[Milsurplus] How Much Vacuum in "Vacuum Tubes?"

[Gregory W. Moore]

GA, All

While I have never made a "circuit usable" vacuum tube from scratch, I 
did, in the early 1970's make a triode
out of parts salvaged from several different tubes, blowing the glass 
envelope myself, and using the services of
a CRT rebuild house to pull the vacuum on said tube.  Now, the tube did 
not have a getter, but I managed
to make a octal base which was similar to another triode ( now that I'm 
59 I forget just what I used) but It
tested "good" in a transconductance tester, and probably would have 
worked in a circuit, except that the
process of testing, I managed to burn the filament out testing higher 
filament voltages.  SO, I would imagine
the concept would have been viable back in the day, I wish I could have 
used the homemade tube in an
actual application to see if the gain, and current curves would have 
matched a commercial manufactured
tube.

OK, I would like to reopen the subject raised in this thread about the 
rebuilding of CRT's.  Being the usual
underpaid Aerospace/Electrical Engineer ;-) as well as a comml pilot,  
with the prospect of layoffs with various administration changes and 
contract completions
as well as contract cancellations (fun to be had by all) I ran a TV 
repair biz for a time, my specialties being tuner rebuilds and also CRT^
replacements using the customer's tube rebuilt.  At the time, late '80's 
thru early '90's there was a company in
northwest Philly, near the old Budd plant that rebuilt all CRT's, both 
color and B/W and if one was lucky enough,
one could not only witness the process, but actively particpate.  
Basically, after breaking off the center of the base
to allow air in, the tube was cut forward of the gun, and another gun, 
with a glass envelope to match was rejoined
to the tube.  The seam showed up as a slight bump, but never once did I 
have a problem with installing the yokes
etc.  after the new gun was installed and sealed in place, the Tube was 
evacuated using a roughing pump and
a  diffusion pump. I never had a bad rebuild job, and my customers 
really appreciated the savings. 

This, incidenally is the same company which I used to manufacture my "GW 
Triode" back in the early 70's
and I actualy used their facilities to blow the glass envelope (made 
from a tube which would have eventually
been destined to hold a new gun for a CRT.  For those who don't know how 
the tubes are sealed off, it's simply
a matter of using an oxy/gas flame on the evacuation tube, and using a 
pair of carbon pieces to neatly seal the
stub off,  I actually performed this a few times and found that it isn't 
that hard, and I never had a failure.

FWIW, I still have a '60's vintage Zenith "portable" which I recieved 
for a wedding gift in '69, and it has been
thru 3 rebuilds on the CRT, has a beautiful pix, and I use it on my 
bench in the basement, on digital cable yet,
and the tuner is absolutely perfect, rejecting adjacent stations without 
peer.

Anyhow, I see no reason why, in the early days of radio, why someone 
couldn't build their own vacuum
tubes, the only drawback being glassblowing facilities, and the vacuum 
pump(s).  Just as an aside, back in the
50's Dr. Hansen from Litton Industries (I used to work for a subsidiary 
of said company) had constructed
an altitude chamber and a pressure suit (the contract from the AF 
specified that "space" or "space suit" couldn't
be mentioned in any way, so the suit was referred to as a "Manipulator 
Station" and the person within was referred
to as the "Inhabitant" The chamber was referred to as the "Inhabited 
High Vacuum facility"

The above info was garnered from :US Space Gear, Outfitting the 
Astronaut" by Lillian D. Koslowski,
Smithsonian Institution Press, Wash DC (c) 1994 ISBN 0-87474-459-8
with library info (I will give the Dewey Nr  (629.47'72-dc20).

At any rate, the info is covered quite well on pp 145-169, as the 
"Manipulator Station" became the prototype
for the Litton "Hard Suit "development. 

OK, let's cut to the chase, according to the account given here, the 
chamber was evacuated to the level of
27,000 ft, at which time, according to the book, Hansen "operated a 
thermionic tube" without a glass
envelope.  The chamber was also used for the development of CRT's in 
which the operation could be
observed without the problem of the evacuated envelope.  Now, I have to 
confess that 27K' seems
low, and a pressure suit would not have been necessary at this altitude, 
an O2 mask would have been
sufficient, and this altitude may have been a misprint.  I have been 
exposed to the same altitude in a
chamber, and have had no problems using just the O2 snout, although the 
"fun and games" resulting
from removing one's mask at the instructor's request, and beginning to 
sign one's name, which seemed
easy at the beginning, showing the overconfidence which incipient 
hypoxia causes, only to find one
"coming to" with the mask back on and the signature running off the 
bottom of the page was a valuable
lesson, never to be forgotten.   I will, since the book states 27,000 as 
the altitude, stay with this, but
I won't go thru the math to figure the PP O2 involved at this altitude, 
etc.  Since the book states the
tube and CRT worked at this altitude, I will stipulate that a ham could 
have built vacuum tubes
with a minimum of equipment.  I also have been checked out at 
significantly higher altitudes
in a pressure suit, but never had the opportunity to try a "naked" tube 
during those chamber runs.

At any rate, it seems plausable to build one's own vacuum tubes, if one 
was really desperate
and had no other source.

Hope this has been helpful..

73 de Greg "GW" Moore WA3IVX/NNN0BVN
green (and gray) radio afficionado///

sdaitch at mor.ibb.gov wrote:

>My problem was visualizing all this, in terms of home production 
>of tubes, that is, placing the unsealed tube in a chamber, 
>drawing down the vacuum, and then trying to seal the tube in
>this vacuum chamber.  I could not figure out how to keep
>the products of the flame from the interior of the tube,
>then it kind of dawned on me, when someone talked about the
>breaking the seal on the CRT for shipping.
>
>DING DING, but then I just had to ask to confirm what 
>I thought the process was. 
>
>The typical tube with the bakelite base, and more than
>9 pins in the subminuture tubes were sealed at the pin
>base, while the typical 7 or 9 pin subminiature is sealed
>at the top of the glass envelope.
>
>Been away from this stuff far too long.
>
>Thanks,
>
>Sheldon
>
>
>
>----- Original Message -----
>From: Bob Camp <ham at cq.nu>
>Date: Thursday, March 29, 2007 11:24 pm
>Subject: Re: [Milsurplus] How Much Vacuum in "Vacuum Tubes?"
>
>  
>
>>Hi
>>
>>That's pretty much it with a couple of refinements.
>>
>>The glass pipe that goes from the pump manifold often will have a 
>>vac- 
>>ion gauge on it. The heat source to melt the "stem" is generally a 
>>
>>hydrogen flame. The tube it's self sits in a furnace while this is 
>>
>>all going on to bake it out.
>>
>>The other stuff that goes on depends on just how the gettering is  
>>being done. Different getters are done different ways.
>>
>>Bob
>>
>>On Mar 29, 2007, at 3:02 PM, sdaitch at mor.ibb.gov wrote:
>>
>>    
>>
>>>How do you seal the tube while under a vacuum?
>>>
>>>Is the seal stem itself hooked to a vacuum pump, and when the  
>>>vacuum is down to the
>>>level desired, the stem is heated by an external heat source, 
>>>      
>>>
>>the  
>>    
>>
>>>stem melts and seals while the vacuum is still being pulled by 
>>>      
>>>
>>the  
>>    
>>
>>>pump?
>>>
>>>I get dense on what is probably very simple.
>>>
>>>73
>>>Sheldon
>>>
>>>----- Original Message -----
>>>From: J Forster <jfor at quik.com>
>>>Date: Thursday, March 29, 2007 6:20 pm
>>>Subject: Re: [Milsurplus] How Much Vacuum in "Vacuum Tubes?"
>>>
>>>      
>>>
>>>> I read a number of articles in old (early 20s and before) radio
>>>> magazines about amateurs making their own tubes. I have a tube
>>>>in my
>>>> pile of stuff that looks very crude and crooked inside, not sure
>>>>if it
>>>>
>>>> could qualify as 'homemade', though.
>>>>
>>>> Maybe it gets down to losing skills due to progress and cost? 
>>>>        
>>>>
>>If you
>>    
>>
>>>> can buy a new one cheaper, why go through the hassle and all that.
>>>>
>>>> ~ Todd,  KA1KAQ
>>>>
>>>>
>>>>Making tubes is not that hard. The first ones were made with the
>>>>technology of roughly 100 years ago. If you can work small 
>>>>        
>>>>
>>pieces of
>>    
>>
>>>>thin metal, blow glass, and pull a vacuum, you can likely make a
>>>>decenttube. Getting them to perform better and be consistent takes
>>>>moreeffort, but obviously can b done.
>>>>
>>>>IMO, it may be easier to build a simple tube from scratch than
>>>>repairingone with a bad filament. Before my time, MIT used to
>>>>teach a 'Tube Lab'
>>>>. They published a set of notes on how to do it which make  
>>>>interesting
>>>>reading.
>>>>
>>>>Making tubes is certainly practical in a small facility. Not so for
>>>>transistors or ICs.
>>>>
>>>>-John
>>>>        
>>>>

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