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Most of those "Retail testors" Eico's Lafeyettes, Knights were just basic emission testers. A voltmeter would tell you where the most likely area to check would be. The switches control what pins get what voltages. so take a known rube like a 12ax7 (which will have two settings 1 for each triode section). Pin one should have a plate voltage ( which on these testers is generally pretty low 150-180 volts). pin 2 will be 0 volts and pin three will have the bias voltage which will be rather low also around 1 to say 5 volts. The second triode pin numbers will be plate -6 grid-7 cathode -8. The filaments are pins 4-5 & 9. If the tester puts out 12.6 volts then pins 4 & 5 will be incircuit with pin 9 open. If the tester puts out 6.3 for the filaments then pins 4 & 9 or pin 5 & 9 or 4-5 & 9 . If all the voltages check out then it is probably the meter and or dirty contacts. When the test button or swithc is closed a small voltage is applied to the grid causing the tube to conduct, which the meter is in circuit to read. These testers are so straight forward I think you will have no problem ferreting out the problem.
Terry
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