After a little diversion, let's get back on track looking at the inside of valves, today we'll look at the ANODE.
A valve anode typically is shaped as a hollow open topped cylinder, which surrounds a central cathode and any other electrodes. Although the cylindrical type is most commonly encountered due to it’s efficiency in both electron capture and heat dissipation, oval or flat pates may also be used.
The anode’s purpose is to receive the electrons emitted from the cathode. This happens because the anode has a positive applied potential, which attracts the negatively charged electrons. Indeed, often the number of electrons drawn from the cathode are in part governed by the applied anode potential.
In small valves, the anode is made of nickel or nickel-plated steel. The power represented by anode current x anode voltage is under a no-signal condition converted into heat as electrode impingement of the anode occurs.
In a power tetrode such as the KT66 operating at no-signal conditions with an anode voltage of 250V and an anode current of 100mA, the power dissipation at the anode is 25W. When you consider that this wattage would boil a tablespoonful of water in just 45 seconds, you get some idea of just how much heat needs to be dissipated for if it isn’t, the anode temperature will progressively rise until the valve is destroyed.
There are a number of heat dissipation methods used, the most commonly seen being anode carbonization in which an anode is first keyed by sand blasting and then sprayed with a colloidal carbon and cellulose admixture, the increased surface area and black body radiation allow enhanced heat dissipation. Alternative heat dissipation methods include making the anode from a wire mesh, fitting the anode with radiant fins or even manufacturing the anode from a less thermally labile and resistive material such as molybdenum, carbon, tantalum or zirconium. In output valves used specifically for transmitting, the anode may form part of the external envelope of the valve, which may then be readily cooled by thermal conduction to an external heat sink or by a circulating-water jacket or an air blast.
In the photo below, you see the deft fingers of Fanny Pincher using a spot welder at the Mullard Blackburn Valve Assembly Department to attach the anode of an EL37 by spot welding: -