I have a Hickok 600 tube tester and use it regularly. I did some poking around on the web and the 600A seems to have loads of information, but I don't find much about the 600. What can anyone tell me about the 600 in comparison to the 600A and others? It seems to work fine for me, but interested in any known short-comings or issues.

Thanks

The model 600 is a good little tester. I still have one here.

Alan Douglas explained this elsewhere: (not a quote)
The 533/ 600 and similar models use a 5 volt grid signal....Good for older tubes.
The "A" suffix models use a 2.5 volts signal. Better for newer tubes....
Neither model group is ideal for tubes requiring a low grid signal, i.e. 12AX7 and others.

As I mentioned elsewhere, the data for the "A" suffix models (bias / shunt / English) is not 100% the same
for the "Non-A" models.

Great - thanks for the info! Follow-on question now that you've piqued my interest... I have an Eico 667 and two emissions testers also (Jackson 648 and Knght KG-600). Which of these would best suited for the 12AX7? I do have a small pile of these to test.

I think the Hickok would still be better for testing a 12AX7 than an emission tester, though perhaps only 20% better.