[Amps] Getting rid of blower noise

[Manfred Mornhinweg]

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.


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