[AMRadio] Testing tubes

[DOXEMF at aol.com]

OK Guys,
I forgot I was addressing Radio ops, LOL !
I should have said most Audio tubes.
I did real and subjective tests on the standard audio
preamp tubes using a Sond Technology 1700 Distortion
analyzer and Variable HT and Bias supply several years ago.
Since it was just personal interest in finding good sounding
tubes I really didn't document anything, just chose the best testing 
lowest noise tubes then did a subjective listening matchup
in my amps.
Now the test gear has been in storage since and I am setting it up
 again to do more advanced and documented testing including square wave and 
pulse behavior.
Will post some actual figures before too long.

But I agree that really there should not be an audible difference
within tubes of the same type operating in the same identical 
voltages. My ears and experience with tube audio tell a 
different story tho and I also would like to see why there 
are noticable fidelity differences in various types of plate
structure and mfg.
I have a few suspicions that some of the difference may be caused by a change 
in plate resistance caused by differences in
cathode composition, quality of mfg., and deterioration which would explain 
why NOS from the "Glory Days" of tube mfg. seem to hold up the best and are the 
most consistent.

An interesting example is the now no longer available
China Mfg 12AX7's that were highly regarded in audio but
when used in guitar amplifiers for a year would still test almost 
as new but would be so dead sounding they were useless.
I will include some of those that I still have in my testing.

A little non tube issue but similiar.
Published spec and actual real world function of many devices
do not necessarily agree with each other as a verifiable
example is in the difference in the sound and behavior 
of the various versions of the common 5532 and 5534 opamp.
Originally a TI device and also Signetics mfg. I have yet 
to find a currently made version from EXAR, NEC, etc that
exibits the same characteristics in regards to output impedance
and current capability, i.e. sound.
Even tho these are "supposed" to be the same device.

Sorry for the subjective length of this but I will follow
up with hard data as soon as I can derive it.

Thanks for listening !
Bill KB3DKS


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