You need to read Eimac document SK-300A Air System Socket. It will
give you the socket and chimney for the 4-400A among other tubes. You
need the SK-406 chimney. Lowering the socket a bit is okay but you
may need to file out the chassis holes some. I was the photo of the
tubes. Obviously chimneys were there once because the clips for
holding them are there. Get a pair of SK406s and put them over the
clips and reattach the anode heat sinks and you should be fine. You
need real SK-406s because they are shaped to force air on to the heat
sinks and are the right height. Running the tubes with only air on
the pins risks having the bulbs get too hot. It's possible for the
vacuum to suck in the glass if it gets soft. Tube glass is pyrex but
it can melt.
73
Rob
K5UJ
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