Matching bias current for an equal grid, plate and screen voltage for a
self-biased class A amp, or matching Gm at equal bias current and Vp, Vs
conditions, for a fixed-bias amp. Maybe both.
Point being, Gm differences have little effect in class A amps, whereas Gm
is best matched in a class AB amp, where equal voltage swings are more
important. Most are fixed biased anyway (since auto bias is impossible
without compression effects), so differences in grid bias voltage has little
effect until clipping.

Physical appearance only matters for a show amp; date codes (not to mention
the differences in mere printing on the tube!) are for fanatics.

Tim

--
"That's for the courts to decide." - Homer Simpson
Website @ http://webpages.charter.net/dawill/tmoranwms

"David L. Stinson" wrote in message
...
Good Morning.

As far as I understand, to "match" tubes,
they must meet these minimum criteria:
No obvious physical defects.
No electrical defects (shorts, gas, microphonics etc.)
GM within single-digit percentages of each other as
measured on a reliable tester that can be calibrated
for GM, such as the TV-7D (GM conversion charts available).
It is also highly desirable that the
manufacture date codes match.

I know there are people who are picky about this small detail
and that small detail, but
is there anything else that is *generally accepted* as
a necessary part of this set of standards?

Thanks very much.