[Radio Wrangler]

You'll find that transmitting valves have no getter. Because of the high voltages involved, the manufacturers didn't want the risk of vaporized metal being sprayed around and any getting onto any insulators.

So they were subjected to a diffusion pump to take them right down to a very low final pressure, and kept there for some time to allow outgassing to proceed before they were sealed off.

Receiving valves got brought down with a rotary pump, allowed to outgas, sealed and then the getter was fired and the exposed reactive metal brought the pressure down the rest of the way. The getter ring was made of channel-shaped high melting point metal and the channel filled with the reactive metal. The whole lot was then plated with non-reactive metal to seal it so it could be stored and handled in air.

Once fired in the sealed valve, the non-reactive plating (usually nickel) boiled off first, before releasing the reactive stuff. So looking ath the glass bulb of a finished valve, you see the nickel alloy against the glass. The reactive stuff forms the inner surface, flypaper for any itinerant gas molecules.

As G-J says, changes to getter loop shape and paint colour may well have happened at the same as other changes... changes which might matter.


David