[Amps] Alpha 87A Tubes 3CX800A7 and the 3CX P 800A7

[Jim Brown]

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