Tube Tester Usefulness and Accuracy!
This is yet another question I am often asked. So I will provide a little insite into these two topics. I am not a writer so give me a break on my communications style and long windedness! I will start with usefulness: The best tube tester is the electronic equipment in which the tube is used in. That is to say if you inserted the appropriate meters and other test equipment into the circuit (of the amplifier, or product the tube is used in) around the tube you want to evaluate, this would be the best tube tester and provide the most accurate and useful information on the tubes condition and performance in the product it is used in. This is just a fact! It is also not practical for anyone, except an engineer, or technician and even then it is costly to do! The next best option would be similar to the vary early days of radio where the tube/set testers were used. In these testers you would, 1) remove the suspect tube from the radio, 2) insert a plug from the tester into the radio tube socket where you just removed the tube to be tested from, 3) you would then plug the tube you removed from the radio into the tester, 4) now you would turn on the radio and the tester, and you would read the condition of the tube on the meters. This worked for the old 4 pin simple rectifier/diodes and the triode amplifier tubes in early radio, but it wont work, or provide enough information in the more modern electronic equipment, radio and TVs of the later generations. Why? Because the circuit configuration and varying operating voltages/currents and signal waveforms would require a lot of specialized test equipment. The cost for the many adaptors and wide range of equipment required would be way to high. Yes, it could be done, if you had the money, but it would not be cost effective for any business and would expose the hobbyist, or customer at home to some serious danger including potential death from an electrical shock. Enter the Service Tester: The service tester was developed for the telephone/radio/TV/communication and the industrial electronics industry to provide basic tube testing capabilities to help technicians and engineers to locate weak, bad, or defective tubes. The early testers were emission testers only. These worked just fine in the early days of the industry before the circuits, and tubes became so sophisticated! Over the years there were many approaches in tube tester designs, and thus the features, accuracy, and the tests available differ by make and model. Not every manufacturer had the same goal. Early on some just wanted to focus on simple low cost units to find weak, or just bad tubes. Latter as the industry grew with many companies jumping into the tube tester market as the number of tubes grew and the consumer electronics industry grew too. In all case of the service testers they were at best a set of balances and trade offs in the evaluation of the tubes vs., the cost of the equipment. Even the best of the service testers made trade offs in design to allow for simple use, features, and accuracy level, balanced by the price for the tester. The service tester was a piece of test equipment to be used by professional engineers and electronic technicians to help in the process of repairing tube equipped electronics. So the equipment was designed with the understanding that those using it were both knowledgeable of tube operation (technology), the equipment the tube to be tested was used in, and how the tube tester worked in evaluating the tube. This is more often not the case today! These testers (any make, or model), are just as useful today in servicing vintage equipment as they were back when they were originally used, and as designed for, to find weak, or bad tubes. There are many types of service testers. The usefulness of each make and model will vary with the type of equipment you are servicing. Of course the age and actual usage, or wear on it will have a big impact on its condition and usefulness in testing tubes today. The condition of key parts like the power transformer, meters, precision components, tubes used in the tester, and capacitors will greatly impact the test results obtained and accuracy level you can expect to get!
The next and last tester is the Laboratory, or quasi-laboratory tester: The quasi-laboratory models were testers that came close to laboratory models, but did not provide true laboratory level testing where all tube parameters could be independently setup for each tube element at either typical circuit operating potentials used, or at the tube data sheet values. This unique group of testers were between the typical high end service tester and the low end of the laboratory testers. Some designs were better than others! Overall some were vary good testers. The true laboratory tester was the closest thing to a tester that could perform a wide range of tests to a high level of test accuracy. However, these testers did not perform all possible tube testing either. Example, no laboratory tester could test for the actual capacitance between tube elements. This level of tube testing had to be done using other laboratory test equipment and was only done in a laboratory environment and never in the service environment. High end laboratory units would allow for the setting of voltages and currents on each tube element and measure the characteristics of the tube at different operating ranges. They could be used to evaluate the tubes actual performance, and measure the tubes specified values when compared directly to the tubes data specification sheet. They could be used to do quality control evaluations in factories where tubes were being produced. In addition tube design engineers could evaluate a new prototype tube design on these testers before approving the tube for production. These are by far the most useful of the tube testers. But they are also vary big, heavy, complicated, and present the greatest potential for operator injury (shock hazzard) and for damaging a tube being tested, if enough care and attention to detail is not used in setting it up. Here again between different makes and models you will find a range of capabilities and features as well. The problem with usefulness today is that testers are not always being used as they were designed to be used, and people have much different expectations and often a lack of knowledge of the testers actual capabilities vs., what they either expect, or actually need them to do. Plus these testers are all quite old now and their actual operating conditions vary widely! So effective usefulness of a tube tester will depend on your needs as to what kind of tubes you are testing, what specifications are important to you, what is the equipment the tube is being used in, and how much accuracy in the test result is required. Then you can decide which tester make and model you choose to use. There is a cost per Gm value at the price some tubes sell for today! So the choice made can affect the costs and actual value you get from the purchase of a tube. Usefulness Example: A service tester that only has a short test using a neon lamp will generally (not all models) only test for shorts in the range of below 300,000 ohms or so. Some other models using lamps to test for shorts and leakage will test to 25 Meg Ohms! So if you are using a low end, or mid range service tester with a neon short lamp to test tubes used in high frequency communications equipment looking for shorts and leakage above 300,000 ohms you would not find a bad tube with inter element leakage well above 300,000 ohms. You would have to either just instal a new tube, or trouble shoot with much more high cost test equipment to locate the problem that a good service tester could have found quickly. In the modern circuit applications of some audio equipment you may have need to test for leakage of as high as 2 meg ohms. The 300,000 ohm test would not catch a bad tube here either. Of course this is, but one example of many! Another typical example: Many testers were not able to accurately measure Gm values at or above 15,000 micro mhos. This even when the scale on the meter went to 30,000 micro mhos. This was a result of a simple design error in some models, or failure to correct for known high mu tubes/tester characteristics. So here again the actual usefulness of any tester will depend on its features, their characteristics (specifications) and how the equipment will be used, and what level of testing is needed. If you buy and sell tubes you will need the highest level of capabilities. If someone in vintage equipment repair needs a high frequency tube (of that date) and you can not test it for high leakage in the meg ohms, you may be selling a bad tube that might work well at low frequencies, but not at the high frequency as it was designed for, or for use in the equipment it was designed to work in!

Summary: The best tester is the equipment the tube is used in. The next best option was just to substitute a new tube back when tubes were plentiful and cheap too. As for the tube tester the laboratory tester would be the best option, the next best is a high end service tester. For easy of use and reasonable accuracy for the overall tube environment the service testers are the most cost effective. The high end units provide the best usefulness with the best test features and specifications. Last, but not least are the low cost mutual conductance (figure of merit) style testers useful for just locating weak, or bad tubes and when you dont need to know the actual Gm value, or need to worry about finding tubes with high inner element leakage issues. However, even in this situation there were some low cost units that did a good job at leakage testing even if they did not provide actual direct Gm test values. Mutual Conductance Testers: Dynamic Mutual Conductance testers: Direct Gm measurement - Best and easiest to get actual Gm values with. Percent quality convertible to actual Gm - Good if you have the charts, or want to do the math. Simple numeric scale convertible to actual Gm - Good if you have the charts, or want to do the math. Arbitrary figure of merit 0-120 convertible to actual Gm via a special chart in the operators manual (not vary accurate) and not as useful. Mutual Conductance measurement circuit design: Arbitrary figure of merit 0-100 or 0-120 Not convertible to actual Gm and not as useful. This method will give good/ ? /bad test results only, and can only be used to do rough comparisons between two tubes. It can not provide direct Gm test values.
Now I will cover accuracy: This subject will by necessity require some math and some technical discussion as well! Basically the accuracy of any equipment is depended on the tolerances of its sub systems and components! The tube tester is made up of several sub system, or individual measuring circuits which other wise could be independent of each other. But the switches in the tester are used to configure, or connect the various sub circuits to measure the appropriate values under test to the level of accuracy designed for. You may notice that many components of +/-10 to 20% in say a laboratory tube tester and it may have an overall accuracy of Gm (mutual conductance) measurement of +/- 1.5%. This is because the measurement circuit will have much tighter tolerance in key components of +/- 1.0% or better, while the supporting circuits need not have this same tolerance value requirements. The most important accuracy measurement in a tube tester is the mutual conductance value of an amplifier tube, followed by the short/leakage tests. Gm is the one specified value of an amplifier tube that tells a lot about its condition, and in a tubes specification data sheet that tells the most about the capabilities of the tube as an amplifier tube. The average accuracy of the better service testers is +/- 10% and for the lesser capable testers is from +/15% to 20% while the quasi-laboratory models will typically be between +/- 10% to +/- 4% and the full laboratory models will be between +/- 4% to +/- 1.5%. When you think of this percentage you need to understand what this means. The actual true, or absolute Gm value of any tube is never really known, because all measuring instruments have a specific measuring tolerance value, and nothing can be actually measured to its absolute value. So we start with a Laboratory tester of +/-1.5% which means the tube measured could have a Gm value within a range of -1.5% to the high of +1.5% from nominal, or a range from low to high of 3.0%

So lets work with a tube with a nominal specification of 6000 Gm. A calibrated laboratory tester with a tolerance of +/-1.5% confirms it measures 6000 Gm. This means the tubes actual Gm could be as low as 5,910 Gm and as high as 6,090 Gm for a range of 180 Gm. Now we measure this tube with a service tester which has a calibration tolerance of +/-10%. This should produce a test result between a low of 5,319 Gm and a high of about 6,699 Gm for a range of 1,380 Gm. What! Why not a low of 5,400 Gm which is -10% from the 6000 nominal Gm of the tube! We have to go back to the confirmed tested Gm of the tube with the Laboratory tester. The worse case low Gm from the lab tester at -1.5% which was 5,910 Gm. As the Bogey tubes measured value with the laboratory tester allows for a 3% Gm range and as the calibration of the service tester is set by the specified Gm value of the Bogey tube you must consider the 3% range of the tubes stated Gm value to understand the actual accuracy range you are dealing with. So when measured on a +/-10% calibrated tester the worse case would be 5,910 Gm less -10% which is equal to 5,319 Gm. Then 6,090 Gm high side plus (+) 10% would be 6,699 Gm, or a range of 1,380 Gm. In the real world this would mean that a tube that had a reject point of 3300 Gm with the above test data would still have a Gm life of 37.96% or at least 2,019 Gm left available worse case, before hitting the reject point of 3300 Gm. Having said all this, from a real world perspective you can ignore the +/- 1.5 % value of the laboratory tester as negligible compared to the bogey tubes own tolerances values. To under stand accuracy you must think of both individual circuits and related circuits that make up a system. A tube tester is a miniature system in that it uses different circuits configured accordingly to provide various measurements. Thus there are inner relationships of parts, circuits, and the over all system values as well. The accuracy of any tube testers specific measurement is controlled by the design standards established based on the degree of required accuracy desired/needed, parts used, and environmental conditions it is used in! Now having an understanding of the accuracy measurement itself, you now need to understand the accuracy of the tester when trying to compare the measured tube value to the tubes data sheet value and the tubes actual value when compared to this data sheet test standard! WOW! I will again use a 6L6 as the example! A 6L6 data sheet calls for a new tube to have a nominal value of 6000 Gm! This is based on the following tube specifications: Plate voltage of 250, Screen Voltage of 250, A DC bias voltage of - 14.0 volts, Tube testers are either of the proportional operating value type, or they are of the actual operating value type. Laboratory testers are of the actual operating value type. Most all service testers are of the proportional value type. That is they apply voltages/currents at a proportional value to the full actual voltage/current values, but at chosen values that will be vary close if not exactly the same Gm values as listed in the tube data sheet, assuming the tube itself was within its nominal tolerances also. A tube has many Gm values depending on the actual voltages applied to each of its elements. There are several points along various operating values at which a tube will have the same Gm (mutual conductance) value. This creates an additional issue of measurement accuracy. That is the difference between the service tube testers measured value, and the actual value of the tube as compared to the data sheet test (from a laboratory tester) when compared to the service testers test result. So a 6L6 ideal nominal bogey tube = 6000 Gm and say it measures 6000 Gm. A service tester should be able to measure this tube at 6000 Gm within +/- 10% to +/- 15% (depending on tester) to meet the typical accuracy test results of a typical service tester. Under this situation you are now comparing apples to apples. The difficult part here is that tubes themselves cause variations in the test results due to some of their own operating characteristics. So if one 6L6 has a different forward grid current, this will effect the DC bias voltage load on the tester and thus the actual DC bias voltage applied to the grid of the tube.

The DC bias voltage works with the signal level to establish the measured Gm value. Services Testers can not set each operating voltage to each tube element separably. So you end up with a compromise of design and accuracy. Back in the day, this was not a major issue as service testers were used to find weak, or bad tubes which they did well. But today they are being used to evaluate, QC (Quality Control) tubes, establish how close to new they are (selling tubes), in addition to finding good and bad tubes.
Summary: A Dynamic Mutual Conductance proportional tester will provide a close Gm (proportional) test result but not a laboratory accurate result. Good is within 10% to 15% depending on make and model of tester! The actual proportional voltages applied will vary depending on the circuit design, parts used, factory calibration accuracy, and the actual condition of the circuits in the tester. Additionally the tube being tested will also have an effect on the voltages applied by the tester. The load characteristic of each specific tube tested will load the testers power supplies to different degrees thus changing the amount of DC voltage on the grid. Only certain model testers with DC bias voltage meters allow you to see this and control it more accurately! As an example: A Hickok tester to test a 6L6 under proportional test values that will measure within the (+/- 10 to 15%) from the nominal 6000 Gm value of a new (ideal) bogey tube would typically require a DC bias voltage of between - 3.0 to -4.0 volts assuming a plate and screen voltage of about 130 volts. This is a range of 1volt in bias and can swing the Gm test result from about 4,900 Gm to about 6,750 Gm based on the tubes actual condition, or characteristics. These values will closely approach the tubes data sheet values based on 250 volts plate and screen at -14.0 volts bias and 72 mA of plate current.
Closing Summary: Now the final and most important thing to understand is that any tube measured, assuming a properly calibrated tester could have an actual Gm value anywhere between the high and low values based on the percent accuracy of the tester and its actual calibration position in its capable accuracy range. Testers out of calibration can be much further off from the examples used here. Because of the actual wear on a tester, direct comparison test results are not practical today unless you either know the actual condition of each tester, or have normalized the operating voltages in both testers to the same values. Then if you calibrated both testers at the same time with the same test equipment following the same procedures you will get the same results on both units. So dont think that two testers just serviced and calibrated separately, or even jointly will necessarily produce the exact same test results. If both were new yes, they could within a few percentage points of each other. But when you consider age, usage/wear, and parts tolerance values you would be lucky to get the same test results exactly! Two testers may be able to be brought into the same operating range if the parts will allow it and if you dont mind spending some extra money to achieve the outcome! In addition related to the usefulness of using a tube tester, any single Gm value test (other than shorts test) may not always tell you if a tube is causing an actual problem in the equipment or not, it does tell you its general condition and its deviation from the ideal nominal specification values. This information with your knowledge and experience will help you establish, if it should be replaced. Evaluating a tube to sell, use, or to purchase for future use is the best application for a tester today as well as locating a generally weak or bad tube. When evaluating a tube there are many characteristics to be considered as well as circuit characteristics and issues as they will effect the tube/circuit inner relationships too! If you are unclear about anything in this article please feel free to email me at vrte@msn.com for clarity!

About the AT-1000 Why was the AT1000 Microprocessor Controlled Automatic Tube Tester developed? The AT1000 was developed over approximately one year by a dedicated team. The reason it exists is simple: The only general purpose vacuum tube testing equipment currently available has not been manufactured for decades. It is only available on the used market, and in variable conditions of age, wear, calibration, etc. Specialized spare parts are no longer available including the complicated switch banks, potentiometers, panel meters and power transformers. Calibration is cumbersome, voltages and currents are unregulated, and they are tedious to use. Roll charts, punched cards, numerous rotary switches and dials are now a thing of the past. Vacuum tube users want to maintain their equipment using state of the art techniques. By applying modern digital and analog technology to the task of vacuum tube testing, the user is freed from the drudgery of manually looking up and inputting test parameters. More precise and repeatable tests can be completed in much less time, with much less trouble. With the old testers, a misplaced rotary switch or loading dial could not only produce invalid results but also damage the tube or the tester. This can't happen with the AT1000. The AT1000 is portable in its own compact carrying case, and is built to tough industrial standards with high quality materials, engineering and manufacturing. And it is extremely simple to use, with only 5 keys plus a power switch. The user is guided through all operations with an easy-to-view 4-line by 40-character, backlit LCD display. What does the AT1000 Tube Tester do? The AT1000 will automatically test diode, triode, tetrode, pentode and heptode vacuum tubes for shorts, emission, mutual conductance (GM), and gas and heater-to-cathode leakage (where applicable). It tests both single and multiple-section tubes. It also tests Electron Ray Indicators (Magic Eye" Tubes). The tester will find use by those persons involved with the repair and maintenance of vintage and modern vacuum tube equipment, as well as designers and engineers who are developing new vacuum tube-based devices. Collectors of vacuum tubes and related equipment will also find the AT1000 useful and easy to use. What physical sizes of tubes will the AT1000 test? The AT1000 accommodates 7 and 9-pin miniature tubes, standard octal tubes, and 4, 5 and 6-pin small base tubes with a filament voltage between 1.0 and 12.6 volts. Tubes with a grid or plate cap can also be tested. Will it test directly-heated tubes, such as the 2A3? Yes. It will test both indirectly heated (heater-cathode) tubes as well as filament tubes like the 2A3, 3Q5, 45 and 300B. Why doesnt the AT1000 test tubes with heater voltages above 12.6? The AT1000 was primarily designed to support modern and vintage vacuum tube hi-fi, guitar amplifiers and associated equipment. Many industrial and mil-spec types are also supported. Tubes with heater voltages above 12.6 were originally intended for the series-heater-string radios and television sets of the 1950s and 60s. Instead, we have concentrated on producing a tube tester targeting vacuum tubes employed in a wide variety of consumer and professional audio and industrial equipment, of both modern and vacuum tube heyday vintage. In addition, a very few tubes with unusual filament/heater pin assignments are not supported.
In detail, what tests does the AT1000 perform? Each tube is tested for emission, which is the ability of the cathode or filament to emit electrons. The reading obtained will reveal the relative remaining useful life of a tube. It then tests the amplifying ability of triode, tetrode, pentode and heptode tubes. Tests are performed at the optimum value of bias for a particular tube, set each time automatically by the microprocessor. A true mutual conductance test is then performed, where the tube amplifies a 1kHz signal, and the AC plate current is read by the microprocessor. The true GM value is then averaged over several seconds and displayed to the user in Milliamperes per Volt (ma/V). 1.00 mA/V equals 1,000 micro-Mhos. Heater-to-cathode leakage is tested and displayed for all tubes that are indirectly heated. Finally, a test is made to detect any excessive gas within the tube. What is the importance of the emission test? The emission test is performed while the microprocessor is adjusting the control grid bias on the tube under test. The bias is automatically adjusted so that the nominal specified plate current is flowing in the tube. Once this has been established, the actual grid voltage and the grid voltage specification are compared. The results are given both in Good-Fair-Poor fashion, as well as the actual voltages. If a tube has very weak (or no) emission, the specification plate current will not be achieved. Such tubes usually also exhibit very low or zero transconductance. What is the importance of the transconductance test? Most tubes with three or more electrodes are employed as either AC or DC amplifiers. Transconductance, otherwise called Mutual Conductance or GM, is a test of the gain, or relative amplification factor that a particular tube is capable of. When performed in the laboratory this test is done after carefully adjusting the filament/heater voltage, grid and plate voltages, and plate current. The AT1000 tests vacuum tubes the same way, using original tube manufacturers specifications. Most other tube testers (even the highly-sought vintage models) do not test tubes using the full voltages and currents that they were designed and specified for, and under which conditions the tubes see in actual use. For example, the popular Hickok 600 tester has only 160V available to test tubes. The AT1000, however, will provide plate voltage at up to 500V and 160mA. This means a much more accurate picture of tube condition and performance is delivered to the user. Flaws that wont show up at the abnormally low voltages and currents used in other testers WILL show up with the AT1000. You just cant properly test a 300B with only 160 volts on the plate! Many other testers do not provide transconductance information in the industry standard units of mA per volt, or micro-Mhos. They provide this information only as good-fair-poor, or, 100 out of a possible 130 or some other arbitrary numbers. The AT1000 displays transconductance in mA/V (which is easily translated to uMhos by simply multiplying by 1000). For quick refernce, the AT1000 also provides a good-fair-poor transconductance indication. What is the purpose of the heater-cathode leakage test? Indirectly heated tubes, (those with a separate cathode) sometimes develop a leakage resistance between the heater and cathode. In the case of a cathode-follower circuit, this resistance appears across the circuits output, causing excessive loading. Heater-to-cathode leakage can also adversely affect a cascode amplifier circuit. Even worse, tubes with heaters operating on AC will couple line frequency noise and hum into the signal path. This is highly undesirable for virtually all applications, especially in low-level audio preamp stages. Most tube testers do not test for heater-to-cathode leakage when the heater is hot. They only test the tube cold, as part of the shorts test before any voltages are applied to the tube. The AT1000 tests the leakage on each heater/cathode assembly in the tube while it is hot. Leakage as low as 1 microampere is detected, which is much more sensitive that the usual neon lamp shorts test of conventional tube testers.

What is the purpose of the gas test? When a vacuum tube is manufactured, atmospheric gasses such as nitrogen, and oxygen are pumped out of the tube. Many tubes employ an active getter, which continues to absorb any residual gas after the tube is sealed off. Gas can come from electrodes within the tube itself, especially if it has been overheated. It can also leak in around the seals at the tubes pins. If this gas is not absorbed by the getter, it interferes with electric fields and electron movements within the tube, disturbing normal operation. Oxygen in a tube can also poison the oxide coating on the filament or cathode. In severe cases, a power tube can run away, where excessive gas pulls the control grid positive. This results in more plate current, more heat, and more gas release. The phenomena thusly becomes self-sustaining. Such a tube may destroy itself and other expensive components in the equipment, such as the output transformer. The gas test of the AT1000 actually measures the effects of the gas, by introducing a higher than normal grid resistance. If gas is present in the tube, it acts against the increased grid resistance, resulting in a plate current increase. The amount of plate current increase is measured and then displayed to the user. What about shorts? Tubes sometimes develop short circuits between elements. Such shorts will make the tube inoperative, and possibly damage equipment. The old, vintage testers of the 1940s and 1950s had a problem with the possibility of a tube short, because it could also damage the tester. Therefore, a test for dead shorts and high resistance shorts was made before applying operating voltages to the tube. It had to be done this way to avoid damaging components inside the tester. The AT1000 does it differently. Tubes are tested for shorts and leakages as part of the other tests. Since the AT1000 power supplies are themselves overload and short circuit protected, they are not damaged should a tube be shorted. Instead, an intermittently shorted tube can be exposed, as the tube is continually being checked for shorts while it undergoes automatic test. Any detected short, even a brief one, will cause the power supplies within the AT1000 to shut down. In addition, the user is advised by a special message on the LCD screen and the test is halted immediately. What information is displayed on the Results Screen(s)? When the test sequence concludes, plate and screen voltage is removed from the tube, while bias and heater voltage are maintained. Then the LCD screen shows the user both the specification and the actual measured values for each section of the tube. The specification grid voltage is presented, along with the grid voltage which the tester determined was necessary to achieve the required plate current. The specification and actual measured transconductance values are also shown. In addition, the measured heater-to-cathode leakage and gas current are displayed. Each tube section is generally shown on a separate screen, navigated simply by using the up and down keys. When the user is finished reviewing the data, the tube can be re-tested immediately, bypassing the heater warm up. This can be repeated several times if necessary, in order to confirm a drifting tube parameter or intermittent fault. What about matching tubes, can the AT1000 do this? Absolutely! Not only will it match tubes by grid voltage (emission), but tubes can further be matched by transconductance. Matching of multi-section tubes is possible too, since the results for each section of such a tube are displayed separately. And most importantly, it will match tubes at normal current and voltage levels, faithfully reproducing the actual operating conditions found in many amplifiers, preamps and other equipment.

What about roll charts, lookup tables, etc? These dont exist with the AT1000! The testers internal FLASH memory contains test data on over 400 types of the most popular tubes, including vintage and high end audio power and preamp types, European types, mil-spec/industrial, and some transmitting types. The tube type numbers are easy to browse through to find your tube, using the intuitive up/down and left/right keys. Pressing Enter then loads all the selected tube's data into the microprocessor, where it is readied for testing the selected tube. Another key press, and the user is prompted as to which socket on the unit in which to insert the tube. A final key press and the test begins. There are no charts to roll, no books to use, or dials or switches to set. All of the tests are performed automatically in sequence, for each tube section, under complete microprocessor control. How long does it take to test a tube? The time can vary, from as little as 40 seconds to about three minutes. It all depends on the voltage and current values being applied during the test, and how many different sections a given tube contains. The heater warm up for directly-heated tubes is 15 seconds, while tubes with a heater-cathode construction are heated for 60 seconds before the tests commence. What if I want to test a tube that isn't listed in the AT1000 Data Table? This is the best part! The AT1000 has been designed with a user-reprogrammable Tube Data memory. The user can edit the tube Tata Table, customize it to his own specifications or preferred tube types, and even add tubes if desired. Most tubes that will fit into one of the sockets, have a heater voltage between 1.0 and 12.6Vat less than 3.5 amperes, and can be tested with 500V or less on their plate and screen can be tested. Tube data is available for free on the internet from original scans of vintage tube data books. All you need to know is the heater voltage, the Class A Plate and Screen voltage and current, and the Grid voltage and Transconductance specified for that class of operation. Tube data is entered or edited on any personal computer (PC) with a standard serial port and text editing software such as Notepad. Data is stored on the computer as a Comma Delimited File (.CSV file extension). The data files are compatible with Microsoft Excel, which makes it extremely easy to change or add to the tube data table. We also include on CD ROM an executable program which will upload a new or modified Tube Data Table to the AT1000 over its built-in RS232 serial port. Once the Tata Table has been uploaded, the RS232 link is disconnected, and the AT1000 again becomes a fully portable unit, but with YOUR customized tube test data stored in its FLASH memory! For tubes that the AT1000 does not have sockets for, socket adapters may be made available if the demand is sufficient. Lacking this, you can even make you own adapters with an old tube base and a tube socket that fits your tube. This would even be applicable to the aforementioned odd tubes that have unusual filament/heater pin assignments. But Im a tube person, and am not into computers. I dont have (or want to use) a computer. Not a problem. We will make you a custom FLASH EEPROM (memory chip) with the tube data you provide, for up to 5 additional tube types. We also include the default tube types (normally shipped with current version testers) and send the EEPROM to you for only $11.95 anywhere within the U.S. (overseas, separately quoted). You will have to open the tester and replace the EEPROM (instructions will be provided), which is positioned in socket. If you are uncomfortable doing this, ask a knowledgeable friend or take the unit to a reputable repair shop.

How about diode tubes? Many tubes have simple diode sections which are sometimes used as AM or FM detectors and peak rectifiers. Examples are the 6AL5, 6T8 and 12SQ7. All of these kinds of tubes can be tested. In addition to the standard transconductance test, a forward and reverse conductance test is done on the diode sections of these tubes. The voltage used is 100V at one milliampere, well within the capabilities of these types of tubes. The test results screen displays not only forward and reverse conduction (in microamperes), but also measured heater-to-cathode leakage for the diode sections. What tests does it perform on Electron Ray Indicator types? The AT1000 is capable of testing tubes such as the 6E5, 6BR5/EM80, 6HU6 and more. The display section of such tubes is tested using the nominal operating voltages, and allows the user to check for brightness and shadow movement. If the tube has a separate triode amplifier section, it is tested for leakage, emission, shorts, transconductance and gas just like other triodes. Tubes operate at high voltages, is the AT1000 safe to use? When the directions, cautions and warnings in the manual are followed, the AT1000 is perfectly safe to use. If you are working with vacuum tubes, you already understand the dangers of their high voltage power supplies. Because the AT1000 operates tubes at the voltage levels they were designed for, it produces up to 500V DC. Tubes must never be inserted or removed while a test is in progress. Plate and grid cap accessories must likewise never be connected or disconnected while a test is in progress, and they should only be handled by their insulating parts with clean and dry hands. Tubes can get very hot during operation. A metal-envelope 6L6 will become uncomfortable to the touch with only its heater lit. Even though the complete test for a 6CA7/EL34 tube lasts under 90 seconds, its envelope can become very hot. Plate and screen voltage is cut off after a test, limiting the temperature rise of a tubes envelope. However, repeated use of the HOT RETEST function will further heat the tube. Following test completion, the tube should be allowed to cool to a safe temperature before removing it from its socket. Appropriate gloves may help in this area. Common sense applies to the use of this equipment, as it does to servicing all vacuum tube electronics. The user must keep his fingers away from any metal contacts while inserting or removing a tube unless the tester is unplugged. This is the same thing he would do when working on his own vacuum tube equipment. What special design features does the tube tester employ? The AT1000 contains independently-adjustable Plate, Screen-grid, Control-grid and Heater Power Supplies. Each power supply is fully regulated to within better than 1% of its full-scale voltage, and is protected against overloads and short circuits. Each of these four power supplies is controlled by a 16-bit D-to-A Converter, interfaced to the microprocessor. The Plate/Screen and Heater supplies are ramped-up over a brief interval, avoiding thermal shock to your valuable tubes. The slow ramp of the heater supply is particularly effective in preventing the bright flash sometimes encountered with cold heaters in miniature tubes. The power supplies are also protected against overheating by thermal protective devices. Should the unit overheat, the user is notified to wait until it cools down, and further tests are inhibited until this occurs. Three separate fuses protect the Microcontroller, Heater and Plate/Screen Power Supplies.