Tom in the link provided, you mention that 811s and 572s like 3-500s use a zirconium gettering material on the plate. Some time ago I was engaged in a debate regarding this very subject. I made the statement that running those three tubes with filament voltage and no anode caps would not be helpful for gettering the tubes. Did I put my foot in my mouth? The 'EE' I was speaking with was adamant that 811s and 572s should be operated with filament voltage only for a few hours.
You can hurt the filament if it runs many days without emission, not hours.
I see contradictory statements at RF Parts:
"The Svetlana 572B..... Temperature-initiated titanium "getter" material is permanently embedded in the surface of the rugged graphite anode for superior gas absorption. This improved "getter" system operates continuously AT HIGH TEMPERATURE, and is over six times more effective than flash getters silvered on tube's glass envelope." -http://www.rfparts.com/tubesvet.html
I guess from that Svetlana changed to titanium, which is about like zirconium. Very few high power transmitting tubes ever used flashed getters so that "six times more effective than flashed getters" is mostly marketing hyperbole. It still has to be very hot to getter the tube.
"Transmitting tubes generally require a "burn in" time to increase emission to its nominal value. In broadcast stations, 1-3 hours at rated filament voltage is allowed for burn in. This initial operation allows the GETTERS, materials which absorb and hold residual gas, to finish the vacuum of the tube in its final operating environment. In an Amateur Radio amplifier, tubes will sometimes require 50-200 hours of accumulated "key down" transmit time to remove the residual gas and condition the filament for full emission. It is well advised to allow tubes to operate in standby mode (filaments on, no TRANSMIT) for 2-3 hours before tune-up.
This is a practice that is rumored to help emission (NOT gas) but I've never seen it work. Not ever. We received a batch of low emission 3CX1200's once and Eimac had us cook the tube filaments at 9 volts or more (I can't remember exactly) for a half hour to an hour, and then run the filament at normal temps overnight. After that the emission was up. About 20 minutes later though, the emission was gone again. I had the same thing happen with 811's some time later.
This problem occurred because the filament surface is not properly "aged" after being manufactured or was manufactured wrong, so emission was low.
The 811A, 572B, 3-500Z, 4-400, etc. utilize a thoriated tungsten filament, which is highly dependent upon sufficient filament temperature to provide adequate electron emission for normal operation. Emission increases as the thorium migrates toward the surface of the filament wire. This is why it is recommended to "burn in" these tubes before operation." -
http://www.rfparts.com/tubeapp.htmlSo it actually appears they agree with what I have been saying, it is NOT a gettering process. It is a emission restoration process, and what they are saying is the tube filament was not cooked in properly at the factory and we need to finish the job for the manufacturer.
The typical manufacturing process would be to heat or flash the filament for just a few moments at a very hot 2500-2800K. After that the filament is aged at 2200K (this is what Eimac had us try) for some minutes to an hour. Then they had us run the filaments at normal voltage overnight (I'm not sure why, except the 3CX1200 being a ceramic external anode tube does getter from the filament heat, and maybe they thought the elevated filament temperatures might have released gas).
I was never successful in the long term restoring emission this way. Probably because I was only doing the later steps when the first step was not done properly.
This has nothing to do with gas or with the tube sitting unused. It is an emission issue.
Degassing of a 811 or 3-500 or 572 would occur only if the tubes is arced internally at very low arc currents, or has the anode heated at least to a very dull red color.
73 Tom
