[Amps] LK-550 Mixing Tubes

[BKStead at aol.com]

I have an LK-550 (3-holer).  One of the tubes recently developed a  screen to 
grid short.  The two existing tubes are regular Eimac 3-500z with  the redish 
ceramic base.  Does my replacement tube need to be an Eimac or  can I 
substitute another brand 3-500z graphite or non graphite?
 
In addition would I encounter any issues running the amplifier with only  two 
tubes?
 
Thanks
Brad
KE0XS
724-684-6157