Tube tester

A tube tester is an electronic instrument designed to test certain characteristics of vacuum tubes (thermionic valves). Tube testers evolved along with the vacuum tube to satisfy the demands of the time, and their evolution ended with the tube era. The first tube testers were simple units designed for specific tubes to be used in the battlefields of World War I by radio operators, so they could easily test the tubes of their communication equipment.

The most modern testers perform a multitude of the below tests and are fully automated. Examples of modern testers include the Amplitrex AT1000, the Space-Tech Lab EasyTubeTester, the Maxi pre-amp tester and the maxi-matcher (power tubes only) by maxi test and the new, and somewhat more primitive, DIVO VT1000 by Orange Amplification. While the AT1000, EasyTubeTester and the Maxi-test brand testers offer precise measurements of transconductance/Gm and emissions/iP at full or near full voltages, the Orange tester offers a very simple numerical quality scale. The EasyTubeTester has a unique feature of quick tube matching +/-percentage display.

The simplest tester is the filament continuity tester, usually with a neon lamp connected in series with the filament/heater and a current limiting resistance fed directly by the mains. There is therefore no need to select the appropriate filament voltage for the particular tube under test, but this equipment will not identify tubes that may be faulty in other (more likely) ways, nor indicate any degree of wear. The same checks can be made with a cheap multimeter's resistance test.

The tube checker is the second-simplest of all tube testers after filament continuity testing. Tubes are used as a low power rectifier, with all elements other than filament connections connected together as the anode, at a fraction of its normal emission. By mistake referred to sometimes as Emission Tester because they are a crude measure of emission in directly heated types (but a measure of unwanted heater-cathode leakage in indirectly heated types). Switches will need to select the correct filament voltage and pins.

Next in complexity is the emission tester, which basically treats any tube as a diode by carefully connecting the cathode to ground, all the grids and plate to B+ voltage, feeding the filament with the correct voltage, and an ammeter in series with either the plate or the cathode. This effectively measures emission, the current which the cathode is capable of emitting, for the given plate voltage, which can usually be controlled by a variable load resistor. Switches will need to select the correct filament voltage plus which pins belong to the filament and cathode(s).

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