[ARC5] A little off-topic: Metal tubes (eg 6SJ7) seem quite valuable ...

Wed Sep 15 23:44:16 EDT 2021

Here is an excerpt from the great book "Tube Lore", written by Ludwell Sibley, on the history of metal tubes. Turns out some (most) are all metal (except for the eyelet seals) but there were metal/glass tubes as well.

THE METAL TUBE

RCA and GE announced a new technology in April 1934:

the metal tube. Developed in secrecy by GE, the new design

used a steel shell, with individual glass-filled eyelets for the leadin

wires, on a new and convenient octal base. This product line

gave GE and other receiver makers a complete tube complement

for the "1935" line of sets. The "original nine" were:

5Z4 rectifier 

6F6 power pentode

6A8 and 6L7 converters 

6H6 duodiode

6C5 and 6F5 triodes 

6J7 and 6K7 pentodes

A tenth type, the 6D5 power triode, was withdrawn before

major production.

The other tube makers were unhappy to have their product

lines and tooling obsoleted. Some, like Raytheon, signed up to

adopt the new line. Others disparaged RCA's invention - the

multiple eyelet seals were sure to leak, they said - and hurried to

repackage their existing big-pin products with octal bases. Yet

others like Arcturus and Triad quickly devised what they termed

the "perfected" metal tube: the metal-glass (MG) design, where

a small conventional stem-sealed glass tube was hidden inside a

steel shell, on the same octal wafer base as GE-RCA used.

The MG push was short-lived. The RMA issued registration

in July, 1935 on the 5Z4MG, 6A8MG, 6C5MG, 6D5MG,

6FSMG, 6F6MG, 6H6MG, 6J7MG, 6K7MG - copies of RCA's

line - plus the 25Z5MG rectifier for AC-DC sets. Later registrations

through March 1936 covered the 6N6MG, 6N7MG,

6Q7MG, and (today unknown) 6Z6MG rectifier. For those

wanting to "collect the set" today, eventual adders were the

6J5MG, 6XSMG, 25A6MG, and 25Z6MG.

A better variant on the MG idea was the Arcturus "Coronet"

type, which used a new glass "button" stem construction, with

the lead-in wires sealed in a circle. These were used in a line of

octal-based replacement types, to be used with an adapter to

"modernize" existing radios. The line was: 24, 27, 51, 56, 57,

58, 75, 77, 78, 80, 85, 2A6, and 25Z5. Inherently octal items

(no adapter needed) were: 5Y3 and 25A6 (also replacing 43

with an adapter). Later octal additions were versions of the

RCA "original nine," 6L6 (!), 6N6, 6N7, 6Q7, 6R7, 6X5, and

25Z6.

Meanwhile RCA overcame great start-up difficulties and got

true metal tubes into full production, introducing additional

types as well. To overcome the high cost of the eyelet base, a

button stem was put into use in 1936. In this design, the glass

stem disc with its lead-in wires was sealed into the bottom of

the metal envelope. The button-stem idea had great benefits in

later tube designs, as we will see. Metal tubes continued to be

registered up to the start of WW II, and even a few variant

types afterward, but other designs took over.

Copies of the original Tube Lore book are hard to come by (they have been out of production for some time) but Sibley released the updated “Tube Lore II” in 2019. You can find it available for purchase from a slew of online book vendors, including Amazon. Great reference.

Dave Rossetti

410-279-0226 (mobile)

-----Original Message-----
From: arc5-bounces at mailman.qth.net <arc5-bounces at mailman.qth.net> On Behalf Of Tom Lee
Sent: Wednesday, September 15, 2021 7:27 PM
To: arc5 at mailman.qth.net
Subject: Re: [ARC5] A little off-topic: Metal tubes (eg 6SJ7) seem quite valuable ...

I would guess that there are no metal-over-glass vacuum tubes (the 0Z4 and other gas-regulator tubes are not, technically speaking, vacuum tubes, so I'm excluding them). I would be happy to learn of exceptions (as well as the engineering reasons for the exceptions, if possible), but from my limited knowledge, it's hard to see a practical reason to build a glass vacuum tube, and then bond metal to the glass. Shielding is far more cheaply provided in other ways, for example.

-- Cheers,

Tom

--

Prof. Thomas H. Lee

Allen Ctr., Rm. 205

350 Jane Stanford Way

Stanford University

Stanford, CA 94305-4070

 <http://www-smirc.stanford.edu> http://www-smirc.stanford.edu

On 9/15/2021 15:37, William Cromwell wrote:

> Hi,

> 

> Looking at the replies regarding glass tubes in metal sleeves it seems 

> that some are and others are not. Maybe most are not. I doubt anybody 

> here has any reason to report other than their experiences opening 

> metal tubes. I may have read some "fake news" once upon a time:)

> 

> I was never interested enough to do a post-mortem on dead metal tubes. 

> Tubes that work should not be 'molested'. I did harvest a few octal 

> plugs from dead glass tubes. Low hanging fruit you know.

> 

> 73,

> 

> Bill KU8H

> 

> bark less - wag more

> 

> On 9/15/21 4:48 PM, Ben Hall wrote:

>> On 9/15/2021 3:24 PM, Tom Lee wrote:

>>> My experience is the same as yours. I’ve opened several over the 

>>> years, and I’ve never seen one that was simply a metal covering over 

>>> an otherwise glass tube.

>> Greetings all,

>> 

>> Years ago I obtained a large quantity of assorted tubes - all just 

>> thrown into cardboard boxes - all of questionable provenance. After 

>> sorting them out, I had about 30 metal octals that were rusty and 

>> unidentifiable.  I try not to waste anything, so I cut them all apart 

>> to salvage the octal bases. (for what?  I don't know...but I will say 

>> I've made a few of them into test adapters, etc...)

>> 

>> Every single one had a glass seal between the octal base and the 

>> metal envelope, but in all of them, there was no further glass. They 

>> were not glass tubes in metal sleeves; once past the glass seal, the 

>> metal envelope was what held the vacuum.  :)

>> 

>> Thanks much and 73,

>> ben, kd5byb

>> 

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