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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. :)
"... 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.
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