There is no such thing as a perfect seal! Vacuum tubes (especially high-power transmitting tubes) not used for a few years might exhibit serious problems if put into service without a prior conditioning of the vacuum. With time, gas molecules leak inside and/or are released by the tube's internal components. With years and years of storage, the vacuum could deteriorate to a dangerous level and once the tube is used for the first time it could "flash-over" - the gas molecules inside will become ionized by the electron flow and this will create a flash of high-temperature plasma between the cathode and anode, damaging the grid(s) and other internal components. A chemical composition, called "getter" is factory deposited inside the tube to complete and maintain the quality of the vacuum (the getter is visible as the shiny, black-metallic area on the inside wall of the glass envelope (in smaller tubes)). In power tubes, the activation of the getter is done by heat. Therefore, it is recommended, before putting into service a power tube with very long on-the-shelf life (more than a couple of years) to condition the vacuum first. This is done by applying power to the filament (cathode heater) only and leaving it on for a period of time. The hot filament will heat up the getter and also will improve the vacuum by itself (gas molecules will react with the hot tungsten filament and the cathode surface, forming chemical bonds and effectively extracting them).
Here is a simple fixture I used to "getter" my newly acquired GU-74b tubes (NOS, manufactured in 1990-92).
The "chimney" is made by cutting the top portion of a plastic soda bottle ("Classic Seltzer Water" sold at Costco to be more specific :). The cooling fan is from computer power supply. The tube MUST (!) be cooled with forced air while being "gettered" or the high temperature will damage the metal-ceramic seals and destroy the tube. The fan is raised about an inch from the surface to allow for air intake. I slowly raised the filament voltage from 3v to 12.5V (12.6 is the nominal voltage) over a period of 5 hours in 5 steps (3v, 5v, 7.5v, 10v, 12.5v) using variable power supply. I, then left the tube running with the nominal filament voltage for about another 8 hours. The fan should run continuously, powered by a separate 12V supply. The current drawn by the heater is around 3.6A (maximum allowable is 3.9A). Absolute maximum voltage for the filament is 13.3V but it should never be reached! Recommended operational voltage for the heater is 12.6v and exceeding this voltage is not healthy for the tube. Measures must be taken to avoid short in the power leads (at the tube's pins) - best is to use proper tube socket but wire-wrapping with solid copper wire (AWG 18 or 16) and heat-shrink tubing insulation could work too.
At the end of the "gettering" procedure (after power is disconnected) is also a good time to conduct a few electrical checks for possible internal short between various tube components - using just a simple ohm-meter, while the tube is still hot (! be careful handling it to avoid burns - use gloves) and one more time when the tube completely cools down. Check for short between the heater and the Cathode, between Cathode and 1st Grid (G1), between G1 and G2 and finally G2 and Anode. While the Cathode is still hot, it is normal to see somewhat lower resistance between the Cathode and the grids or the anode - it is a vacuum tube after all and it will conduct current if electrons are emitted by the hot cathode. This resistance will gradually increase as the tube is cooling down.
After installation in the amplifier it is recommended to start using it at low power and low duty cycle (maybe just starting in SSB mode) and gradually increase the power output.