[AMPS] Airflow

Ian White, G3SEK G3SEK@ifwtech.demon.co.uk
Sat, 23 May 1998 09:18:41 +0100

Steve Thompson wrote:
>Is it permissible to blow air through 3CX800 in the reverse direction to
>'normal', i.e. in at the top and out at the bottom of the anode?
>There's obviously an effect on seal temperatures, but if these are kept
>within acceptable limits, are there any other potential problems?

The only major problem I can think of is the difficulty of getting the
hot air out, without heating the whole box, the output tuned circuit and
the base of the tube in particular.

The cooling of the anode seal would be poorer, because it sees hot exit
air rather than cool inlet air. There's no guessing how much extra air
(or derating) you'd need to compensate for that.

A related question about 3CX800s: how come it's OK to use a wafer socket
that prevents air from contacting the base seals, while 4CX250/350/400s
have this fancy "open-plan" airflow socket?

And a wider question: how come all the HF amplifier builders blow their
tubes from underneath while the VHF/UHF builders have a strong
preference for blowing from the anode side (aka K2RIW-style)?

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)

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