Is there a way to test 12AX7s in circuit ?

[pdf64]

As the tube sound differences don't show in frequency response, they must be due different harmonic content.

This is supported be the data in the link I posted above.

Might the input stage circuit tend to reduce / suppress variance due to the miller effect? In regard of that section’s contribution.
eg extremely low grid circuit source resistance, and a 22pF anode to grid cap.

[Helmholtz]

Might the input stage circuit tend to reduce / suppress variance due to the miller effect? In regard of that section’s contribution.
eg extremely low grid circuit source resistance, and a 22pF anode to grid cap.

Yes, that input wiring will swamp out upper corner frequency variations (due to Cg-a and circuit gain) and extend frequency response to around 40kHz.

Dunno why Nick did that.

Raises the question: What is the correct source resistance?
Easy when there is a buffer or buffered pedal, tricky when the guitar is directly connected.

[Mick Bailey]

Once you get into distortion and cascaded gain stages, then individual triode gain and matching (in a typical twin triode) will affect the harmonics. In addition, the stages may be inclined to clip asymmetrically and unbalanced triode halves could interact with this to produce different levels of distortion at the point of clipping.

[Chuck H]

So, as already covered, there is no life test for tubes. That said my test method has always been as Pete implied. I plug in to V1 position and listen to see if it has proper gain or any noisy issues and I crank up the gain to see if it squeals. If it doesn't squeal I thump it. Most tubes make some kind of sound from a thump under high gain in V1. If it's a single ping or pup that I hear I know it's good. If it makes any sort of rattle or tries to get a brief squeal going for a moment I know it may be turning microphonic soon, though I might use it in a sensitive position like V1 anyway until it does go microphonic. Tubes that are especially good for gain, noise and low microphonics get marked so I don't have to roll the whole stash again finding a good tube. When a tube no longer passes the above microphonic test I move it down the line to a less sensitive position. When a tube no longer passes the gain or noise criteria or becomes grossly microphonic I toss it. Simple as that.

[mozz]

http://www.nutshellhifi.com/library/MostVacuumTubes.pdf

There is a older thread on antique radios website where they talk about this. The link is a tube book and there is a chapter on tube life. Recommended filament reduction is 10% for the test. B&K, Hickok, and many other tube testers had a life test switch.
https://www.antiqueradios.com/forums...c.php?t=160167
Alan Douglas even chimed in, if you know who he is, he wrote few books on tubes, radios and test equipment.

[Helmholtz]

I could not find anything in Tomer's book about a tube life test with reduced heater voltage.

Rather on page 110 he states:
"There is no single measurement of an initial nature which is of any real value in predicting life."

Also (page 120):
"It is almost impossible to estimate a tube's performance from readings alone. It must be tested in the equipment for which it is designed"

Page 122:
"Except in the small percentage of cases where transconductance can be used as an indicator, tube testers do not measure anything that can be truly related to tube quality."

And page 124:
"And it is quite obvious that no tester can predict how well tubes will perform in a given circuit - not even the $10,000 instruments used by tube manufacturers!"

Always best to do some own reading.

[Tom Phillips]

...And page 124:"And it is quite obvious that no tester can predict how well tubes will perform in a given circuit...

This brings to mind the old phrase "If the tube tester says the tube is bad, then it is bad. If it says it's good then it might be bad."

[Chuck H]

This brings to mind the old phrase "If the tube tester says the tube is bad, then it is bad. If it says it's good then it might be bad."

I use to get a lot of UOS (used old stock) tubes from an electronics salvage warehouse in California when I lived there. Shelves and shelves of cardboard boxes with pulled tubes roughly sorted in groupings for shape, physical size and base. Not model. I had lists of all the alias models for tubes used in guitar amps and I would paw through the boxes looking for these. They had a tube tester on site and I would plug my eight or twelve tubes in just to see if they were SHURELY bad. Then I would take them home and plug them in to see if they were ACTUALLY good. They only charged a buck or two per tube and I'll guess about fifty percent were actually useful with a few real gems from makers like Telefunken, Bugle Boy, Mullard, Sylvania, etc.

The fifty percent that tested good but weren't had the problems tube testers can't detect like noise, hum and microphonics.

[mozz]

I guess Hickok was wrong in adding that test in their owners manuals.


c.While holding everything else
constant,reduce the filament voltage and
note new reading.
d.If the meter still reads in the
GREEN(GOOD)sector,the tube has a large
life reserve and will perform
satisfactorily.
e.The filament voltage reductions to
be made are shown in the following table:
NORMAL REDUCE
FIL. VOLTS TO
1.5 1.1
2.0 1.5
2.5 2.0
3.0 2.5
5.0 4.3
6.3 5.0
7.5 6.3
10.0 7.5
12.6 10.0
35.0 25.0
50.0 35.0

Others models used 10% reduction for the test.
Life test: -10% Push button.


15.Life Test.The Model 539CDYNAMIC MUTUAL CONDUCTANCE TUBE TES-
TER is equipped with a special feature to enable Life Test to be made on the tube. In the
center of the control panel is a switch designated CATH .ACT. ,NORM. and TEST.While
holding everything else at normal this switch reduces the filament voltage by 10%.
a. Measure the mutual conductance in the ordinary way with switch set on NOR-
MAL.
b.Throw the CATH. ACT. switch to TEST position.The mutual conductance
should not drop more than 20%.
c.After making life test return the switch to NORMAL for all other tests.

[Helmholtz]

I guess Hickok was wrong in adding that test in their owners manuals.

Fine, but what does the test result tell you about operational life (in the actual application) left in quantitative terms?
A tube that fails the test might have a short life expectancy but what about those passing the test?

Not saying the test is useless, though. It indicates that a good/new tube should have little dependency of gm on heater voltage.
But it's not always low emissivity/gm that limits tube life.