Thanks for all the interesting replies on this. I have had a quick mess
with a variac. Motor starts at about 70 volts and speed seems to
stabilise about 90 to 95 volts. It's a 110 volt motor so I only went up
to 130 but saw no obvious increase. Definite variation in the 70 to 90
volt range though. This is great for slowing the motor down but I need
more speed not less so frequency change seems the only solution. a 60+
Hz oscillator driving a big transformer push pull should do it but
apparently the motor causes a big sag that stops it from starting, hmm?
Seems like a replacement motor is the only sensible answer after all.
Anybody know of a direct replacement that would fit the Alpha 78 and
produce more than the very weak draft I get at the moment?
Actually I'm starting to think I might be better off with moving on both
78's and the Henry, together with various other parts I have there is
probably enough for a serious amp fund. Makes me want to ask another,
unrelated, kinda, question. What would you buy? I've always thought the
Acom 2000 was about ideal for my 'perceived' needs. I did just go in
search of them but not even sure if they still exist or if the 2000 is
still made. Their web site is a disaster.
Cheers
Martin
Martin Sole wrote:
> For some time now I have been thinking about ways to improve the airflow
> through the tubes in my amplifiers. I have a total of four amps, a 30L1,
> two Alpha 78's and a Henry 2K classic. In all cases I am most
> unimpressed by the amount of air being moved and I consider all of them
> to be essentially noise free so there is room to move on that front.
>
> Two factors that seem to contribute to my lack of happiness in this
> matter might be, a) our AC supply is 50Hz so the motors are running at
> 20% less speed than they would on 60Hz (all the motors are marked
> 50/60Hz), and b) the ambient room temperature is about 32-34 degrees
> celsius.
>
> Use of the amplifiers for anything more than a few minutes with
> processed SSB at typical short contact duty cycles, but non contest,
> results in exhaust air which is too hot to hold ones hand over for more
> than a second or two. In standby with the amp on there is enough heat
> from the tubes to warm the airflow. What I desire is to have not more
> than ambient air temperature during standby.
>
> The Collins and the Alphas use simple shaded pole motors, I'm not sure
> about the Henry, it is a Dayton blower. My understanding of these is
> that frequency is the variable that controls speed. How can I vary the
> frequency and drive these motors at say 60 or more Hz? I have seen
> shaded pole motor speed controls for sale and they seem simple enough
> given their price but what is inside them? Can I homebrew something to try?
>
> I don't really want to go the route of extra external fans if it can be
> avoided.
>
> Martin, HS0ZED
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