Vacuum Tube - Cooling

Cooling

Like any electronic device, vacuum tubes produce heat while operating. This waste heat is one of the principal factors that affect tube life. In power amplifiers, the majority of this waste heat originates in the anode though screen grids may also require cooling. For example, the screen grid in an EL34 is cooled by two small radiators or "wings" near the top of the tube. A tube's heater (filament) also contributes to the total waste heat. A tube's data sheet will normally identify the maximum power that each element may safely dissipate.

The method of anode cooling is dependent on the construction of the tube itself. Tubes used in consumer equipment have internal anodes, so cooling occurs through black body radiation from the anode (plate) to the glass envelope; natural convection (air circulation) then removes the heat from the envelope. Tube shields that aided heat dispersal can be retrofitted on certain types of tube; they improve heat conduction from the surface of the tube to the shield itself by means of tens of copper tongues in contact with the glass tube, and have an opaque, black outside finish for improved heat radiation. The ability to remove heat may be further increased by forced-air cooling, and adding an external heat sink attached to the anode through the tube's enclosure. These measures are both implemented in the 4-1000A transmitting tube, whose anode was designed to operate while red hot, dissipating up to one kilowatt.

The amount of heat that may be removed from a tube with an internal anode is limited. Tubes with external anodes may be cooled using forced air, water, vapor, and multiphase. The 3CX10,000A7 is an example of a tube with an external anode cooled by forced air. Water, vapor, and multiphase cooling techniques all depend on the high specific heat and latent heat of water. The water-cooled 80 kg, 1.25 MW 8974 is among the largest commercial tubes available today.

In a water-cooled tube, the anode voltage appears directly on the cooling water surface, thus requiring the water to be an electrical insulator to prevent high voltage leakage through the cooling water to the radiator system. Water as usually supplied has ions which conduct electricity; deionized water, a good insulator, is required. Such systems usually have a built-in water-conductance monitor which will shut down the high-tension supply (often tens of kilovolts) if the conductance {measured in Mhos} becomes too high.