[Jukebox-list] Fw: Question

[Mel Knight]

Hi Ron & David!

           I have seen this happen many times over the years, and it almost always was a NOS 5U4 or other rectifier tube that was at fault! Mostly I believe the tube has been vibrated a lot in someone's service truck for many years, or just a poor job at the factory. What I have found is that either the tube has some air infiltration allowing it to arc or some of the cathode coating has flaked off and is creating a bridge which creates a short when power is applied. Usually the air will cause the tube to glow really weird internally and the loose cathode coating will cause a cool light show, lot's of spitting, sparks and such! These faults almost never show up on a tube tester, as a rectifier tube has lots of voltage across the elements, and moderate current, especially the startup inrush. As for the fuse not blowing in the amp, the tube was probably shorted on one element and was causing a half cycle pulse, the MDL fuse takes several minutes to heat up and blow, as the MDL fuses that Seeburg and other manufacturers used were to prevent "needless" blowing! Needless to say, the newer type slo-blow types available today provide better protection for the transformers......   Mel