Jackson Tube Testers
Another quality tube tester from yesteryear (did I really just say that?) was manufactured by Jackson. The Jackson Electrical Company (of Dayton, Ohio) was not to be confused with Jackson Industries Inc. (of Chicago, Illinois), who actually made TV’s during the 1950’s. The Jackson Electrical Company made quality test equipment for the service technician. Their line-up was emission testers, such as the Model 49. An example can be seen below.
The Model 49 is an older unit, as evidenced by the low two-digit designation. It is an emission tester, sometimes referred to on the panel as a ‘Dynamic Plate Conductance’ tester. This means the ammeter is inserted in the plate of the tube under test. One tell-tale sign of this not being a mutual conductance tester is the absence of complicated switch and control settings. I still really like the white lever switch caps, very reminiscent of a Telecaster’s. Jackson got into ‘dynamic’ conductance testers in a big way (something they ‘invented’), when they started using three-digit model numbers. The word ‘Dynamic’ is usually prominent on all three-digit numbered testers, and the Model 115 below is no exception.
The Model 115 is pretty Jetson-esque, with it’s funky blue paint job and white chicken-head knobs. It’s old enough to test 4-pin tubes, and it keep it’s tube chart where it won’t get misplaced. The bean-counters may have reasoned that having the slide switches saved real estate and coin, but I miss those Telecaster-style lever switches. Next up is another emission tester, the Model 598. There may be many Jackson testers between the Model 115 and the Model 598, but I have none to show you, nor have I seen any. My Jackson 598 can be seen below.
Worth noting is the similar layouts between the Model 49 and the Model 598. The roll chart has moved on up, and the test sockets are over to the left now. Other than that, the similarities are eerie. The Model 598 retains having a roll chart, and it’s a good thing, because some Jackson tube testers do not have any role chart, but rely on a separate setup chart that can easily be misplaced. If you know this tester really well, you can actually guess the settings, as strange as that may sound. The ‘A’ setting is simply the filament voltage, the ‘B’ and ‘C’ settings may take educated guess work, the ‘D’ setting is the plate pin number, and the ‘E’ and ‘F’ settings are the grid pin numbers. If the tube is a triode, you use only the ‘E’ setting. The ‘B’ setting also taps into the filament voltage, and as an example, most 12AX7 type tubes have the ‘B’ setting as ‘4’. The ‘C’ setting I have never been able to figure out, and it’s not something I plan to loose sleep over. It’s still an emission tester, after all. This example is dated June 11th, 1959 inside on a factory schematic.
Here we have another of the early Jackson tube testers; the Model 636 (above). It is old enough to have a 4-pin test socket, but it has quite a few other unique characteristics that make it interesting. First up, it has a round meter movement. I have seen very few round meter movements on any tube tester other than Hickok. Secondly, it had odd-looking burgundy colored ‘chicken head’ pointer knobs; something impossible to replace today if needed. The two black colored pointer knobs make an interesting pair of ‘bookends’ to the burgundy knobs, including the one longer pointer in the center. To add to the color co-ordination, we have white caps on the eleven push-button selectors. Quite a visual feast, but it is still an emission tester, albeit one dressed up real pretty.
Next up is the Jackson Model 637, a very well made ‘tester’. Things get complicated here, and having the manual is a must if you want to get the most out of this unit. This is so because the ‘tester’ also incorporates an analog VOM into the circuit! You can measure voltage, resistance, and current with your Jackson 637. Sample settings are not marked on the panel, as in B&K testers, although you could ‘learn’ them, as the setup procedure is quite similar to a Hickok/Stark tube tester. There are two rows of 11 push buttons on the panel; the top row are to select functions and ranges for the VOM circuit, and the bottom row helps test for shorts, leakage, and the ‘dynamic’ conductance of triodes/tetrodes/pentodes. Banana jacks are provided to connect VOM test leads, and high-impedance headphones to listen in on the tube as it is being tested. Below is a close-up view of the 637 panel, just to give you an idea of the complexity of this ‘tester’.
Other manufacturers tried to combine a VOM with a tube tester, but the idea obviously wasn’t well-accepted, as these testers aren’t very common today. From the picture above, you should note that there are just a few tube sockets to choose from; later day TV tubes are not included. This tester seems to be popular only with the older radio repair technician. It is still well made; with a good ‘Shorts’ test and an adjustment for the line voltage. Soon after the Model 648 came out with a totally different arrangement. My Jackson 648 is seen below. There are many variants to the Model 648, and there will be a letter after the model number (e.g. 648R or 648S), or a hyphenated number (e.g. 648-1). I really do not know what the differences are, and won’t know for sure until I have at least two ‘variants’ of this tester on my bench long enough to compare. To see an example of vintage advertising and a small photo-gallery of a few Jackson 648 tube testers, CLICK HERE.
This is quite a nice tester, and the 32 push buttons are almost large enough that even ham-fisted technicians can accomplish the setup without too many miscues. The one thing I do not feel comfortable with is the Line-Adjust control on this tester. Instead of a wire-wound potentiometer, you get a rotary switch(?) that selects an appropriate tap on the power transformer primary. I have seen many 648 tube testers that never seem to be able to get the needle to sit right at ‘noon’ for the line test; one setting will be slightly below the mark, and the next setting will make the needle park itself slightly above the mark. However, many testers I have with a wire-wound potentiometer for the Line-Adjust do get very warm when the tester has been plugged in for just a few minutes! Maybe the circuit used on this 648 has its benefits after all, seeing as the very slight variation in filament voltage is really not worth worrying about. Thank goodness the roll chart is still all alone up top. I have many tube testers which ‘feature’ roll charts that have become too loose on the axle to move at all or too stiff to turn freely. This takes a skilled and patient hand, but I haven’t failed yet to make every roll chart turn reasonably well. How long they’ll stay that way still remains to be seen.
As I have mentioned before, Jackson seems to have ‘invented’ the ‘Dynamic’ tube testing idea. What this is, in a nutshell, is as follows. Rather than tying all of the tube grids to the plate, and applying an AC voltage to this virtual diode, Jackson decided to uses ‘proportional AC voltages’. This circumvents the shortcomings of the ‘plain’ emission test, and I have a hunch this makes for a better mousetrap. Most Jackson tube testers feature a pretty good ‘Shorts’ test, although many lack an ideal ‘Gas’ test. And, putting higher AC voltages in proportion to each of the tube elements seems ‘smarter’ than using a typical 25VAC or 30VAC on a ‘virtual diode’. Regardless, many people feel any Hickok is vastly superior to any Jackson. Towards the end of the Jackson era, even Jackson possibly conceded that the patent-expired Hickok circuit was better than the ‘Dynamic’ circuit. The result? The only tube tester I know of from Jackson that uses filtered DC voltages on the tube elements. Below is a picture of the Jackson 658A.
What I like about the Jackson 658 is that they use up to a little over 200VDC on the plate of output tubes (depending on the ‘Plate’ setting, and on the condition of your filter capacitors); at least 50VDC higher than any Hickok tube tester. The same very good ‘Shorts’ test is here (although not as good as what the 648 series uses), and the B&K inspired ‘Gas’ test; complete with a 6AT6 under the panel! The only ‘quirk’ is that the filament voltages all seem to be a little high if you follow the set-up guide exactly. Still, this is a very good tester; but unfortunately is quite rare to find. While the 648 seems to have made from the late 1940’s(!) up to the late 1970’s, the 658 seems to have been made only during the late 1960’s or early 1970’s. That’s a shame, because it does perform well. I would still like to see a micromho scale, as opposed to that darn ‘% of gm’ scale. I do sympathize that multiple micromho scales necessitates a range switch, and possibly a different meter movement, so I put up with the Jackson meter ‘as is’. UPDATE! While I cannot find any dedicated advertising for the Jackson 658/658A, I have found a few smaller ads listing different pieces of Jackson test equipment. As a comparison, a small advertisement from the January, 1964 issue of PF Reporter lists the Jackson 648S selling for $169.95, and the Jackson 658A selling for a whopping $234.95! This may explain why there just doesn’t seem to be that many examples of the Model 658/658A around today.
A slightly unusual phenomenon is the practice of issuing the model numbers to your products ‘backwards’, or in a descending numerical sequence. Jackson did it as well, as can be seen in the older model 715 tube tester. Below is a photographic example of this tester.
First appearing during the late 1940’s/early 1950’s, it uses the Jackson ‘Dynamic’ principal, yet does not have a ‘Gas’ test or a ‘Life’ test! In its day this was not an inexpensive tester, with catalogs listing the price at $77.91 retail.
In the end, as with any tube tester, the voltages used fall a little short of the plate voltages seen in your Ampeg SVT. It does not deter me from liking, and collecting, Jackson tube testers. There are many more tube tester models to the long Jackson lineage, of which I have no examples nor catalog inserts to show you. These would include models 103, 427A, 535, 580, 630, 635, and the 680. The last three models listed also had VOM functions, and date from the late 1930’s.