Tested 12AX7 resistance

[surface54]

i tested a couple 12AX7s resistance with my digital multimeter

i dont know what these values mean

Sovtek LPS 16.2 ohms resistance?
Tung Sol reissue 16.9
GT 10.2

does this measures the tubes strength or are all these values useless in telling how much life is still in a tube. just wondering im still learning, and my hunch is a would need a real tube tester to get accurate values.

 

[misterwhizzy]

Re: Tested 12AX7 resistance

Between what two pins did you test? Also, you have to realize that inside each of those 12AX7's is actually two amplification devices. They're dual triodes, and you may not get identical readings across them. Also, (and I can't remember the theory exactly) all the measurements you have go out the window once you start applying voltages on the grid. The real number I think you want to be testing for is the gain, and typical values for 12AX7 are about 100, if I remember correctly.

 

[surface54]

Re: Tested 12AX7 resistance

here is how i tested them
on most of the pins it doesnt register anything.

 

[misterwhizzy]

Re: Tested 12AX7 resistance

This may be the blind leading the blind, but I don't think measuring the resistance will tell you anything about the life of the tube. 12AX7s can last forever, basically, but if you're not liking the sound of a particular tube, you can replace it for less than $15.

I'm not sure about what exactly happens when a tube is dying, but since its main function is as an amplification stage, measuring the gain is really the only way to tell if it's still working (other than your ears, of course). This also supposes that you knew the numbers of the tube BEFORE you were testing it, as a given tube may only have a gain factor of 85. In that case, you might read it saying 85 while expecting 100 and think it's on its way out. This would not, of course, be the case, but you would have to have a reference point.

Some places (cheap plug for Doug's Tubes) will write the gain on the packaging before they send them out to you, so you know beforehand. The only resistances on a 12AX7 datasheet are the plate resistance and the transconductance (which is actually an inverse resistance). I don't think either one of these things is an indicator of tube life, however, since they should be static when there's no signal voltage to be amplified. I think a tube tester is the only way.

 

[surface54]

Re: Tested 12AX7 resistance

This may be the blind leading the blind, but I don't think measuring the resistance will tell you anything about the life of the tube. 12AX7s can last forever, basically, but if you're not liking the sound of a particular tube, you can replace it for less than $15.

I'm not sure about what exactly happens when a tube is dying, but since its main function is as an amplification stage, measuring the gain is really the only way to tell if it's still working (other than your ears, of course). This also supposes that you knew the numbers of the tube BEFORE you were testing it, as a given tube may only have a gain factor of 85. Some places (cheap plug for Doug's Tubes) will write the gain on the packaging before they send them out to you, so you know beforehand.

i have no problems with the tubes ive tested
i have some old ones and just thought whats the value mean in resistance.
thanks for your info.

 

[Koreth]

Re: Tested 12AX7 resistance

The only two pins which you should have any continuity between (any resistance reading at all that isn't infinity) should be pins, 4, 5, an 9. These pins are for the heaters. Between any one of those pins 3 or 8 there should be no reading as there should be an insulative layer between the heather and the cathodes. I highly doubt that a digital volt meter could apply enough voltage across any two other pins for the tube to power up and function, so any reading between any other pins should read as an open circuit.

Either way, the resistance readings of the heaters in the tube is no indication of the strength of the tube itself.

 

[trevorus]

Re: Tested 12AX7 resistance

Your multimeter will tell you almost zilch about your tubes. That's why they build test meters for tubes. It runs them at voltage and tests them that way. Tubes don't conduct until they are hot, hence the reason for the heaters. Oh, and that is what you were testing on those pins, the heaters.

 

[Invader09]

Re: Tested 12AX7 resistance

Buy an old tube tester.

 

[surface54]

Re: Tested 12AX7 resistance

cool thanks do0ds

 

[zzmoore]

Re: Tested 12AX7 resistance

Nice Fluke. I wish my Amprobe could do 10 amps like your meter does....