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I have very strange issues with testing NOS tubes. I have two tester: Hickok 6000A and Heathkit it-3117. I take a NOS 12ax7 tube and test it in Hickok - depending on the time of the day and lunar phase it shows that the tubes is strong or really weak. It's unpredictable. Then I test it in Heathkit (emissions only) the tube is always at 100% of emissions (I calibrate it every time by adjusting the scale to 100% by using a know NOS tube). Is that a know issue with Hickok? I know that they read low for 12ax7, but randomly???
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I would not use one
If it tests good on the same tester, it might be.
From very old memory RCA 12AX7's from the mid-70's often tested below minimum on a Hickok 539B or C but usually worked OK in scopes and test equipment.
The tubes would also pass a life test where the filament voltage was reduced and the test repeated - The Gm would not drop much if at all.
By comparison the Telefunken version would always test at 150 - 200 % of the RCA readings but work just as well as the RCA's in most circuits (not better). Probably a tribute to good circuit design back then.
Since low readings are not your problem but rather consistency of readings, more is at issue with your test set i'm afraid.
You might make a "breakout socket" to add cathode resistors (10 or 100 ohm) and measure tube cathode current with a digital millivolt meter.
If the DC current value stays constant but the Gm reading changes a lot over time I would suspect the AC grid signal level is changing - maybe a capacitor went "west" and needs to be replaced (W A G). The test set uses a precise AC grid voltage that when amplified by the tube produces a signal from which tube Gm is derived and displayed on the meter..
TUBOS
...I have a 752A with loose contacts on the 9 pin socket and it causes intermittent readings similar to what you describe. Traditional contact retensioning is a temporary measure. The real solution is to replace the socket....but that won't be happening anytime soon because it's really a 10 pin socket...nearly unobtainium. Will probably perform a contact transplant procedure when I get the time and nerve.
and on the Hickok, you could have a failing carbon resistor or a cap that needs to be replaced. Could be a number of things.
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I don't think it's contacts since I tried it in another socket with an adapter and still the same behavior. One thing that I need to note that it's only with tubes from one manufacturer. The tester shows me that Tubes is at minimum good, but emissions on the other one are at 100%. For all other tubes if mutual conductance goes down emissions always go down - but not for these tubes. Very strange. And in the amp they behave like champions O_o
Another interesting point, I used in my amp for a couple of days - it was measuring 1.1 millimhos where rejection is 1.2 - worked beautifully. Now I tested it on Hickok and it shows 2.5!!! Every time. Is it possible that it's burning in?????
What brand of tubes are these? It's important to know, because I probably have the explanation - if I know what tubes they are.
These are Ei 12ax7 tubes.
Okay, thanks.
The Ei quality was pretty awful towards the end of production - but I don't know whether that is what you are seeing. You may have tubes with cathode poisoning, and they need to "wake up" (by getting rid of the contaminants on the cathode surface) before they operate normally.
I'd recommend you run them for a few hours before testing them. And remember - the ultimate test of a tube is whether it works properly in a circuit!!
Thank you for all your help. I will continue playing around with the tubes to see if I can detect a pattern :) So far although they show below minimum on Hickok I'm loving them in my amp! Very surprised by the quality of sound.
...Although it should be a gradual phenomenon, not intermittent. I've experienced this with the 12AX7LPS. Another possibility is that the tester is oscillating under certain conditions. This is a lot more likely with hi gm tubes like 6550 and almost impossible with something like a 12AX7. One check is to put your hand over the tube while under test. If it's oscillating, the reading will jump around a lot while you do this. Hickok got around this tendency by using ferrite beads on all socket leads on their later testers. Another point to keep in mind is that a lot of Hickok testers way overdrive hi gain tubes like the X7 while under test. Only the best versions use an appropriate 0.25 or 0.5VAC as the test signal.
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