Here is some excellent advice from Bob Putnak
If you want a nutshell summary, I find room for both machines.
TV-7 pros:
- better test method for amplifying tubes
- military build quality
- highly respected by everyone, although that may be due to it simply being a military product and the military always used "quality"
- grid leakage test is better than the Jackson, although still inferior to a few other testers that excel in that *1* area, but are inferior otherwise.
Jackson pros:
- substantially better rectifier test
- better life test
- model S, which I recommended, has a full socket config of 4-pin thru 12-pin compactron.
- damn near maintenance-free
Less-than-ideal aspects:
- neither has a great leakage test
- the TV-7 shorts test is very limited -- this applies to all Hickok's too. The Jackson shorts test is OK but not foolproof.
- the high AC fields generated by the Jackson can sometimes skew readings of high-mu tubes, such as 7868 or 7591. Generally this is easy to spot because if you cup the tube with your hand and the reading changes, you are seeing oscillation, but that may not always be evident. Oscillation is a issue with all tube testers to some degree, and yet ANOTHER problem that makes "tube testing" an art and NOT a science. The S model is better than earlier models at suppressing oscillation.
- the Jackson test method does not "prove" that a tube is capable of amplification. That sounds worse in theory than reality. The Jackson test method proves that the tube can supply the plate current that is necessary to operate a typical circuit that uses that tube. That is very useful, and it does factor in the control grid and does use high potentials on the plate and screen. That is not the same as supplying a test signal and amplifying it, which is what a Gm tester does. And the end of the day, it seldom makes a difference in "tube testing" as the Jackson is great at finding bad tubes. In the rare situation that the tube would pass a Jackson test but not actually amplify -- a situation that I have NEVER encountered in 20 years -- any novice tech should be able to find that the signal stops at the tube's plate circuit, thereby indicating a tube change was necessary.
In reality, regardless of tube tester, you will find a number of tubes that PASS your tube tester(s) and work SUBSTANDARD (or outright fail) IN-CIRCUIT, and you will find that many tubes you REJECT as "bad" or "weak" would work FINE in circuit if you had actually tried them. That will ALWAYS be a FACT.