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I acquired a Hickok 539c tester a while ago, calibrated by local expert. It has current measuring capability. I test a LOT of tubes but the common 12AX7 is sometimes a mystery. I get tubes that will read high in gm, say 1300 but only pull 0.5 mA current. Others might test 1100 gm but pull 0.9-1.0mA. The 1.3 grid volt setting is sensitive.
Anything below 0.6 mA, I usually toss. But they still sound decent in amp circuits. Which reading is more important for 12AX7, gm or mA?
Thanks for advice.
Follow Ups:
Pete. Nice that your 539C has been enhanced to measure current. I am curious why you apply 0.6ma Ip as the threshold for whether to keep or dump 12ax7's? And what threshold do you apply for GM?
Per your mention the grid voltage setting of your 539C is sensitive. The existence of line and bias meters on the 539C are an excellent feature. But the accuracy of those meters is relative loosey goosey as the small size of each meter's scale provides only a vague understanding of what the line and bias voltage is.
I owned a 539C for a while that was enhanced with many Roger Kennedy upgrades including digital line, bias, and current meters. Should you have the interest upgrade to digital line and bias meters such as Roger uses. The benefit is your test results will be significantly more repeatable.
Nice tube tester!
Well, gm minimum for 12AX7 is very clear at 875, but very few read that low. As a (related) aside, I once got a boxful of used, rejected tubes (mostly Russian of various guitar amp brands) from a local amp repair guy. All the tubes tested at 0.6mA or lower, though gm was still "acceptable". Most were disposed of, but there were a few good ones above .9 in the box. My original question remains, how important is mA draw if the 12AX7 pulls half of new mA at these standard 539C settings? "Half of new" sounds like "bad", regardless of gm over 1100. :)
Pete
"... if the 12AX7 pulls half of new mA at these standard 539C settings? "Half of new" sounds like "bad", regardless of gm over 1100. :)>
You would need to change the cathode resistor to bias the tube back up before making a judgement call on the sound.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
What happens to the gm on the tube that reads 1300gm and .5ma when you re-bias it for 1.0ma?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
That means lowering the grid bias to around .8-1 V. The gm goes way up, to around 1800.
(I can make any poor performing tube read high that way. )
Pete
If a tube sounds great but measures poorly just let friends and family know, it would be a shame to throw out a great sounding tube. Contact me if you need a proper place to discard these tubes.
For commercial sale and use, most people seem to use Gm. The cut off is 1250 for your Hikock If I recall correctly.
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