[Amps] Tube Cooling..

[Ian White GM3SEK]

Carl wrote:
>
>Top down cooling is the norm in many coaxial cavities at VHF/UHF. My 
>Collins 432 MHz military amp with a 2.75KW dissipation tetrode runs 
>that way and it was used as an AM linear at 1000W out. A seperate small 
>blower cools the input side. Due to the TX/RX relay and the coax going 
>to the hardline and antennas I cant run it even close to its capabilities.
>
>I'll also be using that method at 1296 with a pair of  GI-7B's unless I 
>decide to go with water cooling.
>

Do you really mean "downward through the anode cooler", Carl?

A very common method for cooling VHF/UHF amps is to blow cold air into 
the enclosed anode cavity, so the only way out is upward through a 
chimney on top of the anode cooler. This method makes the best possible 
use of the blower's back-pressure capability [1] and blowing cold air 
into the cavity helps to minimize tuning drift (important at VHF/UHF).

The grid/cathode end of the tube needs to be cooled separately, either 
with its own small blower or by allowing 25-30% of the airflow to go 
downward through the base plate.


[1] There is a small spreadsheet on my website to estimate the 
capabilities of squirrel-cage (centrifugal) blowers:
<http://www.ifwtech.co.uk/g3sek/in-prac/blowers.xls>


-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek