Is there a way to test 12AX7s in circuit ?

[daz]

What i mean is if you have several 12AX7s of the exact same model and try them all in circuit, is there a measurement u can check that would tell you which ones are stronger or in NEWER condition? I have a bunch of old 80s chinese ones that are one of my fav 12AX7s and i know some have many hours on them and some much less so. So i'm wondering if theres a way to check.

[pdf64]

As I seen it, all that matters is in circuit performance, whatever some valve tester says seems immaterial. I just check that each section has reasonable gain and isn’t hissy, hummy or microphonic.

[Mick Bailey]

You could set up an amp with a steady fixed signal input (say, 1khz) and measure the AC output of the first preamp tube. Stronger tube=more output. Just a relative check. It won't tell you the age of the tube, though.

[mozz]

You could set up an amp with a steady fixed signal input (say, 1khz) and measure the AC output of the first preamp tube. Stronger tube=more output. Just a relative check. It won't tell you the age of the tube, though.

Exactly, then reduce fil voltage like a life test.

[daz]

You could set up an amp with a steady fixed signal input (say, 1khz) and measure the AC output of the first preamp tube. Stronger tube=more output. Just a relative check. It won't tell you the age of the tube, though.

Thanks, i'll give that a try. Is output the sole difference between a little used tube and one of the same modeled that would be considered worn to the point of being time to replace?

[Helmholtz]

DC voltages in the amp will tell if the tube works, scope measurement can tell about gain and clipping characteristics.
None of these will show if the tube is noisy or microphonic nor how it really sounds.
I don't think there's any tube test that reveals tube life left.

This might be interesting (thanks to nickb):
https://bmamps.com/v01/home/techie-c...parison-tests/

[daz]

Ok, well i hadda ask. I tried all of them and aside from one that was faulty they all sounded close, a few slightly different. But those old chinese are just such neutral sounding tubes that i find them to be the best for most amps overall so i was hoping i could determine how many i really have that aren't old and worn.

[Helmholtz]

Don't forget that tube rolling wears out socket contacts.

[The Dude]

You could set up an amp with a steady fixed signal input (say, 1khz) and measure the AC output of the first preamp tube. Stronger tube=more output. Just a relative check. It won't tell you the age of the tube, though.

^^^^^^ That. Even brand new tubes have a gain variance in a given circuit, so just because a tube has more output doesn't mean it's newer or will last longer. I'll add: using a single frequency test tone won't tell you the frequency response of the tube, which also matters if you're looking for a specific sound.

[Helmholtz]

Tube parameters don't vary with frequency, rather are constant up to Mhz frequencies. So a tube can't have an own frequency response.

[The Dude]

I'm familiar with the theory and data, so I won't argue with you and concede your point. I'll just say that my ears have said otherwise on several occasions.

[g1]

I think tube parameters are usually measured in the tubes normal linear range. I've never seen data on what happens when they are pushed beyond there. Maybe that's where some frequency differences could come into play?

[Mick Bailey]

To expand on what mozz suggests; some old tube testers had a 'life test' function that reduced the heater voltage considerably, the idea being that as a tube ages the emission drops. Reducing the voltage below a certain level gives a clearer indication of how much the emission has dropped, which may otherwise appear OK at the full rated voltage. It's fine as a comparative test, though is dubious in determining the actual remaining life in a tube. My emission tester drops the voltage to 5v and I think 4v. Using a reference tube gives a benchmark. In practice, after playing around with the tester I packed it away and never bothered with it again - I'm not too concerned about tube age so long as a tube sounds good in an amp.

[pdf64]

I think tube parameters are usually measured in the tubes normal linear range. I've never seen data on what happens when they are pushed beyond there. Maybe that's where some frequency differences could come into play?

My take is that tonal differences are due to how the particular valve interacts with the various impedances of the circuit it’s in.
And on the forward biased Vg-k characteristics. eg a sharp transition into grid conduction may tend to cause a greater proportion of higher harmonics than a more rounded transition.

[Helmholtz]

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.