A Look at Sencore Tube Testers
During the 1960’s, Sencore made quality test equipment for the TV service technician. Their line of ‘Mighty-Mite’ emission tube testers were quite popular, and many examples can be found today. What makes it a little confusing is that there were many ‘Mighty-Mite’ tube tester models! The ‘original’ was probably the Model TC109, which is the lowest model number I have seen. Next is most likely the Model TC114, and an advertisement from a 1963 issue of Radio-Electronics can be seen below. Compare photos of the TC114 to the TC109. They are very similar, and neither has a suffix designation! This makes it a little confusing, and a sharp eye is needed to avoid a mix-up.
The ‘Mighty-Mite’ line was not exactly an alternative to other testers in this price range; the Eico 666 ‘kit’ would have been priced similarly, test transistors and is a dynamic tube tester. Also, the Superior TV12 was priced a little cheaper than the TC114, and the TV12 was also not just a simple emission tester. However, Sencore ‘went after’ the service technician by pointing out that their ‘Mighty-Mite’ testers were much more portable, and set up much faster than most other tube testers. As well, the ‘Mighty-Mite’ models utilized a higher test voltage, which attempted to simulate actual voltages found in the typical television sets of the day. The biggest ‘feature’, the handy gas/grid emission test, is a very accurate indicator. This circuit is similar to that found in certain B&K and Seco tube testers. Below is a sample advertisement for the Sencore TC130, or the Mighty-Mite III. It is from the May, 1964 issue of PF Reporter.
What you may not be able to make out clearly from the advertisement are a few points worth considering.
- Lower test voltages are utilized for Nuvistors and frame-grid tubes. This prevents possible damage to the tube. Many ‘older’ tube testers used a single test voltage for all tubes, regardless of whether or not the test current exceeded the tube manufacturer’s maximum ratings! This was evidenced by many tube tester warnings against testing the tube any longer than absolutely necessary to obtain a reading. This makes the Mighty-Mite line already ‘better’ than any other simple emission tester.
- The gas/grid emission tests have a 100 Megohm sensitivity, and are much better than most other tube testers.
- The ‘Shorts’ test will check for inter-element leakage at higher voltages, and check between each individual tube element. Many ‘better’ tube testers do this, while ‘lesser’ testers do not. Still, many of these ‘better’ tube testers utilize a simple neon bulb indicator, and cannot display a leakage higher than 1 Megohm. The real ‘fancy’ tube tester will convert itself to an ohm meter, and show inter-element leakage as a resistance value. When all is said and done, the Mighty-Mite line does have a very good ‘Shorts’ test, although certainly not ‘the best’.
The end result is that the Mighty-Mite would make an excellent tube tester for many people. For the truly anal, the fact that you are just measuring cathode emission is a turn off. I still recommend any later ‘version’ as a top-notch back up unit. Also, you can measure a tube’s transconductance on your fancy-shmancy Hickok, and double-check the gas content on your Mighty-Mite. That makes for a pretty complete test of your tube.
Here we have more advertising from the October, 1967 issue of Radio-Electronics. As we can see, the ‘Mighty-Mite’ model is up to the ‘Mighty-Mite V’ which is designated as the ‘new’ model. You really need to be aware of the model ‘version’. You may notice that the price for the TC142 hasn’t changed since the TC114. Also, note that Sencore has a price range covering the spread of about $74 to $180. Most manufacturers had a similar range, trying to get you to buy one of their testers, regardless of your budget.
The advertisement above is for their top-of-the-line transconductance tester dubbed the ‘Continental’. It is actually the only transconductance tester Sencore manufactured. The Continental has a small mystery that confuses me (which is not a hard thing to do, actually). There is a Continental MU140 and a Continental MU150. There appears to be only very slight case styling alterations between the MU140 and the MU150, but other than that, you can check out the two ‘versions’ yourself below.
Sencore Continental tube testers are excellent choices; hopefully you never have to choose just one of the ‘twins’.
Above left is a sample of the Continental MU140, and above right is the ‘updated’ version, dubbed the Continental MU150. I think of these two models as being one and the same. You may notice that the plate-cap lead has moved to the top of the panel, and the ‘gm’ and ‘Life Test’ pushbuttons are moved down to the lower right-hand side in the MU150. Also note that the instruction sheet and manuals appear the same; they are. There are no extra sockets, and no extra controls to help me determine what the differences are, if any, between the two models. Perhaps if I ever find an advertisement for the Continental MU150 I can decipher this riddle. I still like the few features not found in most, if any, other tube testers. First up is their regulated power supply, courtesy of the 0B2 tube. Next, we have a balanced-bridge amplifier protecting the meter, as seen in every VTVM. This is done with a 12AU7, which feeds a tuning circuit(!), so that only the 5kHz square-wave test signal is measured on the meter. Very sophisticated. Other tube testers I know of that utilize a 5kHz test signal are the B&K 747, the Heathkit TT1, the Triplett 3423 and 3444, and the Weston 981. The 100Meg leakage test is far more sensitive than my Jackson or B&K testers. The six selector switches on the bottom help select the test signal level appropriate for the tube to be tested (something no one else does), sets the plate-screen-grid connections, and adjusts the meter shunt. One of my favorite tube testers, when I need not concern myself with testing an old 4-pin tube like the 2A3. There are only a few drawbacks; one is that the Continental will not read out transconductance on a micromho scale; instead it uses a ‘Good’ or ‘Bad’ scale that always leaves me feeling cheated somehow. The other drawback, and it can be serious, is that this is a complicated tester to calibrate. You need to make test ‘jigs’ to simulate a tube, and an oscilloscope is needed to calibrate the test signal amplifier. If you can take the time and calibrate the Continental properly, it will become a favorite of yours, as well. I have saved the ‘best’ circuit for last; the ‘Automatic Bias Circuit’. How does it work? Check out the diagram below.
The DC bias supplied to the tube is derived as follows. We can think of the resistors R3 and R7 as a single plate-load resistor. Let us assume a plate current of 25mA, which corresponds to a voltage drop of 100VDC across the combination of R3 and R7, resulting in a plate voltage of +110VDC. This would also cause a 326-volt drop across R2 and R6. By using math (ugh!), we can calculate a -7VDC bias on the grid, as seen in the figure above. If your tube tries to ask for an increase to 30mA, there will be an increase in the voltage drop across R3/R7 to 120VDC, and the plate voltage drops to +90VDC. Now there will be 306VDC across R2 and R6, and the grid voltage will be -20VDC. This will tend to bias the tube towards cutoff, resulting in the tube returning to ‘normal’ plate current. The value of R3 is actually the ‘Load’ switch, selecting one of three plate currents; 2mA, 7mA, or 25mA. Very clever.
During this time, Sencore was still pitching their emission testers to the industry. They even jumped in (albeit a little late) with a ‘semi-automatic’ tester of their own. An advertisement for the TC131 can be seen below.
Perhaps Sencore could include traffic lights along with this tester, purported to draw large crowds.
The Sencore TC131 was apparently designed ‘after thousands of requests’ to take the small and compact Mighty Mite tester and enlarge it enough to consume an entire workbench. Note the similarities between the TC131 and other automatic testers; few setup controls, index cards, as well as an ‘auto-scale’ for each test (leakage or shorts and emission). The TC131 did not employ ‘sandwich switches’ to make pin connections, as there is obviously more than one octal socket, as an example. These multiple sockets take up real estate, and make the TC131 a cumbersome piece of equipment. The advertisement above was repeated throughout the mid 1960’s in magazines such as Radio-Electronics.