[Amps] Coupling a blower to an air system socket

[John Lyles]

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 at 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