Tube tester
A tube tester is an electronic instrument designed to test certain characteristics of vacuum tubes. Tube testers evolved along with the vacuum tube to satisfy the demands of the time, and this evolution faded with the tube era, yet with the High-End advent, they became a pricey piece of equipment. The first tube testers were simple units designed for specific tubes to be used in the battle fields of World War I by radio operators, so they could easily test the tubes of their communication equipment.
Types of tube testers
Filament continuity tester
The simplest one is the filament continuity tester, usually with a neon lamp connected in series with the filament and a current limiting resistance fed directly by the mains.
Tube checker
The simplest of all tube testers. Tubes are used as low power rectifier, at a fraction of it's normal emission. By mistake referred to as Emission Tester sometimes.
Emission tester
Next in complexities 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.
Older testers may call themselves Plate Conductance if the ammeter is in series with the plate, or Cathode Conductance if the meter is in series with the cathode.[1]
The problems of emission testers are:
- they do not measure key characteristics of tubes, like transconductance
- they do not perform the tests at real load, voltages and currents
- they test the tube under static conditions, which are not even near the dynamic conditions the tube would work with in a real electronic device
- tubes with grids, might not even show the real emission because of hot spots in the cathode, hidden by the grids under normal conditions
The advantage of an Emission Tester is, from all types of tube testers available, it gives the most reliable warning for tube wear out. If Emission is at 70%, transconductance can be at 90% still, and gain at 100%. Best and most popular Version is the Funke W19.
Short circuit test
Usually, emission testers and better testers have a short circuit test which is just a variation of the continuity tester with a neon lamp, and which allows to identify if there is any shortcut between the different electrodes.
Parametrical tester, DC Type
This tester Applies DC voltage to the Tube under Test, and datasheet values are verified under real conditions. Good examples are Funke W20 and Neuberger RPG375.
Parametrical tester, AC Type
This tester Applies AC voltage to the Tube under Test, and datasheet values are verified under conditions which simulate DC operation. Most popular are the AVO testers.
Mutual conductance tester
The mutual conductance tester tests the tube dynamically by applying bias and an AC voltage to the control grid, and measuring the current obtained on the plate, while maintaining the correct DC voltages on the plate and screen grid. This setup measures the transconductance of the tube, indicated in micromhos.[2]
Dynamic conductance tester
The dynamic conductance tester is just a variation of the emission tester with its implications, where a proportional AC voltage is applied to each electrode. This tester exists thanks to Jackson mainly to avoid infringing the patents of the mutual conductance tester held by Hickok, but obviously do not provide the same measurements.
Oscilloscope tube curve tracer plug-in
A full set of characteristic curves for vacuum tubes, and later for semiconductor devices, could be displayed on an oscilloscope screen by use of a plug-in adaptor, or on a dedicated curve tracer. An example is the Tektronix 570[3]; this instrument as of 2011[update] is rare but still sought after, and sells for thousands of US dollars.
References
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- [1] The idiot's guide to tube testerspl:Tester lamp
