Mel's Web Pages .................................................

Stage / estimated time	Comments (red = afterthoughts - latest 14/06/08)
Preparation 10 minutes	Wash & clean crt bowl Measure & note position of present 'gun - grid / cathode
Allowing in air 20 minutes	A line was scored around the 'tube about 4 cm (1.5") above the base. This was done on the vertical lathe. The glass knives we used were obtained in boxes of about 500 from a local pharmacist (used for opening glass medical phials). We then cracked the glass with a hot wire implemented on tongs from a low voltage source (can't remember what but I have since used a car battery and hot wire to cut the neck of a large pub type charity savings bottle). The cut should be reasonably neat as a new neck extension - about 8cm (3") has to be welded on. Though cracked, the 'tube does not implode or allow gas in due to the vacuum. We then put PVC tape around the cut, gave the end of the 'tube a 'knuckle tap' to crack the cut open. Then peel the tape off SLOWLY until air could be heard 'hissing' SLOWLY into the 'tube. Allow about 15 / 20 minutes before removing the tape and old 'gun assembly.
Removing old electron gun 2 minutes
Extending crt neck 10 minutes	The vertical lathe was used to weld a neck extension of about 8cm (3") onto the 'tube. A rubber 'bung' was inserted into one end of the neck extension. The rubber bung had a flexible rubber tube fitted (glass tube inserts in each end of the rubber tube - one to fit permanently into the bung and the other to be used as a 'mouth piece' - see below. Pre-heat - original & new extension rotating and about 3mm (1/8") apart - to avoid cracking the glass. Then Oxygen heat to form the weld while moving the lower glass extension up to weld with the original 'tube neck'. During this process the rubber tube is 'flopping around' below the bottom chuck. The joint glass wall is now slightly thicker so the bottom extension is drawn back slightly while blowing into the rotating rubber tube to avoid a 'waist' forming. Originally this was done by practice but you can use a carbon block (a large motor / generator carbon brush) held against the rotating 'tube so that the glass wall is blown to the original dimensions. The process is a little tricky at first but - like driving a car - it is relatively easy process with practice.
Fitting new electron gun 20 minutes	The new, (or repaired), electron gun - complete with glass foot and pumping stem - is inserted just below, (vertical lathe) the original position (measured in the preparation stage). About 3mm (1/8") is allowed for the welding process. The bottom chuck (previously holding the glass neck extension with rubber tube passing through) now has a metal 'mandrel' with a central metal rod to locate into the 'gun pumping stem. After pre-heating, an oxygen flame is used to melt the neck just above the 'gun button. The excess length gives weight to allow the excess glass to fall and form a 'waist' onto the glass button. The gun assembly is then lifted about 3mm (1/8") to thicken the weld and form a neat rounded finish.

Pumping stage 1 20 minutes	0il based backing pump - necessary to prevent oil vapour in the diffusion pump process being sucked out ! The backing pump is then turned off and the diffusion pump turned on.
Pumping stage 2 During outgassing	Silicon oil or mercury diffusion pump completed the final vacuum.
Outgassing 2 hours ? Metal cone tubes - particularly rectangular (Mullard MW 43-43 ?) took much longer !	I think the temperature was about 300 degrees C but this IS A COMPLETE GUESS. Occasionally we forgot and, at higher temperatures, the glass softens and is drawn into a 4 vane 'star' shape by the vacuum (or rather by the external air pressure). I remember completing the aging / new base process and picture testing to observe the weird shaped picture ! Prior to seal off the 'gun assembly (not the getters) is heated to a cherry red to drive gas out of the metal. A supply is connected to the crt heaters and taken to well over operating voltage (say from 6.3V to about 10V ???), to drive gasses out of the heater assembly.
Sealing pumping stem 5 minutes	A gas jet can be used but, in our case, the pumping stem passed through a small circular heating element just above the rubber compression seal on the vacuum pumping system. Before turning off the diffusion pump (stage 2) the heating element is turned on and the glass pumping stem 'draws into' the vacuum to form the seal.
Annealing	The ovens were gradually lifted to allow the glass to anneal (cool slowly).
'Firing' Getters	AFTER seal off the getters are heated by an RF heater to open the getter tubes and deposit barium onto the inner wall of the 'tube neck. The getter formed should be a nice silver colour indicating a good vacuum. This will absorb any gas thus extending the 'tubes life. Note that the getters should be heated carefully and should not literally be 'fired' as sometimes happened - the getter tubes then burn open circuit and metal particles can be deposited internally.

Fitting the 'tube base cap	Now off the pumping system and on the bench: Earlier 'tubes had a valve type base which required soldering. Later 'tubes only required a plastic cover over the pins

Ageing	The heater supply is gradually increased in stages in order to reduce the cathode coating from an oxide to a metal (or vice versa - I am not a chemist and can't recall the exact process). Supplies can then be added to the anodes and current measured, (I seem to remember), these supplies are probably not essential.

Picture testing	We used an old TV set up as a test rig - crt base leads extended, eht lead extended and scan coil leads extended - the scan coail were mounted on the bench below the shelf mounted test rig (a cut down TV cabinet).

Further Information
We, (Isherwoods), used a vertical lathe, the chucks having fibre jaws but you MAY no doubt have to utilize a conventional (horizontal) lathe. Obviously gas jets & controls would be required. We used coal gas & oxygen. A large, lagged, oven would be required to outgas the tube while pumping is in progress. RF heating to outgas the 'gun and 'fire' the getters.
Electron Guns	We obtained new electron guns from Mullard, Mazda, Superior Electronics and Sylvania Thorne. Mullard supplied us with heater / cathode assemblies but fitting them was tedious and the complete 'gun only cost 7/6 (35.5p). None of these supplies will now be available - could a small colour crt gun be fitted and ONE of the 'guns used ?? (even colour crt 'gun supplies will dry up shortly !) The coating on the old 'gun cathode could POSSIBLY be renewed