Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx> At 03:09 PM 3/25/2006, Tesla list wrote:
Original poster: Mddeming@xxxxxxx Hi Henry, I agree that the cooler a tube runs, the longer it will last.
This might not be true. Most cathodes are designed to run at a fairly narrow range of temperatures for optimum life. The filament voltage and current are specified to keep it at that temperature under normal operating conditions. If the tube is designed to run glowing, heat radiates both to the outside and to the cathode, so less filament power is needed to keep the cathode at the "right" temperature. (heat transfer would go as proportional to: Thot^4-Tcold^4)
If the plate is running cooler than normal, then the cathode might be running cooler (because it's losing more heat by radiation to the plate), which would change the emission.
There are cathodes (and I can't recall whether it's thoriated or non-thoriated just off hand.. it's been a couple years since I was worrying about perveances and emissions) that really don't like being too cool.
Jim