[Arran]

You have to be careful with measuring some 2.5 volt AC tubes on later tube testers, quite often the heater voltage will be too low under load, even more so with #26 tubes on the 1.5 volt setting. The engineers behind some of these testers didn't take the current draw of early AC tubes into account when they were designing the testers, lighting a battery tube that runs on 50 ma is a lot different then an AC tube that draw 1.25 amps at the same voltage. If the tube filament looks dim on something like a 24A or a #47, measure the actual voltage going to the tube socket, you may be surprised.
Regards
Arran