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?

Doug N4IJ


I built a linear using two 4-400's. I used the Johnson ceramic
sockets (square type) that did have some holes which lined up
with the ones in the bottom of the tubes. I mounted the sockets
under the chassis FLUSH with the bottom of the chassis. I pressurized
the chassis with a 4" boxer type fan mounted under the tube sockets.
This fan was one of the high speed types that could move lots of air.
I also mounted a pair of the same type of fans next to the
bottles on the top of the chassis, blowing air from outside
into the chassis past the tubes. The air flow went through the base holes
and around the glass bottle. This is similar to the way the
SB220 linear was build, as with many HB amps. The tubes did not
suffer any heat effects, while the pair ran near the legal limit.

BTW, 4-400's work ok in GG, but they like more HV than their triode
counterparts. While a pair of 3-500's work well with 2500v, the 4-400
likes more like 3300-3500v to get the same output and IM figures.
So your tank circuit will have more "L" and less "C".