Peter Frenning, OZ1PIF wrote:
>I really doubt that the excellent Gi7b(t) tube will do what Tom is
>stating, I know the tube well and I'm using it in a 144MHz amp. It is,
>as Tom states, a very robust and abuse tolerant tube, but this is
>really pushing it beyond whats credible.
>?
>Specs say 350W, Ft over 1GHz, and 2.5KV in class C. It is a Russian
>military tube built like a tank, and indeed designed for use in tank
>radars. The tube will take a lot af abuse without apparent short term
>ill effect.
>?
>I run a single tube at 2.1KV and 400mA class AB giving out some 425W on
>144MHz when running SSB or CW, and 300W+ when running WSJT (100% output
>, 30s on 30s off), using just an 18w muffin fan for cooling!
Part of the problem with using a GI7 as a 3CX400 substitute in an
existing amp is the massive sand-cast anode cooler, which takes up a lot
of space and is really not very efficient - it only works because it's
so big.
The rest of the tube is also quite a lot taller than the 3CX400, both
above and below the chassis, so I don't know how it might fit into an
Alpha 76 and 78. Then there are the different DC conditions and load
impedances to think about...
However, one thing that might help would be to remove the anode cooler
from the GI7b, and graft on the Eimac anode cooler from one of those
dead 3CX400s.
The cooler of the GI7b simply unscrews, leaving a flat external anode
surface with a threaded post in the middle. The cooler of a dead ceramic
tube can be removed by squeezing the grid ring hard in a vice to crack
the ceramic, and then repeating from a different angle until it cracks
all around. (This is much less likely to damage the cooler than hitting
the tube with a hammer!) Holding the anode cooler in the lathe chuck,
turn off the flared metal flange with broken bits of ceramic attached,
and there you have a nice compact air-cooled heat exchanger that is good
for 400 watts.
I've tried this with a dead 4CX250 (squeeze the screen ring) and it
really is quite easy. The inside of the cooler was the original anode
surface of the vacuum tube, and is a plain round cylinder... or at
least, it was for the 4CX250; some tubes have slots in the anode surface
to act as secondary electron traps, so the rest of this idea wouldn't
work for those.
My idea - which I never got around to completing - was to make a
"top-hat" shaped slug of copper which would be a very close fit inside
the cooler, and had a flat base to bear down on the anode surface of the
GI7. A hole right through the center of the slug would be needed to take
the threaded post and screw the whole thing down firmly.
If this slug was turned to a very good surface finish, and made to be a
shrink fit inside the Eimac cooler (cold slug slid into heated cooler)
it would ensure good thermal contact. In normal use, the slug would
always be the hotter part and would expand to an even tighter fit.
I hope this description makes sense, particularly to anyone who has ever
seen a GI7b or cracked open an Eimac tube. It was only an idea, but it
might be worth trying...
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|