It isn't a F15T8, but something else. I have 3 of them,the Sylvania green etch is worn off  and they are really bright when used as a F5T8, and ages correctly.  Can I use a ballast for a F17/25/32 T8 lamp on this?  

Also can I use F15 and F20 lamps on this ballast? 

Powell

 you can take two F15 in series with F32 ballat they will be OK

 but F20 wil be just under driven as the ballast keep the current constant for F17 lamps. electronic IS ballast
you can run almost any thing on them. just keep the wattage of lamps with in it spec so it dont overheat the transistor. esp on smaller lamps. on the other hand it just drive max current into higher wattage lamps like the F40 lamp but not brightly powered. That when ballast over driving come into place by combinding output lead.
like you would combine transmitters for higher wattage RF output.

I have 3 of these lamps and I can't figure out what the ballast should be. I'm thinking it is 13 watts. They do have these in some refigerated drink machines in WalMart but unfortunately the etch or stamping is not visible. When I get back to the Sylvania place I will ask them.

 if that case you have F10T8 or F13T8 I have some tubes that are 1 foot F10T8 and F13T8 if that the case you can power them up on on PL 13 watt ballast. I also have expermented series 2 x F10T8 on standard F14-20 chokes. they work ok on one FS2 starter and a push button to start the other lamp. as not enough voltage to strike the second glow bottle starter but barely enought OCV to strike the second lamp with a push button.

The etch on  these lamps ( I remember ) start FO 13...... I saw a longer one in a drink machine FO 21.  The 17 watt ones are 24 inches, and this appeared (?) to be a bit longer but certainly not 36 inches. Both are Sylvania.  I ran the 18 inch one in a desk lamp for a long time and had proper aging... but BRIGHT ! ! !