I have a reverb tank which was made in the early to mid '90's by a local builder who has sadly passed. I'm pretty sure he just copied a Fender tank schematic. I used to gig with it with no issues and after sitting for a few years I decided to fire it up and now I'm having a problem with it. I'm trying to isolate the problem which is that within a minute of playing time I just lose power; I have no sound. I want to try a different power tube to see if that's the problem. The tube he installed in the unit is a Ruby 6V6 GTA R. I'm almost surprised that he put a tube like that in because the preamp tubes are a Sylvania and a Bremer. I have some spare tubes and wonder whether or not I can use a JJ 6V6 S or an Electro Harmonix 6V6 EH? Am I OK to do this or do I need a 6V6 GTA R?