am I missing something on my Hickok 539c tube tester
today I stayed in from the heat and decided to test tubes all day.
I got around to testing some 12ax7's and I had some pretty cool old telefunken 12ax7's that looked good but tested a little on the low side at around 800 my tester shows 875 being low
so I wanted to see if any body else was selling any of that type of tube on the low side so I clicked on over to ebay and did a search for (12ax7 telefunken 539c) and I came across this listing that says this guys tubes are coming in on his 539c at 2700.is listing reads
Professional Quality Hickock 539C

Dynamic Mutual Conductance Tested:

Well Matched @

2400/2700 & 2400/2700 !! Gm

2600+ is average new reading

http://www.ebay.com/itm/1954-D-FOIL-RCA ... 257cc48b94

here is a pic of my 539c's roll chart,am I missing something?how can my low side be 875 and his be 2600

2600 is a reasonable Gm but not on the D range of a 539C, which uses an 0.5V grid signal. He must be testing his tubes with another tester.

Thanks Alan that makes sense
while the thread i going I might as wall ask,my 12ax7 tele's are testing on the low side and that hurt my feelings a bit,lol
is there a process or a technique any one has ever figured out short of (opening a weak tubes glass and remelting it back together ) to rejuvenate it back up to better performance ?
I already tried the rain dance over it,I got nothing,lol

Oxide cathodes can't be rejuvenated.

12AX7s normally don't work very hard in a preamp, so weak ones may work fine. They just aren't as saleable.

The guy says he is using a 539c tester, but he must be using a different gm range and making up his own "good" numbers.
I guess it sounds better if the tube tests 2000+ as opposed to in the 800 area.

Only fools pay this much for such an ordinary tube, so it is what a fool believes.

Not sure what that seller was looking at, or for that matter if he had his glasses on when he was testing his tubes. I pulled a couple of Tele's, Brimar 12AX7s and a Sylvania 5751, all NOS and tested them on my 539B. I have verified both the calibration and correlation to other testers. If my curve tracer was up, I would post the values for comparison purposes. On my Hickok 539B the lowest tube "new" GM reading was around 1200, the highest around 1450. Most were around 1250 +/-100. Current was 0.7-0.9 ma, which fits with the plate curves. This GM readings fits with the 539B/C test data with a min. of 875. Some of the other testers will run these tubes with 250V on the plate and a bias of -2.0, the spec is a GM of 1600, current 1.2ma. My recollection was that I normally saw a GM of 1600-2000 when testing these tubes at 250V, do not recall anything higher. I have had some particularly high testing 12AX7 tubes, I also have some of the older (early 40's) RCA 12AX7 tubes, but can't get to them to test.

I think your tubes fit into what would be expected for the low range of GM readings expected, and match with other seller's listing. You might also do a life test to see if there is much change in the GM. The Tele's seem to last a long time, so there are probably a few more miles left on yours. You also might check your tester voltages to see if they are in spec.
mark

Is the roll chart in your 539C the correct one?
The Collums on the roll chart don't seem to match the headings.
There is no heading for the function selector.

Btw, the chart in my KS-15750 calls for a minimum of 950 Micromhos for the 12AX7.

The column headers really line up poorly in that picture, but they are all correct and the roll chart is correct.
I think the camera really distorted the way they line up.

So long as your specimens are performing well, I think you are OK. Other than inter-electrode shorts, and just tuckered out cathodes, these champs keep on going as long as they still light.

The KS-15750 roll chart is not interchangeable with the 539B/C. The KS-15750 bias is filtered DC, the screen voltage does not change with the bias voltage like the 539, signal voltages are different for some of the GM ranges.... Actually the use of filtered DC bias with unfiltered plate DC gives erroneous readings, I have a 580A which also use filtered bias with unfiltered plate voltage, and the plate currents are all off. The GM readings are closer, but without modifications you can not test tubes to book values and get correct readings.

Use the roll chart data setting that is specific to your tube tester. Too many people think they should get the same tube values on different tube testers, and even with the same model you will see quite a bit of variation. If you want to really find out how the tube is functioning, get a utracer tube curve tracer for $250. http://www.dos4ever.com/uTracer3/uTracer3.html. See attached data for a manual jig (regulated PS and digital meters) and uTracer curves for a closely matched reference tube. The uTracer curves are essentially the same. I have tested the same tube (and others) on my AVO CT160 tube testers, with the same plate voltage, it gives almost the exact same values. The little uTracer is quite accurate, and is a fraction of the cost of the Hickok dinosaurs usually going for ridiculous prices, and usually sold as "it came from an old electronics engineer and I am sure it was working when he had it". Yeah, sure!
mark