Tubes of the same type vary in physical size and design, as long as they say '6L6' on em (or whatever else you're running in your F50), there's no need to worry.

Here's a photo of two 6L6 tubes from Wikipedia:

They may differ in size, but are still completely compatible.
And two different 6V6 tubes:

Gilchrist custom
Yamaha SBG500
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100

Cathbard Amplification
My band