I would like to talk a little bit about tube testers and explain why we do not use one here at the shop due to their unreliability and limited usefulness in trouble shooting tube equipment problems.
The absolute best test for a tube is testing it in the actual piece of equipment the tube will be used.
The field service tube tester was developed for the telephone, radio-TV, communications and industrial electronics industries to provide basic tube testing capabilities to help technicians and engineers locate defective tubes. The early testers only tested cathode emission. They worked fine in the early days of the industry before the circuits and tubes became more sophisticated.
Over the years there were many approaches to tube tester design. The features, accuracy, and the tests available differ widely by make and model. Some manufacturers wanted to focus on simple low cost units to find weak or just bad tubes. In all cases tube testers were at best a set of balances and trade-offs in the evaluation of the tubes vs. the cost of the tester.
Even the best service testers made trade-offs in design to allow for simplicity of use, the ability to test many different types of tubes, features, and accuracy, balanced by the price of the tester. The service tester was a piece of test equipment to be used by professional engineers and electronic technicians to aid in the process of repairing tube electronic equipment.
Testers were designed with the understanding that those using them were knowledgeable in tube operation, the equipment the tube was used in, and how the tester worked in evaluating tubes. This is more often not the case today when someone acquires a tube tester and attempts to use it.
There are many types of tube testers and most of them date from the early 1950s to late 1960s. The usefulness of each make and model will vary with the type of equipment you are servicing. Considering the fact that these testers are vintage, the age, actual usage, and condition of the tester will have a big impact of how useful it is in testing tubes today. Over the years, moisture, dust, and dirt can be absorbed by tube sockets, switch wafers, and wiring causing many leakage paths that would contribute to false readings. Calibration of the tester is another factor that will affect accuracy and usefulness of the tester. Many simple testers do not have any provisions to make internal calibration adjustments.
Tube testers fall into two main categories- emission testers and mutual conductance testers. Other types of testers would include laboratory and special purpose testers.
Emission testers are the most common testers. Popular brand names include Heathkit, B&K, and Sencore. There are a few problems using any emission tester. The emission test basically connects all of the elements of the tube together, except the cathode, and tests it as a diode for cathode emission. The meter scale is most commonly labeled “Bad-?-Good”. Most tubes depend on the control of the electron flow rather than the amount of electron flow. This important shortcoming means that emission testers will miss the output tube with a cathode ‘hot spot’ which is hidden when the grid is tied to the plate. When a cathode has a ‘hot spot’ most of the emission current stems from this restricted area on the cathode. A control grid does not have the current control action under this condition and when the tube is placed in an amplifier, it draws excessive current and goes into thermal runaway.
Cheaper emission testers used a low current power supply. A tube tested on a tester with a low current power supply may have enough emission to yield a “Good” reading on the meter, but when the tube is placed in an amplifier that is drawing 150ma to 200ma, the tube may not be able to provide sufficient current for operating at peak efficiency.
Emission testers frequently operate at low voltages, with some testers as low as 30 volts. These testers only provide a static test of the tube that does not represent the conditions the tube will be subject to in an actual circuit in which it would be used.
Some emission testers apply too much current to small signal tubes and when the emission test button is held down on the tester for an extended period of time, the cathode is stripped rendering the tube useless. Another shortcoming of many emission testers is the leakage test. In testers where all of the elements are connected together all of the leakage paths are in measured in parallel, which could cause perfectly functioning tube to fail the leakage test. Some testers were advertised as having the most sensitive leakage test in the industry. This many times was more stringent than circuit application requirements and again causing many functional tubes to fail testing.
Most mutual conductance testers work by applying an AC voltage to the control grid of the tube, while maintaining DC voltages on the plate and screen grid. Most of these testers use the 60Hz line frequency transformer coupled as the input test signal. The cathode can be biased with a small positive DC voltage, or the control grid can have a small negative DC voltage. This setup actually dynamically measures the AC gain of the tube, rather than the actual transconductance.
A variation of this type of tester is the “grid shift” approach. This uses a DC voltage on the control grid that is shifted and the change in plate current is measured. Tube theory says that transconductance is the ratio of change in grid potential to the change in plate current.
Many believe the AC (dynamic) method is superior because it reflects true RMS values, regardless of waveform distortion. If the line voltage is not a true sine wave, which is common in heavy industrialized areas, the dynamic tester will still indicate correct values. Hickok held the patents for and made most of the mutual conductance testers.
Many low priced “conductance” testers used AC voltage on all of the elements can actually damage high transconductance tubes. The heavy rectification current caused by driving the control grid positive can overheat the grid wires, resulting in critical spacing to be upset. The tube then actually loses transconductance.
There are special purpose tube testers that are usually special test fixtures that are used to test a specific tube for a specific purpose. These special test fixtures are usually not available commercially and are usually built by the manufacturer or individual who intends to use them. These are usually found on assembly lines for grading tubes to be used in a certain application.
As was stated at the beginning of this article, the best test for a tube is to test it in the equipment in which it will be used. If you intend to acquire a tube tester, be aware of its limitations and realize that some good tubes will test bad and some bad tubes will test good under certain conditions.