David Axt wrote:

How do "industrial" vacuum tubes compare to "commercial" tubes? Are
they more rugged?

Sometimes, depends on the tube, and what it was optimized for.

Less noise?

Same answer.

Are they less quality because they are
intended to be used in industrial applications not audio applications?

Depends what you mean by "quality." Industrial and military devices tend
to be optimized for ruggedness and long life. In general, how close they
are to "bogey tube" values used to derive the characteristics and curves
were *not* particularly important. So if by "quality" you mean
"matching," then there probably won't be much of a difference. If you
mean abstract or subjective properties like "soundstage transparency,"
then you're on your own. ;-)

For example there are a lot of industrial equivalents (5751, 6057, 6681,
7025, 7494, 7729) to the 12AX7 tube. How are they different?

IIRC, 5751 was optimized for ruggedness, but has a lower published gain
figure than 12AX7. 7025 was optimized for low noise. 6681 was a mobile
communications version, so would again be optimized for ruggedness (and
possibly low microphonics, and high tolerance to filament voltage
changes.) The rest I don't know about. If you can find the actual spec
sheets for the tubes you're interested in, they will usually state what
the intended application was, or other clues as to what they were
optimized for.

Cheers,
Fred
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