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Hi everyone.
Had an interesting failure today which I thought I'd share.
Got a vintage Holden 50 amp on the bench (manufactured in NZ and "final assembly"/sold here in Aus by Wasp Industries, common tactic during the days of tariffs).
Turned up with a blown fuse, quickly found that the installed pair of EL34's were shorted. Few other minor issues addressed before popping in the bench pair of EL34's, hooking it up to a dummy load, powering up on the light bulb limiter and checking rough bias/voltages. All looked good, with both EL34's decently balanced and drawing roughly 15mA (while still attached to the limiter).
So, onto testing into a speaker, where I discovered a static-y noise which had me chasing my tail for a while. Soon discovered that when audio probing, it was present ONLY when the power tubes were installed. With them in place the noise was found through the preamp too, but when they were removed the preamp was nice and clean.
Not sure what prompted me to do so, but I was checking primary resistance, and I was taken aback when I found that one half was open!
How could this be? I'd measured cathode current?
Thinking perhaps my meter was having trouble with the inductive winding (but unsure why one side would measure fine), I tried a different meter, same result. Tried diode test function, again one side open circuit. Pulled out the ESR meter which is handy for low resistance measurements, it's also telling me one side is ~40ohm, the other open circuit.
I put the tubes back in for a sanity check and yep, they were conducting, measuring B+ at the plate.
Deciding I must be mad, I pulled out a 9V battery, resistor, LED and some test leads, and proved that yes, that half was definitely open circuit, and it wasn't some measurement failure.
Finally starting to put the pieces together in my mind, I pulled out my insulation resistance meter (Megger), and hooked it up across that half primary.
100V setting: open circuit.
250V setting: open circuit.
500V setting: ding ding ding! Short circuit.
Seems that wherever the break was, the gap was close enough that somewhere around 500V it was breaking down and arcing, allowing the tube to seemingly "bias up fine" according to the (relatively) slow update rate of my DMM.
I pulled the transformer and cracked the endbells in the hope that perhaps if I was lucky the break might be in the flyout leads or where they were soldered to the windings, but no such luck.
I was able though to use the remaining half primary to check the impedance ratio, and got 212V:22V on the 16ohm winding, meaning a ~6k plate-to-plate primary, not a commonly available type, but something close will do.
Quickly hooked up one of my bench OPT's for testing, and the amp is sounding great.
Anyway, I found this set of symptoms fascinating, it's not something I've ever seen before. Anyone else?
Had an interesting failure today which I thought I'd share.
Got a vintage Holden 50 amp on the bench (manufactured in NZ and "final assembly"/sold here in Aus by Wasp Industries, common tactic during the days of tariffs).
Turned up with a blown fuse, quickly found that the installed pair of EL34's were shorted. Few other minor issues addressed before popping in the bench pair of EL34's, hooking it up to a dummy load, powering up on the light bulb limiter and checking rough bias/voltages. All looked good, with both EL34's decently balanced and drawing roughly 15mA (while still attached to the limiter).
So, onto testing into a speaker, where I discovered a static-y noise which had me chasing my tail for a while. Soon discovered that when audio probing, it was present ONLY when the power tubes were installed. With them in place the noise was found through the preamp too, but when they were removed the preamp was nice and clean.
Not sure what prompted me to do so, but I was checking primary resistance, and I was taken aback when I found that one half was open!
How could this be? I'd measured cathode current?
Thinking perhaps my meter was having trouble with the inductive winding (but unsure why one side would measure fine), I tried a different meter, same result. Tried diode test function, again one side open circuit. Pulled out the ESR meter which is handy for low resistance measurements, it's also telling me one side is ~40ohm, the other open circuit.
I put the tubes back in for a sanity check and yep, they were conducting, measuring B+ at the plate.
Deciding I must be mad, I pulled out a 9V battery, resistor, LED and some test leads, and proved that yes, that half was definitely open circuit, and it wasn't some measurement failure.
Finally starting to put the pieces together in my mind, I pulled out my insulation resistance meter (Megger), and hooked it up across that half primary.
100V setting: open circuit.
250V setting: open circuit.
500V setting: ding ding ding! Short circuit.
Seems that wherever the break was, the gap was close enough that somewhere around 500V it was breaking down and arcing, allowing the tube to seemingly "bias up fine" according to the (relatively) slow update rate of my DMM.
I pulled the transformer and cracked the endbells in the hope that perhaps if I was lucky the break might be in the flyout leads or where they were soldered to the windings, but no such luck.
I was able though to use the remaining half primary to check the impedance ratio, and got 212V:22V on the 16ohm winding, meaning a ~6k plate-to-plate primary, not a commonly available type, but something close will do.
Quickly hooked up one of my bench OPT's for testing, and the amp is sounding great.
Anyway, I found this set of symptoms fascinating, it's not something I've ever seen before. Anyone else?
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