What tube tester to buy?

At that price, probably not. (perhaps an emission tester...of limited value.). Spend $200-300 and get a TV-7 (preferably D/U). Great unit that will test almost anything you ever need to test. Nothing was made just to test 6922, or any particular type like that.

-Ed

These testers (especially the Eico)will be in your price range (see Ebay)and will provide useful info on essentially all (yes I know that there are weird exceptions)modern or semi-modern tubes.