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

[Richard Knoppow]

----- Original Message ----- 
From: "John Lawson" <jpl15 at panix.com>
To: <boatanchors at mailman.qth.net>
Sent: Thursday, December 30, 2010 3:44 PM
Subject: Re: [Boatanchors] 'Ruggedized" 12AX7's ??


>
>
> 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
    I was not responding the "haze" caused by microphonic 
pickup, that is a perfectly valid description of a real 
phenomonon. What I did respond to was the idea that tubes 
can be "graded" as to how well they reproduce some 
frequencies. If the grading is done on the bases is 
mechanical resonances where the tube is subjected to 
vibration that _does_ make sense but the original poster 
does not make any such thing clear. Tubes can certainly 
produce all sorts of effects when subject to vibration. 
That's one reason for not putting the speaker for a receiver 
right on top of it. Of course there are other microphonic 
components, in radio receivers typically the tuning 
capacitors, which outdo tubes by a lot.
    Some tubes were made to have minimum microphonics and 
hum, the 7025 and 5879 are examples. Some specials were 
simply chosen off the line to be minimally microphonic or 
have low hum. The RCA types 1612 (6L7) and 1620 (6J7) are 
examples.
    RCA also made tubes intended for aviation, mobile radio 
and other high vibration purposes and where long life was 
desirable. One of these is the 5751, this is closer to a 
12AT7 than a 12AX7 but might be helpful where resistance to 
vibration is desired. It has a u of 70 compared to 100 for 
the 12AX7.
    Secondary emission can be a serious problem in some 
applications. It is highly dependant on the design of the 
tube and the materials of which its made.

--
Richard Knoppow
Los Angeles, CA, USA
dickburk at ix.netcom.com