Couple more WWII era oddball tubes which can be used with the Clem,
1614 which is essentially a metal envelope 6L6, with a "military" number.
1631 is nearly similar, but has a 12.6V, 0.45A filament and a slightly lower max plate dissipation rating (16W).
Both tubes have the same "7S" basing as a 6L6 / 6L6GC, so no board surgery would be required.
The 1631 would require using the voltage doubler (and a 12SL7 driver) or just use a 12.6VAC filament source and forgo the voltage doubler.
If you like going way out on a limb, the
6DZ7 could be used, it just requires two added jumper wires (1 to 5 and 3 to 6) on the V2 and V3 sockets:
The one "weird" characteristic of the 6DZ7, is that in paralleled operation, the grid resistor has to be significantly lower than useds with most power tubes, when I built my 6DZ7 PSE last year, I found the tubes to be unstable with 470K or 270K grid resistors, and worked the values downward, until it became unconditionally stable, and 150K was the highest value, and 100K oe 120K would probably be better.
The B+ should be approx 365VDC, and the cathode resistors, 200 ohm, 5W, for each 6DZ7.
The 6SL7 driver should still be good with that, just increase the coupling caps to 0.33 uF, from 0.1 or 0.22 uF. (this is described in my "Weekend 6GK6 Project" posting, in the last parts of the topic).
http://diytube.com/phpBB2/viewtopic.php?p=33067#33067
6DZ7s were designed for PP operation, and with the paralleled G2s and cathodes, for some reason, the control grids need the lower resistance value, in parallel operation. My SWAG on this phenomenon is tha the paralleled G2s and cathodes create a rather high G1 capacitance to the adjacent elements, when they are connected in parallel, as opposed to operating in PP, 180 degrees out of phase. ALso, the increased transconductance from paralleling the two sections, would be a major factor in dealing with instability issues. 100K grid resistor takes care of that nicely.
/ed B