[Amps] Power-off tube cooling

[Ian White, G3SEK]

>
>Is not the junction where the metal pins travel through the ceramic
>base and/or glass of a power tube subject to a more rapid mean time 
>between failure rate because of thermal stress?
>
>Would not a power tube's internal metal elements use the pins as a heat 
>sink, whereby the ceramic  in the sealant area will see a concentration
>of heat from inside the tube be conducted down to a larger metal heat 
>sink, namely the tube socket and componentry?
>
>I know in semi-professional cinema projection the equipment requires 
>the lighting device forced air to remain on after the device is 
>extinguished, and this practice started with open-air carbon arc and 
>made its way through the 10KW tungsten multi-filament days into the 
>xenon-arc systems I see now.
>
>It would seem reasonable that the example tube of the former posting to 
>the reflector that consumed over one hundred amps to the heater would 
>be a mighty heat sink and the cooling of the pins might be just the 
>ticket to avoiding premature vacuum leaks around the pins.
>

Metal-ceramic seals will tolerate much more heat, temperature and 
thermal shock than metal-glass. That's one of the main reasons why 
ceramic is used in modern tubes - the other, related, reason being that 
ceramic bounces better than glass.

Eimac invented the air-system socket for the original glass-based 4X150 
and 4X250, because the glass/metal seals of the heater pins would crack 
unless continuously cooled. When the ceramic-based 4CX tubes were 
developed, that problem went away. Eimac continued to sell the existing, 
expensive air-system sockets for reasons that were purely economic, not 
technical.

That info was received direct from an Eimac old-timer, and it rings 
true: you'll notice that later ceramic-based tubes such as the 3CX800A 
and 8877 get along fine with wafer sockets that actually *prevent* the 
base seals from ever receiving any cooling airflow. In such cases, 
switching off air and heater power at the same time must surely be OK.


As others have said, whether or not there's a problem after switch-off 
will ultimately depend on the amount of heat stored by the hot parts 
inside the tube... in other words, on the mass of metal in the anode, 
and the temperature from which it's coming down.

But even if the tube itself can survive air and power being switched off 
at the same time, the stored heat will continue to spread out inside the 
unit, and may damage other components that are more 
temperature-sensitive. That is probably the main reason why a cine 
projector would need to be completely cooled down before switching off 
the fan.



-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek