David C. Hallam dhallam at RAPIDSYS.COM
Wed Nov 26 08:07:51 EST 2008

Lin Robertson wrote:
> At 10:07 PM 11/24/2008 -0500, you wrote:
>>>>  I should have said consistant difference between brands.  There 
>>>> may be a difference between individual tubes but it would be rare 
>>>> to find a consistant difference between bands
>> David
>> KC2JD/4
> According to a series of three articles on fixing problems in the 
> RX-1in Electric Radio, K6AD states the following:
> "Having a good supply of 12AT7 tubes on hand, I measured the drift 
> contributed by various cold tubes in the warmed-up steady state 
> oscillator...For drift, GE tubes seemed the worst, while Sylvania gave 
> the best results."
> For full text of the article, see "New Hope for your Heathkit RX-1"; I 
> think it's Dec 1996-Feb 1997.  (I only have copies, not the originals.)
> In this case there WAS a consistant difference. Q.E.D. : Try it.
> -Lin/KJ6EF
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I wouldn't dispute his findings as I haven't seen them nor looked at his 
documentation of the study.  How many Sylvania tubes did he try?  Were 
they all from the same lot or from different lots?  Was the difference 
statistically significant?

I do know of another case where it seems a certain brand of tube works 
better than others.  The Johnson Invader uses a 6CX8 in the first 
mixer.  A friend of mine says that Sylvania tubes preform better to the 
extent that he has seen another brand that will not work at all in the 
mixer.  But this applies only Sylvania tubes with yellow lettering on 
them.  Sylvania tubes with orange letters do not perform any better than 
other brands.  Or maybe it was the other way around.  It has been 
several years now since we had the discussion and I really don't 
remember which color lettering was the one he said to use.  The carrier 
oscillator drive to the grid of the 6CX8 mixer is marginal at best and 
they can be finicky.  You figure out that one.

I will say that I am a retired metallurgical engineer.  When I started 
my career in the late 1950's my mentor told me all about radio tube 
manufacturing.  He had worked as a consultant for several tube 
manufacturers during WWII as well as for Chance Vought on development of 
the Corsair fighter.

I agree that all tubes are not the same particularly when used in 
certain critical applications as there are manufacturing variations from 
tube to tube and from lot to lot.  But, given that you do not know who 
made a given tube even though it is marked with a manufacturer's name, I 
am not convinced that there is a statistically significant difference 
between manufacturers.


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