This technique (below) is used in a lot of air cooled cavity amplifiers,
esp of the broadcast FM style. The Eimac SK300A socket has too much
openness to offer effective isolation between input and output at 100
MHz. So they came out with the SK3xx series, including the 350, 360 I
believe. They had great low cathode to earth inductance, a lot more
shielding metal, but poor airflow. Modification was made by punching
holes in the solid walls, and voila, the SK355 resulted, excellent for
VHF and not too much air resistance. I think I got the number right,
this was long ago when I was designer of FM TX. This worked with the
4CX3500/5000/7500 sized tetrodes, for example.
By having the inner conductor of the cavity clamped directly to the
anode, and using the same as a duct to the outside (through either the
shorting plane above or through a dielectric shelf in a half-wave
design, pressurization can be applied anywhere below the anode and it
finds its way up the 'chimney'. Getting air onto the filament button on
the bottom of these tubes was tricky. Having a small exhaust pipe that
was directly below the button allowed air to escape this way out the
bottom, and cool the filament well. There are plenty of tricks like this
that work.
Using longer hoses adds air resistance to the flow, so the blower HP and
output must increase to get the same through the tube.
73
John
K5PRO
On 3/16/13 10:00 AM, amps-request@contesting.com wrote:
Another very effective method of cooling is to blow air directly into a
sealed anode compartment. Most of the air flows upward through the anode
cooler and is vented directly to the outside through a chimney ABOVE the
anode cooler. There is NO chimney between the base and the anode cooler.
Meanwhile 25-30% of the air flow is allowed to bleed downward through
the tube socket to cool the base seals. This method reduces the back
pressure on the blower, and allows it to deliver much more air than the
conventional base-upward layout. It has been used very successfully for
decades in VHF and UHF amps - so much so, it is regarded as "the normal
method".
73 from Ian GM3SEK
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