[Boatanchors] General question on substitutions

[James A. (Andy) Moorer]

The reason you are getting this diversity of recommendations is that it 
depends on what you are doing. That is, it depends on what is important in 
the circuit that uses the tube. In some circuits, mu is important, so you 
need to get a tube with similar mu. In others, it might be plate resistance 
or inter-element capacitance. Different substitution guides will recommend 
different tubes based on what parameters they are optimising. Since the 
substitute tube is not identical, some compromise is required on some of the 
specs of the tube. Only the circuit can tell you what is the most important, 
or whether a particular tube is right.

Having said that, there was quite a bit of variability from one tube to the 
next. Often that variability would swamp any effect of substituting the 
tube. Generally, anything that is even vaguely in the ballpark will probably 
sorta work, since engineers in those days designed for the worst-case 
variation in tubes. It may or may not work really well, depending on how 
closely it matches the needs of the circuit.

James A. (Andy) Moorer
www.jamminpower.com

----- Original Message ----- 
From: "Eugene Hertz" <ehertz at tcaf.org>
To: <boatanchors at mailman.qth.net>
Sent: Wednesday, January 17, 2007 1:23 PM
Subject: [Boatanchors] General question on substitutions


I am investigating a problem in my central electronics 100V transmitter. I 
*think* the problem is with the first mixer which is a 12BY7. I began 
looking to get this tube and noticed there are subtitutions available. Other 
than the 12BY7A which is a "slow start" version, some websites list 
substitutions as 12BV7, 12DQ7, 12GN7. However not all websites list all of 
these as subsitutes for each other.

The GE datasheet for the 12BV7 is the only sheet that directly mentions the 
12BY7 saying "In application and characteristics it is related to the 12BY7"

Some sites like NJ7P list a "preferred" substitute as the 12DQ7 and a 
general substitute (not preferred) as the 12GN7 but does not mention the 
12BV7 (again which is mentioned as a "related" tube in the GE datasheet.

So what does it all mean? Am I making much ado about nothing here? Do I just 
pick whichever I can find in my tube spares and try it? If it works leave 
well enough alone? Is it possible that one of these is lower noise than the 
others and could be beneficial? Or any other benefit? (i.e. Oh yes, people 
stopped using the 12BY7 in favor of the 12xx7 long ago because of its 
superior properties...)