[Amps] Blower noise

[Manfred Mornhinweg]

Mike,

> Slowing a blower can significantly reduce the back pressure. Be careful
> that you have enough pressure to get the required airflow for the rated
> anode dissipation.

I'm very well aware of that. I ordered a blower that has an impeller that's 
pretty much the same size of the original one, in a somewhat narrower case (the 
original has a case much wider than the impeller), and that runs at 4500 RPM at 
full voltage. The original at 60Hz must run at 3400 RPM or so. The spec sheet of 
the blower I ordered gives a flow/pressure curve that offers much more capacity 
than I need.

My intention is to install this blower, along with a power supply that switches 
its voltage between TX and RX, perhaps with a time delay after TX ends, so that 
in TX I get all the airflow these tubes need for full power, whatever noise that 
takes, while in RX the blower runs slowly enough to make almost no noise, 
whatever airflow results. This low airflow is probably still enough to keep the 
tubes cool during RX, since the filament power of these tubes is quite small. 
It's likely that they could even run without any forced air, while only the 
filaments are on.

I expect this to require speeds of about 3800 and 2200 RPM, respectively.

Yesterday I measured the actual pressure delivered by the original blower, 
installed in the amp: It's just a tad over 5mm of water column! That equates to 
pretty exactly 0.2 inches of water. According to these tube's datasheet, this 
would be barely enough for 270 watts dissipation per tube, at sea level, and 
slightly less at my altitude.

The amp's manual warns that at 50Hz the cooling is impaired, and the amp should 
only be used at reduced power (one more reason to change the blower!). But I 
didn't expect the pressure to be that low! For full power rating of these tubes, 
more than twice this pressure is required, along with the correspondingly higher 
flow rate. And I strongly doubt that the difference between 50 and 60Hz can make 
this blower deliver more than twice the pressure! Instead I would expect both 
the speed and pressure to be essentially proportional to frequency. From which 
it follows that this blower is probably a bit weak for its task, even at 60Hz! 
It has plenty of airflow capability, but not enough pressure for these tubes.

Does anybody have the original specs of the NCL-2000's blower? I tried googling 
them by brand and model number, but didn't find anything.

Manfred


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