On Sun, 5 Mar 2006, DOUGLAS SNOWDEN wrote:

I have seen 3-500Z tubes mounted in the johnson ceramic sockets. The Heath
SB-220 uses them.
Will the 4-400 tube handle the airflow the same way? Usually you see the air
system sockets used so that the air will flow up through the bottom. Will
the filament connections remain solid in a 275 ceramic socket with a muffin
fan blowing across the tube?
? I have seen the sockets raised a bit so that the filaments get cooling.
Just wondering if the 4-400 would work that way as well? Any experience
with this?

My recollection from Eimac tube spec sheets is that there must be
forced-air directed upwards from the non-tube side of the tube socket and
there are specifications for flow rate and pressure. Both the 3-500 and
4-400 (100 watts less plate dissipation) call for 5 v at 15 amps (IIRC) on
the filament and that is a lot of heat. I don't know if the 3-500 is
pushed closer to the limits on this but Eimac made a special socket (and
chimney) for the 4-400 but many ARRL handbook amplifiers went for a much
simpler socket. And, the Johnson Thunderbolt (I had two of them at one
time) used a simple ceramic socket plus an under-chassis phonograph motor
type of fan to blow air up from the bottom as well as an above chassis fan
motor to blow more air around the pair of 4-400s for a 1 KW DC plate input
level (with dull red cherry glow on cw carrier and a little less glow on
SSB voice).

Beyond that, if you are thinking of running 4-400s instead of the 3-500z,
you also need to worry about bias voltage a little and you would have to
resolder the socket pins because the 3-500 is a triode and the 4-400 is a
tetrode (need to ground the screen, as well as control grid). But I have
run 4-400s in GG in a homebrew amp in the past.

Doug N4IJ