Tube Tester Types mikeday at backwater dot com 24 July 2017 - revised 8 July 2022 -med So, you are interested in playing with tubes, or maybe you bought a tube amplifier and want to know if the tubes are ok. So what do you do? You could find a friend who has a tube tester and have him check your tubes for you. But, if you are serious about getting involved with tubes, you will need a tube tester as it is an essential tool that is used to check the operational condition of a tube. If you aren't interested in doing this yourself, then find someone you can rely on to do it for you. If however you are truely interested in playing with tubes then read on. Tester Types: So what sort of tester should you get? There are five basic types of tube testers you will encounter 1. Filament Tester 2. Emissions Tester 3. Dynamic Conductance (Transconductance) Tester 4. Mutual Conductance Tester 5. Curve Tracer 1. Filament Tester The Filament tester is the simplest and cheapest tube tester. They don't really test the tube, they just check to see if the filament is working. They don't even really do that very well. The vast majority just connect a neon bulb in series with the heater and connect it to household AC power. If the bulb lights then the filament has conductivity. It the bulb doesn't light, then the filament is burned out. The problem is that these checkers generally only test a very limited number of tubes as they normally use fixed wired sockets for 7 pin, 9 pin and octal base tubes. If the tube heater is not connected to the expected heater pins, then the tester will falsely say the tube is bad, or even might say it is ok when the heater is actually open. The testers are only really useful to a serviceman to do a very quick initial test of a tube to see if the heater is burned out without having to drag out and setup a real tube tester to find out. It still doesn't really tell you if the tube is working, only if it is not working due to a bad heater. For glass tubes you can usually visually see if the heater is working by looking at the tube while it is running since the tube heater will glow when it is on. Although for series strung radios and TVs there is no way to tell which tube is bad since if one is bad, none of them will glow. It should also be noted that low power battery tubes may not show a glowing heater when they are running. So the visual check is not always a valid check for battery operated tubes. In addition, while the filament can be seen inside most glass tubes, some have the heater hidden behind other tube elements, or in some cases the getter flash may make it difficult or even impossible to see the filament glow. Generally you should avoid the filament tester if you are serious about testing tubes as the filament tester does not provide adequate information about the condition of the tube. 2. Emissions Tester The Emissions Tester is the next level up from a filament tester. This is the bare minimum tester to get. If you are on a tight budget or just not sure if you are really interested in playing with tubes but want to try, then an emissions tester can be a good first tester to buy. The advantage of an Emissions tester is that the design is very simple. There are no active circuits to wear out. So testers 50 to 70 years old still work fine as long as they were cared for properly. Although the older ones may not be able to test the more modern tubes you may need to test, so look for a newer model. Something made in the the 1950s or later would be best. Things to look for in an emissions tester. The tester should have a bunch of switches that allow you to configure the tester to test many different tubes. It should also have a "sensitivity" control that allows you to adjust the tester for measuring plate current. This can sometimes be a rotary switch, but usually is a potentiometer (which is the preferred method). Some testers will have multiple sockets that are preconfigured for testing the most common tubes without having to set the tester's multitude of switches. This is ok as long as it has the switches to allow you to test tubes that the tester is not pre-configured to test. How to tell if it is an emissions tester. If it says it is, then it is. However since an emissions only tester is considered less desirable than a dynamic or mutual conductance tester, most won't say they are an emissions only tester. The thing to look for is; does the tester have a "Grid" control (sometimes marked as "Bias"). If it does NOT have the control, then it is an emissions tester. So, is an Emissions only tester bad? Not overly so. The reality is that at the end of the day, all tube testers are checking the tube emissions. If the tube has not been physically damaged, then the only change that occurs in a used tube verses a new tube is the emissions. For most situations, the emission is all you need to know to determine if the tube still has life left in it or not. The other thing to look for is a "Shorts" indicator light. Most (but not all) emissions testers have a shorts test light. The shorts test looks for a short between the internal elements. It is important to have the shorts test, and you should always do that test first as a shorted tube can potentially damage the tube tester beyond repair (as well as any equipment you might put the tube in). An emissions tester connects the tube to act like a rectifier (all tubes are rectifiers). By adding grids, the rectification current can be controlled, which is what makes an amplifier tube different from a plain rectifier. To convert an amplifier tube into a rectifier, you tie the control grid and suppressor grid to the cathode and the screen grid to the plate. (Some testers just tie all the grids to the cathode, which is ok as long as the maximum screen to plate voltage difference is not exceeded.) A few just leave the grids floating. Calibration of an emissions tester is seldom an issue since there are no active components to require compensation. Unless you have the tester repaired, once it is calibrated it stays calibrated. Example Emissions tester: The Heathkit TC1 is an emissions tester. 3. Dynamic Conductance (Transconductance) Tester The next step up from an Emissions tester is the Dynamic Conductance (sometimes called a Transconductance) tester. A Dynamic Conductance tester adds additional control to the tester. Specifically it applies a control signal to the grid. The grid signal amplitiude is controlled by a knob marked "Grid" or something similar. Some Dynamic Conductance testers only provide a simple shorts test using a neon bulb like the emissions testers use. Others like the Eico testers provide a leakage test instead which is a much better test because it does not just provide a go/no-go shorts test, but rather provides information of how much leakage there is between the tube element and the ground reference. It is a much more informative test that can provide clues to odd tube behavior such as excess noise, hum or distortion. Because the tester does more to check the tube, the tests are a bit more complicated than with an emissions tester. Although the testers need to be calibrated, generally it is simple and straight forward. Some purests do not consider the testers like the Eico to be valid since they don't provide a Gm reading, but instead indicate a percentage of emission which provides an indication of how much life is left in the tube. The reality is that the Dynamic conductance tester will provide all the information a hobbiest will normally need in a tube tester. While it does not provide a Gm reading, the percentage reading provided is directly related to the Gm reading of the tube. Since the tester is providing a signal to the grid and measuring the resulting plate current, the information is directly relatable to the Gm reading. If you know the tube characteristics, it is even possible to compute the Gm reading from the emissions reading. So why didn't Eico just build a mutual conductance tester? The problem was that Hickok held the patent for the mutual conductance tester, so Eico came up with the dynamic conductance tester as an alternative test method. Example Dynamic Conductance Tester: The Eico 666/667 is a Dynamic Conductance Tester. 4. Mutual Conductance Tester The Mutual Conductance tester generally provides the most detailed information about a tube and generally are the most expensive tester type. This tester is mostly dominated by the Hickok tester brand because they held the patent for the Mutual Conductance test. During WWII the government had other manufacturers make the testers to fill the war demands, but that stopped once the war was over. After the patent expired other manufacturers like B&K started to make Mutual Conductance testers of their own. The main difference with the mutual conductance tester is that instead of measuring the plate emission directly, it presents a signal to the control grid of the tube and checks the resulting amplification of the signal. This makes the tester more complicated, but gives a more comprehensive test result. The imporant aspect here is understanding what is being measured in the tube. A tube has two key aspects that make up the primary characteristics of the tube. The dynamic plate resistance and the gain of the tube. The gain of the tube is set at the time the tube is made. It does not change (not even as it wears out). It is related to the physical construction of the tube. The plate emission is the part that wears out over time. It is essentially a measurement of how many electrons the cathode gives off given specified voltages on the tube. As the tube wears out, the cathode will give off fewer electrons resulting in the plate current for the given voltage to be less. So if we are only really just measuring the plate current for the tube, why pay extra for a mutual conductance tester? The basic answer is the dynamic conductance tester in most cases is perfectly adequate. The key to understanding the mutual conductance tester is that it also tests the tube gain. This can change from tube to tube since it is derived from the physical construction of the tube and no two tubes are exactly the same. But also if the tube gets dropped or otherwise abused, the gain may change because the internal elements may shift position. An emissons based tester may not catch that problem, but a mutual conductance tester will. The reality is that in most cases the gain is not different enough to make any significant difference in operation. It is the emissions that determine tube life. The down side is that the gain and dynamic plate resistance are munged together into a single reading called Mutual Conductance. So you don't know if it is the gain that is the problem or the tube emissions is the problem. The only way to know that is to use a much more advanced piece of equipment called a curve tracer. But those are rather expensive and requires an engineer to interpret the results. For more information on what mutual conductance is measuring, see the file "mutualconductance.txt". In most case though, it is the plate emissions that is what changes in a tube, so a dynamic transconductance or mutual conductance tester is sufficient to test if a tube is working and how much life is left in the tube. Example Mutual Conductance Tester: The Hickok testers are Mutual Conductance Testers. 5. Curve Tracer: The curve tracer is the rarest tester. Generally it is only used by engineers to develope or verify the characteristics of a tube. A key problem with the previously described testers is that they do not test the tube in the conditions in which it is operated. This is because those testers must be designed to test as many different tubes as possible and thus must compromise how the testing is done. A Curve Tracer tests the tube under multiple conditions, normally between the extremes of its load limits. It provides a graphical display showing what the response of the tube is to the various conditions that were tested. This is the most comprehensive test that can be done to a tube. So why are Curve Tracers rare? Because it requires a specially trained engineer to interpret the results to any sort of meaning. The previous testers described all provide simple single readings that can be defined in go/nogo terms. The tube is working acceptably, or it is time to replace it. This was the purpose for which those testers were designed to be used. A Curve Tracer is designed for an engineer to be able to see and understand the full operating characteristics of the tube so that they can properly design the circuit in which the tube is used. A Curve Tracer does not provide a simple answer. Rather it provides a graphical display of the active operation of the tube. To interpert that to whether the tube is operating adequately for the circuit in which it is used requires additional knowledge about the circuit design and how that interacts with the operation of the tube. Meaning the engineer who designed the circuit or one who understands the circuit design. While a Curve Tracer can provide a wealth of information about a tube, it is not something that a casual user needs, and in fact very likely is not going to be much use since it requires additional detailed knowledge about tubes and circuit designs to understand what the test results mean. Or to put it bluntly, if you don't have that training, you don't want to buy a Curve Tracer. You will just be wasting your money buying something that you can't use. Unless of course your intent is to have a conversation piece to brag to others that you have one. Example Curve Tracer: Curve Tracers are pretty much all custom designs intended for engineers and were not normally sold to the general public. No two designs are alike since they are purpose built. As such, there is no real common "example" reference. A final note about tube testers: As was indicated in the Curve Tracer notes, the common tube testers (emissions, dynamic conductance and mutual conductance) do not test the tubes in the condition in which they are actually used. As a result, the tester may declare a tube is bad when it is not, or good when it would not function properly in the actual circuit. This is especially true with high power tubes which most testers are unable to operate at the power levels they normally function. The tube may also load down the tester causing it's output to be weak. Although it may tell you if the tube is operational, it may not indicate if it is operating within acceptable limits. In addition, sometimes the setup guide for the tube on the tester may not be accurate resulting in the tube not being properly tested, or in a few cases, could actually destroy the tube. When testing a tube for the first time, it is always a good idea to cross check the tester settings, especially the filament setting, against the information provided in a tube essential characteristics manual. Also when buying a tube tester, make sure it will be able to test the tubes that you will be testing. Tube testers generally cannot test all tubes. Tube pin configurations changed over the years, and some tubes have non-standard connections. For more information about tubes and testers see our companion files: http://www.fourwater.com/tubeinfo.htm http://www.fourwater.com/files/666-667-mod.png http://www.fourwater.com/files/eicotesting.txt http://www.fourwater.com/files/eico666-667-repair.txt http://www.fourwater.com/files/mutualconductance.txt http://www.fourwater.com/files/eico666meter-power-notes.txt http://www.fourwater.com/files/eico666tester-meter-check.txt http://www.fourwater.com/files/testertypes.txt Visit us at http://www.MDBVentures.com - Great prices on great tubes!