Tube Testers Exposed
How good is your Tube Tester ?
Foreword:
Unfortunately, it appears the website ‘hickoktubetesters.com’ is no long operating. This is a shame, as the article mentioned below was well-researched, well-written, and a valuable insight into the problems of trusting a simple tube tester. In the interest of keeping the information alive, I will highlight a few key points, and ‘reprint’ some pertinent test results. Hopefully, the website will be back up and running soon. In the meantime, we’ll have to make do with this Reader’s Digest version.
Testing the 12au7 on various Tube Testers
Introduction
Take a batch of tube testers, all in good working order and carefully calibrated following the manufacturer’s instructions. Separate them by type – mutual conductance testers in one group, emission testers in another. Wouldn’t you think that if you ran the same tube through all of them, you’d get comparable readings from all of the testers within their respective groups? That would be logical – and wrong. As it happens, each tester applies unique conditions to a tube under test, so readings from tester to tester, even on the same model, will not necessarily be comparable. Not only that: readings obtained on mutual conductance testers other than laboratory models usually won’t match those published in standard tube manuals. Why? Because manuals assume a set of operating conditions that most testers simply cannot duplicate.
The tube chosen to serve as the standard was an Amperex NOS 12AU7, selected because this manufacturer’s tubes have a deserved reputation for ruggedness and a lack of microphonics. The 12AU7 is first tested on an RCA WT100A, one of the best laboratory-grade tube testers ever made.
The results:
- Section 1: 2400 micromhos
- Section 2: 2480 micromhos
Then the 12AU7 is checked on the testers listed below. All had been rebuilt, calibrated using the manufacturer’s procedures (except Jackson and Precision testers, for which instructions were unavailable), and tested. Some mutual conductance testers read in micromhos, others (and emission testers) have a scale with index numbers such as ‘0’ to ‘100’. Readings taken from numbered scales are given in the appropriate column (e.g. “99”) and the tester’s full-scale reading is provided (e.g. FS = “130”). The ‘reject point’ is also provided (e.g. RP = 50).
Observations
Testers within a given family read quite close to each other. Achieving this degree of repeatability between units is obtained by precise setting of the signal voltage. The Hickok models showed greater than expected variation in readings among the different units. The positioning of certain components, such as the electrolytic capacitor across the quality meter, seemed to change. While component values may have remained essentially the same, even slight alterations in lead dress could affect operation. Indirect evidence of this is Hickok’s use of ferrite beads on its commercial testers over time. These beads were an attempt to cure oscillation problems in the testers. This kind of doctoring is hardly an exact science and may not have resulted in total cures. Oscillation in tube testers will generally cause a bad tube to read much higher than it should due to regenerative feedback.
Practical Applications
This article shows the range of results that can be obtained in testing a single tube. It should also furnish a means of comparison between two testers of different types. There are two additional factors to consider when comparing tube tester readings. On such testers as the Sencore and B&K models, there is no manual adjustments to compensate for AC line voltage variation. Differences in AC input voltage to these testers are compensated for by circuits within the testers themselves. These systems are imperfect, and if you vary the AC input to the testers from 110 volts to 125 volts, you will observe a change in quality reading for the tube under test of about 5 to 10 percent. An additional factor to consider is how long the tester has been turned on. Many models are not temperature stable. If you test a tube when the unit has been on for 10 minutes and test the same tube 2 hours later, your results will probably be significantly different. For checking calibration and making other critical tests, a minimum half-hour warm-up is advisable.
Article by Chris Haedt – March 2002
Postscript
While not addressed above, another factor to consider is the following. Some people set or reset their ‘Line Adjust’ control (if your tester has such an adjustment) after the tube under test has warmed up for a time, while others set the control at the beginning of the testing procedure, and do not readjust it. This does make a difference. Many testers, -like Hickok, give chart numbers assuming you do not reset the ‘Line Adjust’ control; you set it immediately after inserting the tube (this gives you something to do while the tube is warming up!), and leave this setting alone. Readjusting the control will alter your readings. This isn’t a life-or-death catastrophe if your readings are ‘off’ a few percentage points, but the anal audiophile will most certainly be upset to learn this fact. Don’t tell him! Always keep in mind that no tube tester was designed to be a final judge, jury, and executioner of any tube. The lowly tube tester is used to check for leakage, emission, and/or to verify AC amplification capabilities. That all, folks.