I've gotten some quotes on tubes. The best price so far on tested, guaranteed
medical-pull 8877's is $270 (yes, I've used the telephone). A pair of Eimac
3-500Z's, also medical pulls, is $150. Both types of tube are tested, full
output, supplied with curve-tracings and hi-pot tested. I haven't found any
cheap 3CX1200A7's yet (but I'm still looking).
The 3-500Z's get some points for being instant-on (I almost hate to bring this
up again) and $130 cheaper. The 8877 gets points for producing 1500 watts at
2300 volts, and for being compact.
I'm looking at the 3-500Z data sheet. The maximum rated plate current in any
kind of service appears to be 400 ma. If a pair is operated at 2500 volts and
800 ma, this gives 2KW input * 0.65 = 1300 watts. If we drive it hard enough
to get 925 ma, that gives 2310 watts * 0.65 = 1502 watts. That's still only 808
watts plate dissipation.
Am I missing something here? Would I be risking damage to the filaments from
drawing this much current? Would it require enough drive to exceed the grid
dissipation limit? I assume a 50% short-term duty cycle for CW (an electronic
switch will cut off the tubes completely when not keyed), and CW operators don't
care about IMD.
If it's possible to design the amp for full output at 2500v, the 3-500Z's look
very attractive.
One more question: may I get your opinions on cooling 3-500Z's? Is it better to
use chimneys and a pressurized chassis, or the 'sb-220 method' with a muffin fan
blowing across the tubes?
73,
Vic, K2VCO
Fresno CA
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