Hi Tim,
No, it was in a pentode from heater to cathode (then all hell broke loose in the tube!) in a Conrad Johnson Premier 1 power amplifier. That heater lost 1/2 of the bias string and was forced to ground potential. I think there was a capacitor as well which is the thing that allowed the arc to be created. Then 6 x KT-88 tubes sparked as the $12 600V fuse opened in the B+ supply. I can't remember the rating of it, but the energy was impressive. It took forever to figure this out and the local C-J distributor couldn't find it either.
It was heartbreaking as the variac passed 90 VAC (120 V mains here) and all those tubes flashed at once. The tube that failed was the screen grid regulator for that channel.
Anyway, a 6X5 won't normally fail unless the input capacitance is simply too high. A popular trend these days.
-Chris
No, it was in a pentode from heater to cathode (then all hell broke loose in the tube!) in a Conrad Johnson Premier 1 power amplifier. That heater lost 1/2 of the bias string and was forced to ground potential. I think there was a capacitor as well which is the thing that allowed the arc to be created. Then 6 x KT-88 tubes sparked as the $12 600V fuse opened in the B+ supply. I can't remember the rating of it, but the energy was impressive. It took forever to figure this out and the local C-J distributor couldn't find it either.
It was heartbreaking as the variac passed 90 VAC (120 V mains here) and all those tubes flashed at once. The tube that failed was the screen grid regulator for that channel.
Anyway, a 6X5 won't normally fail unless the input capacitance is simply too high. A popular trend these days.
-Chris
The 6X5 failure mode from excessive first filter capacitance would initially be, as I understand it, from too high a current density being pulled off a hot cathode, leading to I guess degraded cathode surface (perhaps increasing on-resistance) and possibly then gas emission (which could lead to arcing from anode to anode or anode to cathode, and then stressing the power transformer).
Yes, and we also have to consider that a higher 1st filter capacitor leads to shorter, thus higher repeatedly charging pulses. Thjis *imho* is the main course of excessive cathode wear.
There also are indirectly heated rectifiers with the demand of connecting the heater to the cathode. The Philips EZ4 is an example (datasheet: http://www.mif.pg.gda.pl/homepages/frank/sheets/046/e/EZ4.pdf).
About four and a half decades ago, when I didn't yet know about this fact, I replaced the gassy EZ12 rectifier in a Grundig 475W radio (pictures here: Repairing a radio enclosure) that has it's heater fed from the same winding as the other tubes by an EZ4 thatI had laying around. Fortunately nothing happened, the device worked happily. As the grandson of the previous owner gave this radio to me some months ago, I was in some hurry to plug in the right tube, though
.Best regards!
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