[Amps] tube cooling

[Ian White, G3SEK]

Mark Beckwith wrote:
>Vic, you're the second person to make this observation, so I am taking
>seriously the thought that MY alpha may be the odd one.  I am checking into
>a few things.
>
>My Alpha has the "auxiliary fan" mounted outboard behind the transformer.
>This could be part of the problem.

Ahhh... that makes more sense now. The auxiliary fan is a flat 
Muffin-style computer fan, and these can give a very wide range of noise 
outputs. Find out what type it is, and then check a good component 
catalog that actually gives noise outputs, to see if there is an 
equivalent type that could easily be 10-20dBA quieter.

There are various sources of noise in an air-cooled amp. Bearing and 
blade noise from the blower and/or auxiliary fan are things you can work 
on - especially with the Muffin fans which are a commodity item with 
standard sizes and some range of choice. The more modern quiet fans have 
better bearings and more rounded blade profiles. You can also often 
reduce the transmission of low-frequency rumble through the metalwork, 
and prevent the cabinet from acting as a sounding-board.

The one noise you can't reduce much is the rushing sound of turbulent 
air... well, you can reduce it, but only at the expense of cooling 
efficiency shortened tube life and generally reduced reliability.

Turbulence is vital for effective cooling. If the air flow over a hot 
surface is gentle (laminar) there is always a stagnant boundary layer of 
hot air which effectively insulates the surface from the bulk of cool 
air flowing past it. Turbulence scrubs away the boundary layer, so it 
greatly increases the heat transfer.

The key point about the "Blow or suck?" discussion is that turbulence 
only exists *downstream* from the blower or fan blades - so there is a 
real difference.

In desktop amps with the Alpha-type through flow cooling system, the 
blower is buried deep inside (which helps reduce noise) and it delivers 
highly turbulent air direct to the tubes. However, the inward air flow 
from the rear of the amp is much more gentle, almost laminar, so the 
power supply components don't enjoy the same effective cooling. The 
transformer is the hottest single power supply component, and most 
manufacturers place it right at the air intake so at least it has the 
coolest possible air... but the airflow over the transformer is still 
almost laminar, and there can be stagnant corners in the power supply 
compartment where air hardly circulates at all.

Contrary to popular belief, the auxiliary fan at the air intake doesn't 
greatly increase the air flow rate through the whole amplifier. It works 
almost entirely by making the incoming air highly turbulent. It's the 
turbulence that gives the dramatic improvement in cooling for the 
transformer and other power supply components.

However, when adding an auxiliary fan, you do have to be careful that 
it's big enough to keep up with the existing blower, or else it could 
actually be choking the air flow and making the cooling worse! The most 
reliable test (tnx K6GT) is to time how long it takes to inflate a 
lightweight plastic bag covering the air outlet over the tube. If it 
takes the same or less time with the auxiliary fan than it did before, 
then your amp is in good shape.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)

http://www.ifwtech.co.uk/g3sek