[Amps] 3-500Z Tubes

[Bee Walton]

The 3-500Z tubes in my AMP INT 3-500ZC amplifier are shorted between
filament and grid.  There may be more wrong but this is a start.

Question.  What is my best source for obtaining the 3-500Z tubes?  Are
Eimac tubes still available?  Do I want the ZG with the graphite anode? 
Will the G require any changes in the amp?  Do I need a matched pair? 
What about non-USA made tubes?

Thanks again,

Bee, K4JLD