The Eico Tube Tester Range
Among the many popular tube tester brands to find at swap meets, Eico has to be one of the more common. Eico was one of the companies who supplied their test equipment either ‘prewired’ or as a kit. They were in direct competition with companies like Dyna, Heath, and Knight. In case you didn’t know, Eico was an acronym for Electronic Instrument COmpany, and was located in Brooklyn, New York.
Eico made a full line of test equipment, and a chart below lists only a sampling of their many offerings. Well, there are 31 items, to be exact. This is still only a partial listing of their test equipment; during the same time period Eico even sold Stereo Hi-Fi equipment in a kit form!
It is the Eico lineup of tube testers that interests me, and that is what we will discuss here. The earliest were ‘simple’ emission testers, and we’ll start with a pair of mine that can be seen ‘again’ below.
A very early and quite popular emission tester from Eico was the Model 625. While it is a basic emission tester, the Model 625 is very compact and easy to use. The Model 625 is also one of the few (only?) Eico tube testers I know of that has a vacuum tube under the panel. A 6H6 rectifies one of the AC filament voltage for the neon bulb used in the ‘Shorts’ test. I’m not sure why this same model of tube tester was available in two different case-styles (and colors), and if any reader can offer some kind of explanation, I will include it in a later update. These examples of the 625 can be dated fairly accurately by the fact it has test sockets for 4 and 5-pin tubes. I don’t believe it will test TV picture tubes, as I have never seen an adaptor for such a test. Perhaps the two case stylings differentiate the ‘kit’ as opposed to the factory assembled unit? My only other guess is that since the 625 above right has a case style that resembles my Model 666 and Model 667, perhaps the ‘older’ version of the Model 625 has the odd color, and the later day units were ‘upgraded’ to conform to the ‘new’ look. Magazine advertisements from around 1953(!) show the ‘Blue’ version, and designate the model number as 625K. Note also that the older ‘Blue Bonnet’ version is more portable, vis-à-vis the smaller case, and lack of hinged lid means it was possibly a counter-top model. I just love a good mystery. The only remaining oddity (at first glance) is the seemingly absent AC cord! You know this is an AC operated unit, so where is the AC cord? It is in the back of the case, and pushes in/pulls out through a rubber grommet. The Model 625 was ‘updated to accommodate newer tubes developed during the 1950’s, and was christened the Model 628. A photographic example can be seen below.
Still a simple emission tester, the Model 628 continues with the new ‘modern’ look to the line of Eico test equipment. While the array of controls looks intimidating, they really are not. To the left you may make out the meter ‘Shunt’ and the ‘Line Adjust’. To the right we have the filament voltage switch and the ‘selector’ switch, which begins to arrange the socket to the ‘virtual diode’ circuit, and place the meter in the appropriate path. The lever switches on the bottom complete the setup, and also perform the ‘Shorts’ test. This tester is very similar in performance and set up to the Heathkit IT17, which is also from the same era.
Another one of the more popular emission testers, the Model 635 came with an adaptor to test TV picture tubes. However, the 635 was still an emission tester, and lost favor among audio technicians when mutual conductance testers became available. Eico got in on the act, albeit a bit differently, with their Model 666, or as I call it, ‘Satan’s Choice’. I have no idea why no one suggested the model number might have negative connotations, but the ‘666’ was still a very good tester. It had the unique distinction of using ‘proportional’ AC voltages for the grid, plate, and screen supplies. Jackson is the only other tube tester I know of, or have seen, that uses this type of circuit. I have a theory that this makes for a better mousetrap, but other tube tester ‘authorities’ disagree. If there is any ‘problem’ with the Eico 666, or the ‘Dynamic’ circuit in general, it is on the roll chart! That will be explained on the Eico 666 page, which can be found by HERE. Below is a sample of the Model 666, which first appeared on the market in about 1956.
You may be able to spot the control labeled ‘Grid’ in the lower left corner on the picture shown above. This sets the grid bias on the tube, and ranges from -5VDC all the way up to -45VDC. Under the hood, there are no tubes to worry about in these testers. The ‘solid state’ bias supply is furnished via the single diode. Below is a picture of the ‘guts’ of my Model 666. Highlighted in yellow is the filtered DC supply (for the transistor test) on the right and the adjustment for leakage indication on the left. You can set how ‘high’ a leakage exists between elements of the tubes that would be ‘failed’ on the leakage test. Looking at the ‘guts’ of any tube tester is the best indicator of whether or not you have a simple emission tester.
Overall, this tester ‘sets up’ like a fancy-shmancy Hickok (each lever corresponds to a tube pin-number, and must be connected to ‘something’), but separate sections of a multiunit tube are tested by depressing a separate test button. This puts the indicating meter in series with the section to be tested, while the section(s) not under test are ‘grounded’. This unit tests transistors as well, and has a meter scale to show the beta (gain) of the transistor under test. The Model 666 has provisions to test the ‘old’ tubes (2A3), and is a very ‘complete’ tester. It was replaced by the non-demonic sounding Model 667, which first appeared in the mid 1960’s. My sample is seen below. In a 1965 advertisement, the Model 667 is listed at $79.95 for the unassembled kit, and $129.95 for the factory-wired unit.
The Model 667 does not have quite as many test sockets as the Model 666; the 4-pin, 5-pin, and 6-pin sockets are not included. This means you cannot test say a 2A3 or a 6D6, but pretty well every other ‘modern’ tube-type can be tested. You may also note that there is an external fuse holder mounted to the panel where the ‘obsolete’ tube sockets used to be. While the settings would naturally be assumed as being identical, as the manuals are almost carbon copies of each other, there are differences. As well, the settings on the 667 have to be adjusted to test the second triode of a 12AX7. I still think of the Model 666 and the Model 667 as pretty much the same tube tester.
Here is the schematic and a few troubleshooting tips for the Eico 667. Apparently the 666 and 667 manuals are easily available online, but the 667 manual is without the schematic. Here it is, in a very large but easy-to-read 150K GIF file. You can check out the schematic HERE.
These Eico tube testers calibrate almost too easily; there is a procedure to calibrate the ‘Line Adjust’ test as well as the ‘Leakage’ sensitivity. As I mentioned earlier, you are even instructed how to adjust the calibration to ‘fail’ tubes with an even higher leakage than the factory suggested setting.
I like Eico tube testers. I still don’t know if the AC voltages used on the tubes under test are not as reliable an indicator compared to using DC voltages, but so far the reading have been comparable to other testers I have. Also, their instruction manuals are the most thorough, giving a little tube tester circuit theory (as it relates to Eico testers) as a bonus. Here is a sample of how the Model 666/667 tests the first half of a 12AX7. It is taken from the accompanying instruction manual, and is identical in either the Model 666 or the Model 667 manual.
As you can see, the section of the tube under test has the AC meter movement inserted in series with the plate connection; we are measuring the AC voltage (and therefore the gain, in a convoluted way) in that section of the tube when a fixed AC voltage is applied to the grid. Depressing the button to test the second half moves the meter connection into the circuit of that triode. Pretty simple in theory, yet every Hickok I have seen and/or owned makes you reset the switches to connect all relevant voltages and the meter movement to each section. I like the momentary push button idea; it is very fast and easy to compare the gain of each section, should you be anal enough to want a ‘matched’ 12AX7. You may want a matched phase inverter tube, but that’s another story. Back to the Eico Model 666, pentodes are tested as follows;
As we can see, the plate, screen, and grid voltages are set via the external controls, and the meter movement in the plate once again measures the AC voltage gain of the tube under test. The instruction manual gives a great theory lesson on the design of these testers, and how the switches selected affect the range of voltages on the tube and the reading as a result. Still, in the end the voltages used are a little short of the B+ found in your average Marshall amplifier, and many output tube troubles show up only at high plate potentials. That said, I still like, and collect, Eico tube testers.