[Johnson] 12AT7 Tubes

[Roy Morgan]

On May 31, 2008, at 9:11 AM, David C. Hallam wrote:

> ... just because a certain name was on a
> tube didn't mean that manufacturer made the tube.... when one type  
> was run they would mark them with many different manufacturers' name.

This is certainly true for many tube types.  There are folks who know  
a lot about who made which tube and when and so on. (I'm not one of  
them.)  Eric Barbour is one.  He's published many articles about the  
history and characteristics of tubes well known in the audio field and  
of others not so well knows.  (Is Vacuum Tube Valley still being  
published?)

Some tubes were made only by a particular maker: the Tung Sol 6550 may  
hae been of them, and certainly many of the Western Electric types  
were made only by WE. Some of the reasons for that were
-WE used odd filament voltages (10 volts, for instance) for use in  
remote sites where battery backup was important
-The telephone and microwave applications that WE had needed extreme  
reliability.  One early example is amplifier tubes put into submarine  
cables and placed on the ocean floor in the middle of the sea.
-WE found, no doubt that they needed some other characteristics that  
were simply not available.

Tube manufacturing is a fascinating topic.  Apparently a factory would  
tool up for a particular type and start the production line very  
slowly. The yield (of satisfactory product) at first was very low.   
They'd tweak the machines, mixtures, and processes, train or re-train  
the workers, and then as yield went up, they'd increase the production  
rate till the whole place was running at speed.  They would run  
continuously three shifts a day, till they'd made 3 to 5 years worth  
of tubes of that type.  That might take weeks or a couple of months.   
Then they'd shut down and shift to another tube type.  There would be  
a warehouse full of tubes ready to be sold to whatever company needed  
them, or had made contracts earlier for part of the batch.

Roy


Roy Morgan
k1lky at earthlink.net
Lovettsville, VA 20180