The British (and a few other armies') D11 and D13 transmitters cooled 4-125
(D11) and 4-250 (D13) by blowing air from the top. They suffered badly from
this socket problem. If you figure that Eimac didn't sell chimneys for 4-400s
because they weren't needed, you can reach the conclusion that they were meant
to be blown from below. Air goes through the holes in the ceramic sockets and
out through the holes in the metal base ring, cooling the filament (and other
pins) on the way, and then goes over the envelope and past the heat dissipating
plate connector before exhausting. Tubes like the European QY4 250 etc don't
have the metal base ring, and so are much harder to cool. As was said, one
answer is to use small fans cooling the socket side, while another is to
arrange cooling to come from underneath and use a chimney. I use a 4 inch
Muffin fan on each tube base of my 2 tube 4-250 amp, as well as an 8 inch fan
blowing straight down on the plate caps. Seems to work.
Peter W6/G3RZP
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