On Thu, 18 Mar 1999 21:43:53 -0800 Vic Rosenthal <rakefet@rakefet.com>
writes:
>
>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).
Too bad I tossed that phone #.
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).
Whats Ameritron getting for new ones?
>
>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.
The 8877 can get along fine with a five blade muffin fan at that power.
>
>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?
More power requires more voltage.
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.
The tube will only deliver so much power before it goes into P1dB
compression. Excessive drive will just heat up the grids.
>
>If it's possible to design the amp for full output at 2500v, the
>3-500Z's look
>very attractive.
So whats the difference between 1300W and 1500W?
>
>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?
The 220 method works well as long as you direct some air at the socket
pins. The Harbach fan makes this an attractive ( cheap) option to
chimneys. You could also use a muffin fan if you are careful in the
selection and positioning.
73 Carl KM1H
>
>73,
>Vic, K2VCO
>Fresno CA
>
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>
>
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