I'm using an older Heathkit SB-200 power amp. This forty-year-old PA is equipped with two 572b glass triodes and works really well. However, years of operation have resulted in increased fan noise. It is a 120V motor with a metal propeller that swirls the air in the space of the tubes. Not only the noise, but also the vibrations caused by the fan are unpleasant. In addition, this method of air flow is not the most advantageous - the air mainly swirls in the space of the tubes. You can find an excellently written article on tube power stage cooling at www.ok1kvk.cz
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Harbach Electronics commercial solution
Harbach Electronics offers a new fan, but its price including shipping is high. The use of axial fans would require major mechanical modifications. In GM Electronic, however, turbines with the exhaust aside are available, the so-called blower SUNON. This allows for sub-chassis mounting along with the air tunnel. This will ensure the flow of air through the holes in the chassis into the space of the tubes and then the outflow through the top cover. As the air does not swirl, but surrounds the tubes, the cooling efficiency increases.
Additional source 12V
The SB-200 must first be supplemented with a 12V/300mA power source. A suitable space will be created after replacing the old electrolytic capacitors in the anode source. New types of capacitors have smaller dimensions. Small network transformer with a rectifier and filter capacitor can be placed above them. Another option is to place the switch source in the subchassis space next to the turbine.
Air tunnel
The source can also be used for the soft-start circuit, or powering other accessories in the final stage. The air tunnel is actually a box into which the outlet from the turbine opens. The turbine blows air into the box. An increase in pressure causes the air to be forced out in the only possible direction – into the space of the tubes. The box must therefore ensure a tight connection with the turbine and chassis so that air cannot escape elsewhere.
In amateur conditions, it is probably easiest to solder a cuprextite box. It is then screwed onto the chassis. A layer of silicone was applied to the contact surfaces. Silicone also seals the passage of the turbine - air tunnel.
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Turbine speed regulation
It is advantageous to add turbine speed regulation. It is unnecessary for the turbine to run at full speed in the idle state (unlocked, just glowing). It is possible to use various continuous electronic regulators with comparators. Another solution is bimetal switch. The contacts of the switch are bridged with a resistor of such a value (approx. 10 ohms) that the turbine rotates slowly. The switch is attached to the chassis. When the temperature rises, the switch closes and the turbine spins at full speed.
The efficiency of cooling in the described way is noticeably higher. In the case of PA with ceramic tubes, the use of a turbine is even a must.