If you're getting into tube-type gear, sooner or later you'll want a tube tester. Fact is, you may eventually want more than one. Even if you're lucky (or blessed) enough to score the legendary TV-7 or the Hickok 752A, you'll likely arrive at wanting more than one tester.
Why? Mostly because of the variety of jobs you'll be using your tester(s) for. Checking a few tubes from a patient on your shop bench is one job. A quick triage on a bunch of used tubes you scored at the swap meet or hamfest is another matter. Selecting the best available tube from a dozen candidates in your stash is yet another matter.
Apart from all that, your tester may not be capable, for example, of grid emission or leakage tests, and you'd like to have one that could do such tests. It may be too old to handle the newer tubes, or too new to handle the older ones. And so it goes. Even if you have one of the big-leaguers like the TV-7 or 752A, you may get tired of wearing it out on jobs that could be performed adequately by simpler and less expensive units (and God knows the simpler ones are expensive enough these days!
You may have got far enough into the RCA Receiving Tube Manual to find these forbidding lines:
"The tube tester...cannot be looked upon as a final authority in determining whether or not a tube is always satisfactory. Actual operating test in the equipment in which the tube is to be used will give the best possible indication of a tube's worth."
True enough. But on the other hand, nice work if you can get it. It's not always possible to plug newly-acquired tubes straight into a piece of gear, and when you're vetting a boxload of new arrivals from the hamfest or swap meet, you may not care to test them "live" anyhow, especially where rectifiers and power output tubes are concerned. You may damage the gear you're using as a tester, perhaps seriously, perhaps mortally!
So tube testers are more than worth having around. But what types are you looking at?
There are two basic kinds. The simpler one of the two is the emission tester; the more complicated one, the mutual conductance tester.
The emission tester tests the most important aspect of a tube's function, cathode emission. It is the "quick-and-dirty" tester, but it may serve as much as 80% of your needs. Those boxy drugstore and supermarket testers of yesteryear were emission testers. So were most of the testers you could buy as kits. If you walked into any radio-TV repair shop of that era, you'd almost certainly find a tester like this one on their bench:
This is a Superior Instruments (SICO) model TW-11 tester from about 1957, simple and straightforward to set up and use. It will test for shorts, leakage, and even microphonics, with a magnetic headphone plugged into the service jack. It may or may not catch grid emission (that would show up as leakage), but it is still adequate for most jobs that don't require precision testing. Since it may be set up and used quickly, it's the ideal thing for vetting that boxload of new arrivals from the fest.
This "bad-?-good" scale is typical of emission testers. Note that there is a supplementary "B" scale, meant for low-drain tubes (mostly detector diodes and battery tubes). Quality is read on a scale of 100% as ideal, although in practice, very few tubes will ever approach that 100% reading.
Now for the mutual conductance tester, which reads the best all-around measurement of a tube's usefulness as an amplifier, its mutual conductance, better known as transconductance.
Transconductance amounts to the change in plate current
divided by the change in control grid voltage
that causes the plate current change (the plate voltage being fixed at a desired value). The greater the transconductance of a tube, the greater the possible amplification.
Mutual conductance testers come in two flavors. One (the older type) is the "static" type. In this tester, a tube is set up under controlled operating conditions. A plate-current measurement is taken. The control grid bias is then changed, and a second plate-current measurement is taken. The difference between the two plate-current measurements is indicative of the tube's transconductance.
On the other hand is the more modern "dynamic" type. In this tester, the tube is tested under conditions approaching real life. It is set up in a fundamental amplifier circuit and fed an AC signal, most often from an AC transformer feed, but sometimes from a dedicated oscillator. The AC component of the plate current is measured to arrive at the tube's transconductance.
This is a Weston 981 dynamic MC tester from 1954. You can instantly see that this tester requires more "setup" than the little SICO emission tester pictured above. It will check for leakage as well as shorts. It gets its test signal from a type 3A4 oscillator. Like the SICO, it is too old for Compactrons, Novars or Nuvistors, but both testers will handle subminiatures. The Weston will also handle acorn tubes.
Notice that on this Weston and the SICO as well, the 9-pin test socket has been replaced. If you do a high volume of testing, then get in the habit of using a "socket saver" adapter with your testers, especially with the miniature sockets, which tend to wear out IN A HURRY
The meter is scaled in micromhos, the unit of transconductance. Again, diodes/rectifiers have their own reading scale (since they receive a different kind of test, being incapable of amplification). VR tubes also have their own scale, measured in volts.
Many tubes would read off the transconductance scale on this meter if the tester did a "straight" reading. Instead, it often does a proportional reading, and you are instructed to multiply the resultant "X2," "X3" etc. Thus, a reading of 1200 with an "X3" instruction would amount to 3600 micromhos.
This is a B&K 707 "Dynajet" hybrid tester from 1965. This tester was meant for the service shop bench and is very easily set up, compared to the Weston. It has an upper rank of "preset" sockets covering the types most likely to be encountered on the bench. These get transconductance tests. A lower rank of sockets and controls deal with other, less frequently encountered tubes, which get only emissions tests. Although it has no sockets for the old 4-5-6-7-pin tubes, it will handle Novars, Compactrons and Nuvistors.
This tester, being a hybrid, uses the "bad-?-good" meter of straight emission testers, and doesn't give you any actual transconductance measurements. It does, however, have a grid emission scale.
Now for a reading from all three testers on the same tube, a weak 6F6G pulled from a recent "patient on the bench."
First the SICO, with a reading just over the threshold of "bad:"
Then the Weston 981, showing a reading of roughly 620 against a designated rejection point of 830. With an instruction for "X2" reading, this translates out to an actual reading of 1240 micromhos (against the tester's specified rejection point of 1660). Against a designated (tube manual) new-tube value of 2500 to 2550 (depending on circuit), this reading translates to just under 50% of specified new-tube capacity.
And now the B&K Dynajet, showing a reading roughly equivalent to that of the SICO.
Although the three testers were in general agreement on this tube, you'll often get dissimilar readings from the same tube on different testers. However, there's nothing to worry about as long as the readings are in the same ballpark. Remember, the emissions tester gives a rough-and-ready reading, and the MC tester gives a more accurate one.
It is worth remembering that while mistakes on the tester "roll chart" and/or printed supplements are very rare, they do crop up from time to time. One of these three testers would go crazy whenever I tried to test a 6BQ5 according to the roll chart settings. I managed to get hold of later printed updates, and sure enough, corrected settings for the 6BQ5 were included. I tried these, and everything went OK. Be advised that it is ALWAYS worthwhile to pick up everything in print about your particular tester, supplemental tube data and calibration instructions above all.
It is also worth remembering that whenever testers have any line voltage adjustments (as many do), these have to be reconciled with existing AC line voltage in order to get accurate readings. Some testers have a compensating switch (the SICO, above, has a high-low switch), while others have a pot adjustment (as does the Weston 981 above).
Like any other piece of gear, testers need service from time to time. They have switches, pots and sockets that can get dirty; sockets can become worn and lose their grip, requiring replacement; they have internal parts such as transformers, tubes, resistors, caps, rectifiers etc. that can fail. You should periodically give your tester a parts checkup, prophylaxis and calibration, if you want to keep its readings honest.
It would be hard to pick one of these testers in an "if-I-could-keep-only-one" situation, for each of them is more useful than the others in different respects. The little SICO, for example, will check many older tubes that don't show up on the Weston and B&K specs, and it's very quick and handy when I have a lot of tubes to check fast. The Weston is slower to set up than either of the other testers, and it won't check the later tubes, but on the other hand, it gives me actual transconductance readings, and thus I prefer it for every job it can handle, while I have the B&K to handle newer tubes and special chores such as grid emission tests.
As you get farther and farther into the hobby, don't be surprised to find your work dividing itself among multiple testers in the same way...
Hope this helps and happy hobbying,