Guide: Which Tube Tester should I get?
I am often asked this question, but usually after the purchase of the wrong tester! I will provide some help with this article and limit the techno-babble as much as I can. I will provide a technical version of this in another document.
I will only cover a few specific models in this document, mostly the better makes and models! I will relate to others in general by either name, model number, or type tester they are. This is only a primer on the topic and not an all encompassing manual. It is to provide you some guidance and questions to ask yourself on the issue to help you end up with the tester you can use and will meet most if not all your needs and requirements.
Questions you must first ask yourself
- What is the primary reason for the tester? To buy and sell tubes? Repairs to vintage radio/TV's, Ham / CB type equipment? To purchase expensive tubes for your own use? And if so for what kind of product are you testing tube for, audio in general, power amps, receivers, high end power amps or guitar amps?
- Do you use only a small specific set of tubes, or do you need to cover a wide range of tubes?
- Is it possible you may do more tube testing in the future over what you do now and over a wider range of tubes?
- How important is the accuracy of the tester to you and your needs?
- Is the physical size of the tester an issue?
- What test features do you want or need?
- Finally how much are you willing to spend. This questions will for the most part determine the accuracy, reliability, and flexibility of the tester you end up with!
I am happy to provide this general tube tester acquisition guide, but in the end the final decision is yours. I can tell you the advantages, disadvantages, features, and accuracy and ease of use, but the main limiting factor will be the price you are willing to pay. I know as I have helped a lot of people with this issue. The internet is full of misleading information, half-truths, out right wrong information and non technical opinions, beliefs, or perceptions - few based in engineering facts or truths. I deal only in technical and engineering facts!
Without knowing the answers to the above questions here is a general bit of information:
At this point I will address the measurement issue of Gm, or Mutual Conductance, also refered to as transconductance, the base measurement unit is conductance, or the modern term is siemens.
I would not buy any tester that is an Emission-only tester. If accuracy is important only look for a Mutual Conductance (Gm) tester. But not all Gm testers are equal. As for emission models there is no technical data established by the original tube manufacturers on tube emissions. So you can not compare to any official tube manufacturing data to your test result in the emision models. Emission alone does not tell the full story of a tube's condition. Each tester manufacturer established their own test criteria.
If you want greater accuracy get a tester that has fixed Gm ranges (switch selectable) like 3000, 6000, and 15000. Not units like the Hickok 600 series, 800 series, or 6000 series, which use a variable control to set the ranges. The accuracy and repeatability of the test results are not as good on these testers. These testers have an overall accuracy of +/- 15% This relates to a 30% variance range from high to low. So a tube which has a true Gm of say 6000 could read a low of 5100 Gm to a high 6900 Gm (range of 1800 Gm), and would be within the accuracy range of the testers calibration standards.
These testers are good for vintage radios/TV and service work. Situations which they were designed for like finding bad or weak tubes, but not for evaluating tubes for accuracy, or for purchasing expensive tubes or for selling tubes! If you need or want an accurate test result with repeatability these are not the models for you!
So what testers are the best to obtain?
I will provide a list of testers that are the better choices. However, you may want, or need different test features which will drive your available options and price.
Let me start this section with a realistic note and fact: Hickok, all but owned the tube tester market back in the day! They made most all their testers as true Mutual Conductance testers that read the test result directly in Gm. Many other manufacturers called their testers Dynamic Mutual Conductance Testers. There is a difference and it can be a big one for some of you and not as critical for others. This is a technical discussion which I promised I would not go into in this guide. If you need to know more about this you will need to read my other article, or contact me directly for the facts!
The Hickok models
Hickok low-cost types like the 533, 533A, 534, 534A, are good testers for most service type work and for testing tubes with Gm values below 15,000 micromho's (Mutual Conductance Values). They have one test signal level, one power transformer, and 3 Gm test ranges as mentioned above. They have a short test, and noise test features as well. This is a good, reliable, accurate tester with the ability to be upgraded with some modern technology for more reliability and greater accuracy as well! These models are good all-around testers and lower in price.
The Hickok models 536, 538, 538A, 539 and the 539A, which was the predecessor to the 539B/C are much better. All these units have the added feature of having two power transformers. One to supply all the heater (Filament) current and the other to provide all the test measurement voltage/current. So the Tube Under Test (TUT) load is divided by the heater and test transformers also improving the accuracy of the tests, and reduces the load on the test circuit and its power transformer. They also have another advantage of having a separate AC line set volt meter. This helps by reducing the effects of the line sag (accuracy issue). Sorry, some techno-babble is needed! In addition these models also have more than one test signal level allowing for more accurate test results on a wider range of tubes. They have a short test, and noise test features as well. These testers are also not too expensive, but may be harder to find. However the weak point is the Gm meter and its technical specification. If you get a bad meter special added circuits are needed if you have to install a new Gm meter if the original one is not repairable. These models are good, reliable, accurate testers with the ability to be upgraded with some modern technology for more reliability and greater accuracy as well!
The Hickok 750, 752 and 752A are a little more costly. All these also have noise tests, and short/leakage tests too! More than one test signal is provided. The 752/752A also allow you to test both sections of a dual section tube without re-setting all the switches for that section of the TUT. In addition the 752/752A do not have a short test lamp they use the main Gm meter as a ohm meter and measure leakage in ohms. The 750 uses the short lamp circuit only. All these models have only one power transformer. These do not have a separate AC line set meter so the line sag issues is more prevalent. Again a good, reliable, accurate testers with the ability to be upgraded with some modern technology for more reliability and greater accuracy!
The Hickok 539B and the 539C are the top of the line in the Hickok service tester category. If you are concerned with testing a wide range of tubes, getting accurate readings, and want to accurately/correctly match power tubes then this is the tester for you. It has both the short lamp testing method and the ability to use the Gm meter to measure leakage in ohms as well. It has the standard Hickok noise test. It provides 7 test signal levels and measures Gm to 60,000 Micromho's! You can add a plate current meter and heater current meter as well with the test post terminals on the top panel. You to use self basis by adding external cathode resistors to terminal strips also located on the top panel.
The Hickok 580 and 580A: These models are part service and part laboratory or quasi-laboratory testers. These use 3 power transformers; one for the heaters, one for the screen voltages and one for the plate/Gm circuit measurement. With three transformers they draw almost 800 mA of current at idle from the AC line. This series had some technical issues and require some upgrades to make them reliable and accurate. If serviced, upgraded and calibrated they are good testers. More expensive to obtain and to service as the parts count is much higher than the earlier Hickok models.
The following Hickok models I do not generally recommended as they do not provide a higher degrees of accuracy, or repeatability. And one is only an emission tester for the hobbyists of the day, this was the model 250. I do not recommend the 600, 600A, 605, 605A, 800, 800A, 800K, or the 6000, 6000A, 60005 the 6000B except as a service tester where you only need to look for weak or bad tubes. The overall calibration tolerances of these models is +/- 15% They are good for what they were designed for, but not for accurately measuring Gm! These models have some other issues which are not addressed in this document. If you just want to look for good and bad tubes and accuracy and actual Gm values are not an issue for you, these units are very good. Quick and simple to use.
The Military Models
Most all military testers utilized the Hickok circuit design. TV-2 series, TV-3 series, TV-7, and TV-10 series. And the AN/USM-118A/B series Cardmatic models as well.
The TV-2 is very good tester. Time consuming and a little complicated to setup. All the parts were military grade and built for battle so the testers are rugged. This is a very good tester if you don't mind the setup procedure. The meter displays percent quality which is not direct Gm but the actual Gm value can be obtained by the use of a chart or some basic math calculations.
The TV-3 series are also good testers. They also contain a built in VOM (volt ohm meter) which in today's environment is mostly useless. However, it does display Gm directly and measures shorts/leakage to 300K ohms and has three Gm range scales: 3000, 6000, 15,000
The TV-7 series is a good and popular tester. The meter displays in a single numeric scale which can be directly converted to actual Gm by using a chart or doing the math. This unit has up to 5 Gm ranges depending on the model you obtain. The TV-7 D/U was the final model built and has the most Gm ranges with the latest upgrades and best overall performance of the TV-7 series.
The TV-10 series is yet another good tester. The TV-3 and TV-10 are almost identical except the TV-10 series does not have the built in VOM operation. This unit measures directly in Gm.
The military cardmatics AN/USM-118A or B versions are the same as the commercial Hickok 1234A or B version or the Western Electric KS-17xxxxxx L1 or L2 versions. The only difference is the military grade components and the cost to make them, which the US taxpayers covered. This is an excellent tester with a +/- 4% accuracy in readings when properly serviced and calibrated. You do need to obtain, or punch the cards for the tubes you will be testing. The meter reading is percent qualty but the tube reference manual that came with the unit makes it easy to calculate the actual Gm test value. No knobs to switch all operation set up by the tube test card you put in.
The Triplett models
Triplett 3423 is a good low end model. It uses AC plate supply voltages and DC bias voltages but it does provide an accurate Gm test result and reads directly in Gm. A good low cost tester.
The Triplett 3444 and 3444A are the top-end versions! Both are excellent tube testers. The difference between them are small. The 3444 is tube operated in that the measurement amp and oscillator are tubes. The maximume plate current measured on the meter is 50 mA. The 3444A is fully solid state operated and the maximume plate current measured on its meter is 150mA. Otherwise these two units ate all but the same. Either would be an excellent choice, but both are more expensive and the 3444a is more costly than the 3444.
The B&K models
All are good service testers but do not directly measure Gm, nor can you get to Gm with a chart or math either. The better models are the 747, 747B, 707, These models have some other issues which are not addressed in this document. If you just want to look for good and bad tubes and accuracy and actual Gm values are not an issue for you theses unit are very good. Quick and simple to use.
The Sencore models
The only units I would consider would be the MU-140 and the MU-150, but both like the B&K units above are Dynamic mutual conductance models and do not directly measure in Gm. You can get to Gm with a chart but not with math. Using the chart information contained in the operation manual will get you close, but not to +/-10% or better. These models have some other issues which are not addressed in this document. Again, if you just want to look for good, and bad tubes, and actual Gm values and test accuracy to 10% or better are not an issue for you theses unit are very good Quick and simple to use.
Additional facts and information
Tube testers improved in design and capability over the years, but other than laboratory models all testers were service testers and most all service testers were proportional testers. That is they test tube at operating voltages below tube specification data sheet values, but the operating values chosen were proportionally close in test result value to the stated data sheet values so they were valid.
These testers have a calibrated accuracy of +/- 10% which is a 20% range from high to low. So that same 6000 Gm tube from above could be from a low of 5400 Gm to a high of 6600 Gm (range of 1200 Gm) when measured on a typical service tester.There are a range of advantages and disadvantages by tester type and by feature too. Things that must be considered are:
- Measurement method and type. Direct Gm, arbitrary merit value, percent quality, method, and then design type does it use, AC, DC, AC/DC voltages on TUT (Tube Under Test)
- Operating voltages available on tester, how many different plate voltage steps are available based on you needs,
- Short test method
- Leakage test available and test method used
- Gas test circuit and accuracy
- Noise tests available and method used
- Life test, also called cathode activation test
- Tube sockets available on the tester
- Measurement ranges available, how may Gm ranges are used, or Good/Bad only or Gm with Good/Bad also
- Test signal frequency and signal levels used
- One, two or more power transformers used in unit
- How the bias voltage is set and monitored with a simple dial or with a volt meter
- The effect of the AC line sag on the test results/accuracy
- How the AC line voltage is set and monitored
- Tube operated or solid state operated, (calibration timing issue)
- Simple to use or more complicated to use
- Accuracy requirements, +/- 10 to 15% for less accurate models/versions
- Physical size of tester
Then you have to consider the tubes you plan to test, what level of testing is needed for your requirements, and the needed level of test accuracy you want/require too.
Can the tester do the line of tubes you plan on testing, tube sockets, operating voltages, test data available. But it comes down to features, functionality, accuracy, repeatability and price!
All tube testers are test equipment and will require maintenance and calibration from time to time. If this is not done then accuracy and trust in the test results is useless. Testers are not like radios where you buy them take them home and use then till they stop working. So be prepared to spend some on repair and calibration on any tester you buy unless it is sold as repaired and calibrated.
What features and their specifications will depend on your needs and expectations in the final accuracy of the test results.