[Boatanchors] 'Ruggedized" 12AX7's ??

[John Lawson]

On Thu, 30 Dec 2010, Richard Knoppow wrote:

>    That makes no sense to me at all.



   Consider the total environment that the tube is operating in.  In this 
case, the 12AX7 is subjected to a rather intense acoustic sound field - 
even in a 'small' guitar amplifier, it is inches from the speaker, and 
thus the sound pressure levels (SPLs) are high.  Vibrations are imposed on 
the tube structures from the glass envelope and from the chassis and 
scoket via the pins and base.

   These vibrations are communicated to the elements, the plate and grid 
mostly - providing a direct feedback path, by varying the element's 
spacing at the frequencies of the sound field - and this feedback is 
highly modified and distorted, first mechanically by the responses and the 
resonances of the affected structures, and then electronically by the 
effect these tube-element vibrations have on the transfer function of the 
tube - how it responds, how it amplifies, it's 'fidelity' to its input 
signals - the signal throughput is changed by the acoustically-induced 
vibrations.

(My Viking Valiant had a nasty tendency for the audio input tubes to pick 
up mechanical vibrations from the mod iron...  SQUEEEEEELLLL!)

   So when someone says a certain tube produces a "haze" that 'colors' the 
sound of the instrument - the report is subjectively accurate, and this 
particular distortion regime can be (and has been!) measured objectively 
on the bench.

   Thus, when used in an acoustic, high-sound-level environment, various 
manufacturer's 12AX7s will definitely produce hear-able and measurable 
differences in the overall response and 'sound' of the amplifier in 
question. Substituting a "ruggedized" or "JAN" (mil-spec) device will 
most likely make an audible difference over a 'vanilla' tube. And 
similarly many other types of tubes when used in such an environment will 
produce similar effects.


   Then there's "Secondary Emission" - but thats a topic for later...


Cheers and Best of the Season!


John KB6SCO
Carson City
DM09fg