I recently worked on set with a 6BK4 HV regulator/shunt tube. How I know when these are bad???????? My tube tester doesn't have test data for this tube. Tube chart says just 6BK4, I found out later a 6BK4-A-B is available and a 6BK4-C version. Which tube do I use???, what are symtons of bad tube since can't test???
