Kevin Dooks wrote:

Sorry, I should have given more info. This home-brew CA
( Canoe Anchor ) covers 40 thru 10M, CW and phone and runs 400 ish
volts on the plate. There are space limitations, height including
envelope and base con't exceed 3 inches max and envelope diameter
can't exceed 1.65 inches due to cabinet restrictions.

Hi,

If you can't submount the octal socket, even a 5881 won't fit.
Have you considered a pair of 5763 9 pin miniature
transmitting tubes in the final? Each has 12W max plate dissipation,
so a pair would be rated for the same power as a 6L6.
They are small, so not as forgiving of an overload, though.

An 8121 tube would fit easily, but it requires a 12V heater,
and the sockets can be expensive. It's finned for a transverse
blower, but at 400V class C, it might not need a blower.
I've bought 8121's on ebay for around $10, a bargain
compared to the pricey 8122, which is the same tube
with a bigger plate dissipation rating (and which requires
through-the-socket cooling).

If you are willing to pressurize the chassis, a 4X150A
or 4CX250B would fit fine, but your blower might be as large
as the rest of the rig.

73,
Ed Knobloch