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[Amps] Use of 60Hz blowers on 50Hz mains for cooling ceramic triodes/tet

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Subject: [Amps] Use of 60Hz blowers on 50Hz mains for cooling ceramic triodes/tetrodes
From: (Michael J. Tubby B.Sc. (Hons) G8TIC)
Date: Tue, 13 Aug 2002 20:43:35 +0100

As I recently posted, my Commander-II amp went up in smoke
after an extended period operating WSJT meteor scatter during
Perseids, even though I was running it at under half max power.

I've now stripped it down to get a new transformer made, and
while I was at it I decided to take a look at the blower that cools
the 3CX800 (and the whole amp). The blower is made by Revcor
in the USA and is model Q30B (guess it might be 30 CFM looking
at its size and model number).

The thing is that it has a synchronous motor on it, and while the
Commander-II was strapped for 234VAC operation, the motor
is 110/115VAC and was across half the primary and operated in
auto-transformer mode - this is okay as far as obtaining the right
volts, but doesn't address my other concern...

If the motor is a synchonous type then its rotational speed is related
to the AC supply frequency. If the motor/blower operates as specified
on 60Hz in the US then when its moved to the UK/Europe it goes
only 5/6ths as fast (83.3%). Now if my recollection of physics is
correct then the volume of air moved will drop by the cube of the
drop in speed = 0.833 x 0.833 x 0.833 = 0.578 or in otherwords
57.8% of air (by volume) - this appears to be a significant drop in
cooling - and somewhat worrying if I am correct.

I've looked inside my friend's Tempo 2002A - it also has a 60Hz
motor and (for my opinion) isn't well cooled either - doesn't get
anywhere near Rich's requirement to nearly blow the tube out of
the socket :-) and is certainly much less cooling than what I used
to use on my homebrew apir of 4CX250Bs some 20-or-so years

Revcor appear to make 230/240VAC 50Hz versions of their
products for the UK/European market.

When supplying amps to the export market, do US manufactures
not fit an appopriate alternative blower?


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