Generally, there are 2 kinds of tube testers. The more-basic one tests for emissions, while the type that tests actual output measures 'transconductance'. Most manufacturers made both types; the emissions testers (ETs) were less expensive, smaller, and more portable, so there are a lot of them around. Most tubeheads prefer a transconductance type (TT), but each of those had limitations. I strongly recommend you buy a copy of Alan Douglas's 'Tube Testers and Classic Electronic Test Gear', still available new.
One meaningful limitation of TTs is that most if not all of them have a limited number of bias Voltages. If one of those Voltages isn't all suitable to the tube you're testing, the TT will test the tube for emissions. I think one question your should ask is whether the tester in question tests both your tubes for transconductance or for just emissions; the tube chart will indicate. FWIW my Mercury model 1000 tests both for transconductance.
One other requirement--don't even think about buying a tester without an operating manual AND a tube chart.
Good luck, and try to not get too frustrated in your quest.
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One meaningful limitation of TTs is that most if not all of them have a limited number of bias Voltages. If one of those Voltages isn't all suitable to the tube you're testing, the TT will test the tube for emissions. I think one question your should ask is whether the tester in question tests both your tubes for transconductance or for just emissions; the tube chart will indicate. FWIW my Mercury model 1000 tests both for transconductance.
One other requirement--don't even think about buying a tester without an operating manual AND a tube chart.
Good luck, and try to not get too frustrated in your quest.
.