Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!mcgill-vision!snorkelwacker!apple!julius.cs.uiuc.edu!wuarchive!emory!ogicse!milton!whit
From: whit@milton.u.washington.edu (John Whitmore)
Newsgroups: sci.electronics
Subject: Re: Ridiculous Impedance
Message-ID: <7445@milton.u.washington.edu>
Date: 12 Sep 90 23:45:19 GMT
References: <1770013@otter.hpl.hp.com> <1990Sep12.035908.6083@mlb.semi.harris.com> <4022@kitty.UUCP>
Organization: University of Washington, Seattle
Lines: 37

In article <4022@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes:
>In article <1990Sep12.035908.6083@mlb.semi.harris.com>, del@thrush.mlb.semi.ha\
rris.com (Don Lewis) writes:
>> > ... these vacuum tubes had flexible stranded wire leads which were
>> >soldered directly into the circuit - with no connectors being used.
>>
>> How did they produce a vacuum tight seal with stranded leads
>
>       I believe the stranded leads were welded to solid metal posts
>which actually penetrated the glass envelope.
>
>       Incidentally, the glass-to-metal seal area of a vacuum tube
>usually uses a glass of different composition than that of the envelope
>itself.  Such glass, which is usually borosilicate in nature, is specially
>formulated to match the thermal expansion characteristics of
> [special metal alloys] with two common tradenames being Dumet and Kovar.


        I would like to add that the thermal stresses, while minimized
by material selection, still often kill vacuum tubes.  Some
feedthroughs use a thin metal tube (instead of a rod) so that the
glass/metal interface has less resistance to expansion/contraction.
We use such feedthroughs with thermocouple wire (plugging one
end with solder), so that our thermocouple alloy remains constant
and our temperature measurements accurate across the vacuum boundary.

        By 1963, there were good quality ceramic feedthroughs (see,
for instance, Nuvistor-type vacuum tubes).  According to our
glassblower (who still custom-builds the odd tube), these are
superior to the soft-glass and hard-glass seals.  Possibly the
tubes for the transatlantic cable of 1963 weren't glass at all,
but metal with ceramic/metal feedthroughs.  
	
	I know Nuvistors were available in 1963; were the
ceramic/metal tubes considered reliable then?

I am known for my brilliance,             John Whitmore
 by those who do not know me well.