A couple of things you might try to help:
A filter of open cell foam sheet placed over the inlet of the blower will
reduce some of the noise, maybe a lot of it.
If there is a wall behind the amplifier you can place a sheet of thick foam
closed cell material on the wall behind the area where the blower inlet is.
This will keep some of the noise from reflecting off the wall.
You can place a temperature switch above the tubes where the air exits to
switch a capacitor or resistor in series with the blower motor to run it
slow until the heat rises then the switch shorts out the capacitor and runs
the blower full speed.
I did all of the above on an amplifier that I have that has a large blower
on it that was terribly noisy. Still get air noise from the outlet of the
tube but the inlet side of the blower seemed to cause the most noise.
If you have a smaller high speed blower then changing to a larger slower
speed blower may be quieter and still give the same required amount of air
and back pressure.
Another thing to try, place a large hose on the inlet of the blower and run
it down below the desk. Place a couple of bends (curves) in the hose. That
will reduce the noise.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Manfred Mornhinweg
> Sent: Monday, February 19, 2007 1:05 PM
> To: amps@contesting.com
> Subject: [Amps] Getting rid of blower noise
>
> Hi all,
>
> this time I have questions, rather than answers.
>
> My present amplifier is an ages-old National Radio NCL-2000. I
> refurbished it when I obtained it very cheaply a few years ago. Picking
> up another thread, it did have equalizing resistors across the filter
> capacitors that were severely out of spec (between 15 and 60%!), and a
> lot of other minor issues, mostly in the bias circuit. I fixed all that,
> replaced the non-original and very badly assembled rectifier by a new
> one (I simply put in strings of five 1N5408 in series, without resistors
> nor capacitors in parallel, as is usual practice with modern diodes),
> and since then the amplifier has been working very well indeed.
>
> But there is one problem, for which I have been tempted, more than a few
> times, to throw the beast out the window! This problem is noise. Loud,
> disturbing, permanent whirr from its blower. It upsets me so much that I
> use the amplifier rather rarely, for this sole reason!
>
> This amplifier uses a pair of 8122 tetrodes. They are good, quite clean,
> highly efficient, sensitive, but very small, and so they require a
> pretty stiff air flow. They get it from a centrifugal blower with steel
> impeller and induction motor.
>
> Can any list member suggest some way of quieting down this beast? Of
> course, I know, I could bore a hole through the wall and mount the
> blower outside the room. But that's not very practical, since I live in
> a third-floor appartment and the wall is made of thick concrete. Space
> restrictions also prevent me from mounting the whole amplifier in a
> noise-insulated cabinet.
>
> Are there any low noise blowers available? Ideally one that would fit in
> the same space of the original one? About 13 CFM at 1/2" water column of
> backpressure are required. So it's comparatively little air and rather a
> lot of pressure, compared to what larger tubes need.
>
> And the other question: Can I get away with rewiring the amp so that the
> blower stops during RX periods? Perhaps stopping with delayed action to
> cool down the tubes after each transmission? These tubes draw about 18
> watts of filament power each. Can I leave them without forced air during
> prolonged RX, or will the base heat up too much from the filament alone?
>
> Hoping for any good ideas,
> Manfred.
>
>
> ----------------------------
> Visit my hobby website!
> http://ludens.cl
> ----------------------------
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