That helps Terry. I guess I will follow pretty much the way you described it
except will try and put a chimney on the tubes. I plan on driving the crap
out of them! Naw! Just joking. I try to build and use to last.

Doug N4IJ

wrote in message
oups.com...
Doug, Here's what I remember from several RCA and Collins Broadcast
transmitters and a KW amp I built: Those ceramic sockets SHOULD have
holes in them of about 1/2" diameter. Line up a 4-400A with the socket.
Notice that the BASE of the 4-400A has holes that line up with the
socket holes. Notice that the 4-400A has a metal base which has a
series of holes around it. It's very important to put lots of air thru
those holes to flow around the tube base pins. If you're going to run
lots of dissipation (definite color on plate) then the usual approach
is to mount the socket just below the chassis on standoffs or about
3/16 to 1/4" as I recall. THe SIZE of the hole in the chassis then is
set to have about 3/16 to 1/4" of space around the outside of the metal
tube base, so there is additional airflow which will hopefully stay
fairly laminar around the upper part of the tube envelope. NOTE:
Normally that metal base ring is grounded by several metal clips.

SOME systems used the "Air System Chimney" (A pyrex glass open-topped
"bottle" around the tube) to direct the air around the tube envelope,
although 1000W OUT Broadcast transmitters from Collins and RCA did not
use them, and their high level modulators put out 750 watts of audio,
at, say, 50 percent efficiency, so there was about 375 watts of plate
dissipation per 4-400A at 100% sinewave modulation. Which at one time
was OK, because the actual duty cycle on "Typical Program Material" was
much lower. But about 1970, as I recall, watching the plate color
during "Magic Carpet Ride" by Steppenwolf, with an asymetrical limiter,
20 db of syllable-rate compression, plus the pre-processing of the
airplay cart by a demented Program Director, the plates looked a lot
like 100% modulation during an Audio Proof of Performance test :-)

Regards, Terry King ...On The Mediterranean in Carthage