There are thousands upon thousands of SB-220 amps that still have the original tubes. The filament voltage is fine in those amps. Neither poor de gassing or filament voltage would generally cause a G-F short. G-F shorts happen as a fact of life and often times inside the warranty period especially on Chinese tubes. They should cover it under warranty however they usually make you jump through fired hoops before they do.
Since you didn't mention anything about the current(s) or power measured during this dance, we might assume you don't know them. Which would, along with the usual BS about running the tubes untill they start glowing some or another shade of red or yellow, Would lead me to believe that the tube failed from abuse and not some sort of manufacturing defect. Rege
Well I do know them and they were not out of the normal even with the shorted tube only thing that was noticed was loss of output. I don't abuse my equipment the backup set of tubes are original to the amp and they are still perfectly fine just a little less on output but they are 40 years old. I honestly think it was just a bad tube and it' really not a big deal even if they don't want to warranty it. 73'S K4ATR And god less
