[krikor]

Question for you tube gurus...

What does it mean if a tube seller states that a dual triode input tube tested good at say "106, 108 / 56" for one tube and say "78, 65 / 56" for another tube. Does that mean the first tube tested above (106% and 108% per section) what would be expected of an unused new tube? What does the 56 indicate?

Any enlightenment you can provide would be greatly appreciated.

Thanks.

[G Georgopoulos]

I havent got a clue,but i think it's the mu of the tube...

[sfox7076]

It usually means the tube's hypothetical tube value (either the NOS number or the 60% number (depending on the tester)) is 56, but then with testing, the actual number is 106 for one side, 108 for the other.  To clarify, it isn't a %, it is a proportion: 106/56 and 108/56...

[galyons]

What does it mean if a tube seller states that a dual triode input tube tested good at say "106, 108 / 56" for one tube and say "78, 65 / 56" for another tube.

Different testers measure tubes in different manners. The major choices are emissions or transconductance.  There is no consistency from different brands/models of testers.  That means the scales on a B&K tester have no direct relationship to the scales on a Hickock tester. 

Emissions testers give results in terms of "Bad, ???,  Good" or numeric scales, as an example 1 to 100.  1 is bad, 100 would typically be considered "tests as new". There are many variations on the theme. 

Transconductance testers usually give a minimum usable result, (usually around 40- 60% of the transconductance of that tube when new).  Again no consistency of scale between different brands/testers.

In you examples you give, the tubes are dual triodes, literally two tubes in one glass envelope.  So, the "106, 108 / 56"  example that you gave, 56 is likely the minimum transconductance. (minimum does mean you can use it, it just has less life left)  The two triodes measure at almost twice the minimum, so would likely be considered "measures as new". The two triodes are closely matched, as well.   The other example, "78, 65 / 56"  means the two triodes are above minimum, have good life left, but the two triodes are not well matched. Whether matching is important or not depends on the circuit in which they will be used.

Hope this helps.

Cheers,
Geary

[krikor]

Different testers measure tubes in different manners. The major choices are emissions or transconductance.  There is no consistency from different brands/models of testers.  That means the scales on a B&K tester have no direct relationship to the scales on a Hickock tester. 

Emissions testers give results in terms of "Bad, ???,  Good" or numeric scales, as an example 1 to 100.  1 is bad, 100 would typically be considered "tests as new". There are many variations on the theme. 

Transconductance testers usually give a minimum usable result, (usually around 40- 60% of the transconductance of that tube when new).  Again no consistency of scale between different brands/testers.

In you examples you give, the tubes are dual triodes, literally two tubes in one glass envelope.  So, the "106, 108 / 56"  example that you gave, 56 is likely the minimum transconductance. (minimum does mean you can use it, it just has less life left)  The two triodes measure at almost twice the minimum, so would likely be considered "measures as new". The two triodes are closely matched, as well.   The other example, "78, 65 / 56"  means the two triodes are above minimum, have good life left, but the two triodes are not well matched. Whether matching is important or not depends on the circuit in which they will be used.

Hope this helps.

Cheers,
Geary

Thanks a bunch! That helps a lot. If it matters, the tester being used is a TV7.

[FullRangeMan]

What is the tube?

[krikor]

12AU7

[FullRangeMan]

12AU7 sport a Gain=20, so 56 is not Gain.
Datasheet:
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12AU7A.pdf
Usually these values vendors inform are plate emission per Triode.
56 seems Plate resistance imo.

[galyons]

Thanks a bunch! That helps a lot. If it matters, the tester being used is a TV7.

A very good transconductance tester. The TV7 has its own scale, (1-120).  I own its big brother the TV10D/U and it measures in microhms, (mutual transconductance). Both were Hickock designs and most, but not all, were manufactured under government contract by Hickock.  Sorry, TMI!!!!   

Cheers,
Geary

[JakeJ]

Galyons is spot on!  I have the TV-7 D/U and it indeed has a its own scale of 0-120 and the "56" is lowest acceptable value for that tube.  I personally would change a tube that tests that low.

Most assuredly all the military tube testers were not all built by Hickok but, in fact, a plethora of manufacturers built them to the gov't's spec.  Thus there is minor variation among the military units.

Oh, I think you meant "micromhos" as mutual transconductance is a measurement that is the reciprocal of ohms.

[G Georgopoulos]

This doesn't say anything about tube life,I would guess the military had this testers to prevent tube failure in serious situations/emergencies and such,I have a 60 year old tube antique radio and it still works fine.If the sound is good why change the tube unless you want to make it new again...

[Steve]

I have found the only way to really test a tube is in the actual circuit of use. One will quickly differentiate
the quality of tubes, at least that has been my experience.

Cheers
Steve