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 ponúka nový ventilátor, ale jeho cena s poštovným je vysoká. Použitie axiálnych ventilátorov by si vyžiadalo veľké mechanické úpravy. V GM Electronic sú však dostupné turbíny s výfukom stranou, tzv. 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
SB-200 je potrebné najprv doplniť o zdroj 12V/300mA. 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 transformátor s usmerňovačom a filtračným kondenzátorom je možné umiestniť nad ne. Iná možnosť je umiestniť spínaný zdroj do priestoru subšasi vedľa turbíny.
Air tunnel
Zdroj je možné využiť aj na obvod soft-startu, resp. napájanie ďalších doplnkov v koncovom stupni. Vzduchový tunel je vlastne box do ktorého ústi vývod z turbíny. 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
Výhodné je doplniť ešte reguláciu otáčok turbíny. 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 bimetalový spínač. Kontakty spínača sa preklenú rezistorom takej hodnoty (cca.10ohm), aby sa turbína otáčala pomaly. Spínač sa pripevní na šasi. Pri zvýšení teploty spínač zopne a turbína sa roztočí naplno.
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.