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
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