is exceeded. Ken runs flat out legal all the
time, and they do occasionally kick out, especially if you QSY
without retuning the amp. I have no idea as to the age of his tubes,
and he's switching between multi-element antennas with matching
networks but mostly without tuners.
73,
Jim K9YC
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