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Re: Cool DIY Vacuum Tube Video
Posted: Thu Jul 10, 2008 2:57 am
by John Futter
Steven
From my earlier training as an instrument maker 30 odd years ago I did as part of it 3 months of scientific glass blowing.
depends on what glass the final is but an intermediate with differing thermal expansion is required between the tungsten wire and the final glass or is it Kovar wire which would require a different intermediate.
From memory platinum wire is compatible with pyrex directly-- or was it soda glass or quartz
don't quote me
for the first time in thirty years I did a little glass bending 3 months ago to make some annealing supports for our substrates in the vacuum furnace for annealling at 600 degrees, made of pyrex.
ruined a few days later by the students who thought it was made of quartz and took it to 1000 degrees celsius melting it to the furnace quartz tube which imploded during cooldown because of the differing thermal expansions.
I'll go and talk to the glass blower next week and find out about it and if he has any.
Re: Cool DIY Vacuum Tube Video
Posted: Thu Jul 10, 2008 10:45 am
by Richard Hull
A useful tip relates to sealing electrodes to glass.
Platinum is compatible with most all glass as is, provided it is of small diameter.
Likewise, very thin tunsten welding rod is also useable with glasses, but it has to be treated with sodium nitrite at high temps. .010 rod stock is good. Tim Raney of our group here does this a lot. Tim now tends to use the classic graded seal using small dabs of uranium glass beads.
U glass is readily found on larger used hydrogen thyratrons and other large vacuum tubes at hamfests. Smash them and either keep the graded seal or just the U glass.
Cheaper still might be vaseline glassware. I haven't tried it yet as a graded seal, but about $40.00 will get you a large item from an antique store that would make thosuands of such seals. The U glass marbles that used to be on ebay might serve also.
Graded seals are real pretty and can take a good deal of heating (expansion and contraction without failure as witnessed in the old glass H2 thyratrons that run really hot. Some number to look for 4C35, 5C22. The 5C22 is still avaialble commercially for $1195.000. I found two at a recent hamfest of $2.00 each.
Richard Hull
Re: Cool DIY Vacuum Tube Video
Posted: Thu Jul 10, 2008 2:44 pm
by DaveC
The standard metals for inexpensive glass feedthroughs are Kovar and Dumet which more or less match the thermal expansion of soft glasses. As Richard notes small diameter Tungsten is also a close match for soft glass seals, as is Patinum. As I recall, the key is to have a wettable outer surface for the wire lead. For Tungsten, this is some sort of oxide.
Rosebury' Book on Vacuum Tube materials and Techniques, has some useful tables of thermal expansion for various glasses as well as the metal types the match reasonably well.
One reasonably good source for feedthrough stock, is any discarded incandescent bulb. Not sure how good a seal this is for extended high vac applications, but definitely works for about 1000 hrs more or less.
I think, you can still buy graded seals, but can't remember where... sorry... But the price is a lot more than that of a stray U glass jar from a junk shop. A lead borate glass is routinely used in CRT e-gun bases.
Dave Cooper
Re: Cool DIY Vacuum Tube Video
Posted: Tue Jul 15, 2008 12:41 pm
by teslacoil.it
Re: Cool DIY Vacuum Tube Video
Posted: Tue Jul 15, 2008 11:04 pm
by Steven Sesselmann
Fabio,
Nice setup, I am impressed!
Have you considered building a sealed glass fusor?
Steven
Re: Cool DIY Vacuum Tube Video
Posted: Wed Jul 16, 2008 7:18 am
by teslacoil.it
YES of course!
But these tubes are my really first glass works with positive results, actually i haven't neither the skill nor the material/equipment required to blow a larger tube, generally the glass crack during the cooling down defeating all the work!
anyhow, work in progress, i recovered some unused getters, these are able to achieve high degrees of vacuum inside of a permanently sealed tube even with a simple dual stage pump; a sealed-glass fusor (deuterium filled!) will made when i'll be able to achieve it.
Ciao!
Fabio.
