[Amps] Getting rid of blower noise

[Paul E. Cater]

Good day Manfred,

In a couple different amplifiers, I have gone to the hardware and 
purchased some rubber washers.  I place these on the mounting screws of 
the blower giving the assembly a little cushioning instead of direct 
metal-to-metal contact.  It calms down the vibration and some of the 
noise. 

Paul


Manfred Mornhinweg wrote:
> 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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