Care of the rare beasts.

The part most vulnerable isn’t the glass envelope but the filamentary cathode (filament) of the battery or direct cathode valve and secondarily the alloy or oxide coating of the filament or of the the cathode. The next most critical aspect is the heater of the indirect cathode.

The glass can break and it’s important to clean corrosion from pins or wires and avoid bending them close to the seal. With time a large CRT may gain some helium which can be a problem. No glass / metal seal is perfect and even the glass or metal can is can be porous. The remaining getter is eventually used up. The metal electrodes and glass can slowly release gas, but good news is that the heat of operation may cause certain electrode metals to absorb gases and improve the vacuum.

If the voltages are such to have too much cathode current (where “too much” can be a function also of  cathode or filamentary cathode temperature) then the oxide coating is damaged.  To  a certain extent Thoriated Cathodes can be “regenerated” by an appropriate temperature of filament (an exact over voltage) but without drawing current (or very little). The potential to reactivate an oxide coated filament or cathode is much poorer, but also can be achieved to increase emission of  an apparently dead tube.

A filament or heater can of course  just fail, open circuit, like a light bulb. This is due to two mechanisms, one is evaporation of the metal into the vacuum with eventual failure and the other is the filament or heater fracturing. Tungsten is extremely brittle, not helped either by drawing it into wires. Nickel is much less common but was also used in 1.4V “all dry” battery valves (tubes). Platinum was also used for filaments but very rarely and only in some early types.

What voltage really?

Tubes (valve) heaters or filaments are described as 2V, 4V, 5V, 1.4V, 1.2V 6.3V etc. But what is the ideal voltage and tolerance for acceptable performance and longest life?