I didn't know a thing about Seco tube testers, not having even heard of them, until recently. I had received a few email enquiries asking if these were 'worthwhile' bidding on, as there were quite a few Seco 107 testers on eBay recently. Flattered that someone thought I was any type of authority, I still took a cautious approach. I looked over the auction photographs, and quickly glanced through a few Radio-Electronics magazines from about 1958 through 1961 (the 'era' I estimated these testers to be originally from). The above ad was repeated quite a few times, and they do look like they are a quality tester. The first clue is the elaborate control panels. Both sides of the case are used, similar to my B&K 650. Also, close-up photographs reveal settings to adjust bias ('Grid') and the micromho scale used on the meter movement. Lastly, the Model 107 uses a 'magic eye' tube to read leakage; very unique. The Model 78, seen above at the bottom of the advertisement, is a simple emission tester. It states that the Model 78 still incorporates the 'patented Seco Grid Circuit Test', which is probably similar to the circuit used in B&K tube testers. It also utilizes a 'two stage DC amplifier' to isolate the meter movement from the tube being tested, something I think is a very clever idea. I still wonder why no one else used this principle; it would make calibration and the actual reading much more accurate. The 'upgraded' version of the Model 78 is most likely the Model 98, and a sample advertisement will be seen below. Seco tube testers seemed a little more expensive than their competitors, and the Model 107 ($139.50) would compare to the Eico 666 ($109.95), the B&K 500 ($1119.95), and the Superior TV-12 ($72.50). The retail price of the Seco 107 compares to that of the Hickok 800 ($159.50), although I have a hunch that the Hickok is more sought after today. Below is a pair of full page advertisements describing the Seco 107A and Seco 107B in quite expanded terms. The Model 107B simply has a few extra test sockets to accommodate the newly developed tubes. I warn you now; they are both large 'files', and may load slowly in your browser. I did this purposely to try and let you see them in greater detail. The Model 107B advertisement is from the January, 1964 issue of PF Reporter.
Let's go through these advertisements slowly, and see what we can deduce just from reading between the lines. Keep in mind I do not own a Seco 107, so this is all just speculative guess work. But, from experience, the actual tester seldom lives up to the advertising, and that includes any brand. Many come close to being as good as they say it is, but I'll wager Seco tube testers certainly aren't going to exceed their claims. 'Modesty' and 'Advertising' go together like oysters and peanut butter.
All told, there is nothing to make the Seco 107 a stand-out tester. I have no doubt that it does an adequate job of testing your tubes. But it isn't a high-end tube tester, so to pay high-end prices makes no sense to me. I have seen prices on eBay range from the 'too-good-to-be-true' of $20 all the way up to the 'what-the-Hell-was-I-thinking?' $100+ mark. The Model 98 seems to be loosely based on the popular B&K 600/606 or the Sencore 'Mighty-Mite' tube testers. These are all simple emission testers, but 'feature' a very good gas/grid emission test. Below is a sample advertisement, as seen is the May, 1964 issue of PF Reporter.
What I like about the Seco Model 98 is that the grid current is read directly on a 100uA meter movement. Also, carried over from the Model 78 is that 'two stage DC amplifier' to isolate the meter movement from the tube being tested, something I still think is a very clever idea. This is probably a very good tester, as long as the test voltages and currents are 'realistic'. It is mentioned that because of the DC amplifiers utilized, higher test voltages are not needed(!), which might actually damage sensitive frame-grid tubes. Truth, or fiction? Experiment with a few similar testers, and decide if you can trust any of them, including the Seco 98.
I have recently 'discovered' that the Seco Model 107 uses a very low plate voltage, and low plate current, to test most tubes. This is definitely not the preferred 'way to go'. Also, many 'common' tubes are tested for mutual conductance, but others are tested for emission only. I would therefore advise if you really want to own a Seco Model 107 tube tester, not to pay exorbitant prices, and do not use it as your 'go to' tester. I still think it is a well-built unit, and seems to work. I would only add the proviso of not to put a lot of faith in the readings, or to compare them to readings from your Hickok 539c. After reading the advertisement for the Model 98, I believe that may actually be the preferable tester from the Seco line.