Here we have a common advertisement for the Precise Model 111 tube tester. It is a well written sales pitch, and explains exactly why this tube tester really does do a better job than much of its competition. The above advertisement is from the January, 1956 issue of Radio-Electronics. What is so special about this tube tester? A few things are worth mentioning.

• A tube can be tester for either mutual conductance or emission. Yep; a 6L6 can be tested for AC amplification, or connected as a virtual diode and tested for plate conductance. Why you would need to do this is a mystery, but I like the option of testing the amplification capabilities and/or the condition of the cathode coating.
• Rather than a single plate voltage (as seen on most other tube testers), the Precise 111 has six different plate voltages, and you select the one most appropriate (or the one most likely to be found in the circuit) for the specific tube under test. This includes selecting voltages to mimic the 'proportional AC voltages' found in Jackson tube testers. These voltages are still a little less than what is in a Marshall Major, but a high transconductance tube is not tested at the same voltages as a low transconductance tube. This makes for far more accurate readings.
• Also along the path of 'different voltages for different tubes' is the fact that you select the appropriate Grid signal as well. Most Hickok tube testers just use one 5VAC test signal; far too much for high transconductance tubes like the 12AX7. Some testers, like the B&K 747, will always test a 12AX7 on the lower end of the 'scale'. This is because the excessive Grid signal forces the Grid to start drawing current, and this loads down the test signal! The reading is therefore lower than what would be expected. This is compensated for in the tube chart. Look for yourself on most any B&K tube tester (700, 707, 747); the listing for the 12AX7 will have the note: "Tube 'Good' if it reads '22' or more."
• Grid bias and filament current(!) are also measured on the meter. This helps you determine if the tube is working under conditions that it would experience in a real circuit. The filament current test is useful if you were a 1950's TV repair technician, and needed to test the behavior of series-string 600mA tubes.
• Because you can meter everything happening with any tube under test, you can do a better job of 'matching' output tubes or dual-triode 'halves'.

This all adds up to one very good tube tester. However, in the practical world, the Precise 111 has a few things going against it as well. What could be the downside to such a complete tube tester?

• As I'll explain below, you absolutely must have the manual to make the Precise 111 work for you.
• With all the things the Precise 111 will measure, we need to calibrate everything. There are multiple 'gm' scales, and each needs to be calibrated! We also have to calibrate the bias voltages, the filament current(!), emission current, etc., again on each 'gm' scale. This is a pain-in-the-ass, as tweaking up each of the 1,209 potentiometers seems to 'upset' the others. Ouch. If you have a lot of patience, the Precise 111 can be calibrated in less than an afternoon, and seems to work well. If you do not need accuracy down to +.000001%, you can calibrate the Precise 111 in about an hour. Below is a photograph of the interior of the Precise 111, along with six of the aforementioned calibration potentiometers. You may also note the two rectifier tubes used in the circuit.

• The weirdest part about calibrating the Precise 111 is that we need 'reference tubes' to do the calibrating with! If you do not have the luxury of owning a 'reference tube', you can follow these instructions according to the large owner's manual; "It is suggested that you take the average of several new tubes - preferably of different manufacturers." Thanks.

All in all, I really like the Precise 111, and use it as one of my 'go-to' tube testers. To really check a tube over, I will put it through its paces on my Precise 111, my B&K 747, and my Hickok 752A. I know that's overkill, but I do a better job at finding borderline tubes with gas, microphonics, or leakage. Plus I can really match 12AX7 'halves'; always handy for critical phase inverter applications. It ain't how Leo would have done it; but with today's tubes it is a necessary evil. As a few final notes; the Precise 111 is also available as the Realistic TK133. Precise also made a 'little sister' to the Model 111, which was dubbed the Model 116. An example can be seen below.

As you may note, the panel is much simpler, with fewer controls. There are numerous octal sockets, as an example, so the 'Selector' switch must be set to the appropriate circuit. Also, there is no roll chart this time around; instead a printed set-up chart is affixed to the top lid. You cannot meter the bias voltage, nor the filament current, and there is one 'gm' scale for all tube readings. But you are measuring the transconductance, and that's the main objective. I have no advertising for the Precise Model 116, so I cannot tell you a list price, nor when it debuted on the market. But it is safe to say it was a simpler, less costly version of the Model 111.