John,
If this tube is being used as a bias rectifier (like in the Johnson Ranger) then you will need a series dropping resistor to get the voltage back in range. If it is for a noise limiter in a receiver, a pair of cheap diodes is all you need.
I am not sure I have ever replaced a 6AL5 in any of the gear I own. They are very reliable so unless there is an unusual circumstance requiring substitution I would just plug in another 6AL5.
What he said ! ! ! !
Why?? Other than a very rare filament failure, they never crap out. If you just need to replace one, PM me your address and I'll send you a few.