Tung-Sol.com:: FAQs What is the truth about tube testers?

FAQs

Question:
What is the truth about tube testers?
Answer:
The best test for a tube is in the actual piece of equipment the tube will be used. It is common for design engineers to build a mockup of the circuit being designed with meters and oscilloscope connections in order to evaluate the performance of the tube under the various operating parameters encountered. Early in the development of radio special tube/set testers were used where the tube was removed from the radio, the tester was plugged into the tube socket, and the tube was plugged into the tester socket. The radio and tester were turned on and the condition of the tube was read on the tester meters. This worked well on old 4 pin simple rectifiers and triodes. As more tube types were developed and circuits became more complex, these simple testers did not work or provide enough information to properly evaluate the condition of tubes operating at wide ranges of voltage, current, and signal waveform. The high cost for many adaptors and wide range of equipment required became impractical and not cost effective.

The 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 service 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 emission testers do not have any provisions to make internal calibration adjustments. Another interesting note concerns the military TV-series of mutual conductance testers designed by Hickok. The TV-series testers were ruggedized military versions of commercial Hickok testers. When purchased from military surplus the buyer would usually find a sticker affixed to the tester that reads "CALIBRATION NOT REQUIRED-NOT USED FOR QUANTITATIVE MEASUREMENT". The only time these testers were calibrated was when they were manufactured or when they were repaired or overhauled.

Service 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 Eico, Heathkit, Mercury, 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 other than diodes 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. Also, cheaper emission testers used a low current power supply. A 5U4 rectifier tested on a tester with a 100ma 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, such as the Sencore Mighty-Mite series 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 (usually 1 volt) and the change in plate current is measured. Tube theory tells us that transconductance is the ratio of change in grid potential to the change in plate current. The "grid shift" method is a static test. Engineers 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. Hickok also designed testers for Western Electric, the military TV-series of tube testers, and licensed patents to Stark in Canada. 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.

The remaining types of tube testers include laboratory grade testers and special purpose testers. Included in this group of testers is the Hickok Cardmatic. This was a sophisticated tester used primarily by manufacturers and the military that employed punched cards instead of switches to provide tube test setup. A punched card was provided for each type of tube to be tested. An example of a top end laboratory tester is the New London Instruments Model 901A Transconductance Analyzer. This tester is setup by using a tube manual. Tube pin connections are selected by a set of push-button switches. Voltages to all of the elements can be adjusted and current of each element can be monitored on separate meters. It also directly measures transconductance. A very complete analysis of the condition of a tube can be made on this device. This instrument has jumpers that can be removed for each tube element so it can actually be used as a design platform for tube circuits. The flexibility of this tester allows it to be used to plot tube curves. Special purpose tube testers include those types that are used for a specific purpose, such as the small signal tube testers made by George Kaye Audio Labs and Vacuum Tube Valley. These are used to test preamp tubes for noise, microphonics, gain, and triode balance. Other special purpose tube testers 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 earlier, the best test for a tube is in the equipment in which it will be used. If you intend to acquire a tube tester, be aware of its limitations and do not take every tube tester reading as the gospel truth. Some good tubes will test bad and some bad tubes will test good under certain conditions, as noted above in the descriptions of the various types of testers. If you are in doubt concerning a tube tester reading, substitute a known good tube in the piece of equipment the tube is used in. Tubes and vintage tube testers are analog devices. If you avoid using a "digital" frame of mind when using these analog devices, you will find tube testers to be very useful.

<<back