Here are but a few samples of 'emission' testers. You will see these testers frequently on eBay, and the prices are not that much lower than B&K 'transconductance' testers, as one example. This is a prime example of sellers only understanding that there is money to be made on eBay, and not how the very tester they are selling works, or if it works at all! You'll see many auctions claiming 'I don't know anything about these things, so it is being sold "AS IS".' Or else something along the lines of 'Untested, and being sold "AS IS".', or I have no tubes to test this thing out with, so it is being sold "AS IS".' To be fair, I've acquired many tube testers from the original owners, who insisted that the tester was fully functioning, only to get it home and discover it was anything but. Unless you walk around carrying a tube in your pocket should the opportunity arise, it is always a gamble.
You can often spot an emission tester by the absence of fancy switching arrangements. Typically, there will be a filament voltage selector, and perhaps two other controls; one could labeled 'Load', and the other 'Circuit'. Some testers simply labeled these controls 'A', 'B', and 'C'. Also look out for faceplate labeling along the lines of 'Qwik-Check' or 'Dyna-Test', or 'Rapid-Test', or something similar. You may also notice emission testers usually have a simple 'GOOD'/'BAD' scale, and a percentage rating designation. Mutual conductance testers usually have the scale marked in 'micromhos'. There is also usually a range selector switch, seeing as all tubes can't read accurately on just one 0-15,000 micromho scale.
Even when mutual conductance testers became vogue, the emission testers fought back by advertising how fast you could test a tube on an emission tester! As a piece of useless trivia; Radios did the same thing, as the very first sets had multiple tuning knobs that had to be adjusted at the same time! Later, when multi-ganged tuning capacitors became available, ads popped up stating how fast and accurate a certain radio could be tuned. Tube testers followed suit with those cute nicknames and ads stating how much time the service technician can save by using that tube tester. "No more wasting valuable time setting multiple switches! No more cumbersome roll charts to fight with! Just two simple controls to set and any tube is fully tested in under 10 seconds!". The latest emission testers sometimes advertised that they tested all tubes 'under full load', meaning they simply applied higher voltages to the virtual diode when called for by setting the selector switches. Sometimes this control is even labeled 'LOAD'. While this is 'better', as the tube's emission can 'sag' under heavy current demands, it's still an emission tester. A final problem with cheap emission testers (and cheap transconductance testers, as well) is with the 'Shorts' test. This necessary test is made with an AC voltage that is applied between each tube element, where leakage will pass enough current to activate an indicator in series with the circuit. It is important not to further test any tube which fails a 'Shorts' test. Doing so usually results in damage to the meter movement. In the 'Shorts' test circuit, a series resistance must be used to keep the lamp from burning out when there is a 'direct' short between two tube elements. This series resistance places a limitation on the maximum resistive short that can be indicated to about 1 Meg. There are TV and radio circuits where 1 Meg of leakage resistance will have absolutely no adverse effects, and there are others where this would disable the circuit and cause confusing symptoms. This leads to many TV technicians deciding after the second or third time he was led astray by his tube tester to put it away, and use tube substitution as a repair technique. A crude idea of how this particular test circuit works is shown below.
Fancier emission and even transconductance testers (like my Hickok 752A) sometimes have an ohm-meter scale on the meter movement; utilizing the 'Shorts' tests converts the meter to the ohms scale, and a leakage reading is done. This is much more sensitive, and is usually only found on more elaborate (and more expensive) testers. A crude representation of how this might be implemented is shown below.
In conclusion; emission testers are suitable for testing diodes and rectifiers only (as these tubes have no 'gain factor' to measure, and can only be tested for cathode emission). A 'good' emission tester will still find 75% of your bad tubes; providing the tester uses a heavy-duty transformer, and your tube is 'suffering' from low emission. If you want an emission tester for having an emergency 'backup' unit, that's perfectly fine. Beware of their limitations (don't ever try and 'match' output tubes on one), and don't pay hundreds of dollars for any emission tester.