Jackson 648 tester, recently calibrated, giving very high readings for tubes.

[TankAudio]

Hi Guys!

I didn't think the 648 was much of a looker at first glance, but now that I've had time with it I've really warmed up! I have invested about 7 hours learning about it, and installing all new resistors within tolerance and .1 cap, after using the unit and finding that it was already in good shape and worth keeping.

Here is the guide I used to calibrate and service the tube tester
TubeSound >> Blog Archive >> Jackson 648-series Rebuild & Calibration

The first 20 12ax7's I tested have given readings between 93 and 125% of relative micromhos. My former tube tester could test a good/bad tube, but the meter always pinned MAX when something was functional. So I could not use that tester to confirm any of these new readings.

Does anyone know if readings over 100% are very common for 12ax7s? The bulk of the tubes I used were new Mullards and vintage USA tubes.

 

[nigelwright7557]

12ax7's have a gain of around 100.
They are very sensitive.
I tend to go for 12au7 to keep noise/gain down a bit.

 

[TankAudio]

From the website:

The explanation you must read

It’s the RP-value, NOT the MU-value that’s Important!

To many people, the most important characteristic of a vacuum tube is its Amplification Factor (mu). Unfortunately, this is not true.

Of the vacuum tubes’ three operating characteristics – Dynamic Plate Resistance (rp), Transconductance (gm) and Amplification Factor (mu) - two are explicit values and one is an implicit value. The gm and rp values are explicit because they come directly from the tubes’ actual operating voltages and currents. The mu value, however, is an implicit value that is mathematically-derived from the product of the rp and gm values:

This means that even though a 12ax7 is understood to have a gain of 100, I should still test and rate the tube based on its Gm.



The line voltage adjustment is calibrated, using a solid state solution to replace the 1S5 vacuum tube inside the Jackson 648. This leads to long-term stability and better accuracy of the voltages inside.

All resistors have been tested out of circuit or replaced so that all 1% resistors are now .5%, and all 5/10% values are within 2% tolerance. All contacts have been thoroughly cleaned, the pots have been opened up and cleaned manually.

The meter adjustment has been calibrated using a small DC PS and a Fluke current meter.

I know the 648 is very popular, I will see if I can contact the original author of the calibration web link.