Hi,
A SSE amp that I’ve built almost two years ago and working properly since then, started a fire in the power transformer. It’s an Edcor XPWR-059 ( the 220v version of the recommended one ). The only reason I believe could be is that as there were little space between the top shelve and little ventilation, the hot went up so high that the power transformer meltdown.
Can anybody provide any clue?, could anything be broken afterwards?
Regards
Jorge
A SSE amp that I’ve built almost two years ago and working properly since then, started a fire in the power transformer. It’s an Edcor XPWR-059 ( the 220v version of the recommended one ). The only reason I believe could be is that as there were little space between the top shelve and little ventilation, the hot went up so high that the power transformer meltdown.
Can anybody provide any clue?, could anything be broken afterwards?
Regards
Jorge
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Unlikely that the surrounding temperature had anything to do with it. It is far more likely that it was not sized properly and that the internal temperature became too high. I always check after a build by measuring the resistance before and after it has been running how high the internal temperature is - you can find the formula's in the RDH4.
The XPWR059 should be fine for an SSE unless the output tubes were biased unusually high, in which case they would have been glowing hot themselves.
The output tubes in an SSE usually run at 50 to 70 mA depending on tube choice, and each half of the 12AT7 eats about 7 to 9 mA. So worse case HV consumption should could have been 160 mA or so.
A serious fault like a shorted cap or rectifier should have blown the fuse, so should a shorted transformer. In any event, even a bad transformer should have blown the fuse before erupting in flames. The fuse in a 240 volt SSE should be 1 amp, 2 amps in a USA spec 120 volt SSE.
It is possible for a gassy tube to draw excessive current, but a that is gassy enough to cook a power transformer should have exhibited obvious distortion, and probably a blue / purple glow.
There have been a few instances with Sovtek / EH / Tung Sol reissue KT-88 tubes running away due to gas after a year or two of use, but they tend to show signs like loss of getter and distortion before that happens. My 10 year old EH KT88's still work fine.
A runaway tube that went to failure usually blows the fuse, but can take out a cathode bypass cap or even the cathode resistor or rectifier tube.
Wrong fuse already...
Hi,
The fuse was wrong.... it was way up of 1 amp.....I’m embarrassed....
The bias resistors are the stated 560 Ohms and the tubes used are EL-34.
Just in case there weren’t any fuse, could be something else wrong along with the transformer, taking for granted that the power transformer meltdown due to excessive heat?
Regards
Jorge
Hi,
The fuse was wrong.... it was way up of 1 amp.....I’m embarrassed....
The bias resistors are the stated 560 Ohms and the tubes used are EL-34.
Just in case there weren’t any fuse, could be something else wrong along with the transformer, taking for granted that the power transformer meltdown due to excessive heat?
Regards
Jorge
The ultimate cause of a power transformer failure is excessive heat. This comes from drawing too much current from a transformer, or an internal short inside the transformer itself, usually from a winding defect in manufacture.
If a single (or a few) turn of wire in a transformer were to short out due to a bad spot in its insulation, or a kink in the wire when wound, it would get very hot. The localized heat will eventually damage the insulation of neighboring wire causing more shorted turns and more heat, leading to eventual thermal runaway and a fried transformer.
An overload condition on one or more windings will cause a lesser amount of excessive heat, but spread over a larger number of turns, causing a heat buildup deep in the winding. The HV winding is worse case, followed by the primary itself, because there are more turns of thinner wire, usually buried closer to the transformer core. The internal temperature deep inside these windings could get high enough to cause an insulation failure leading to one or more shorted turns and eventual thermal runaway.
This used to be fairly common in consumer grade equipment from the 40's through 60's where the wire insulation was enamel and the winding insulation was paper. Modern transformers use high temperature materials that can withstand some serious heat.
It is possible for a bad (gassy) output tube to draw excessive current and slowly cook the transformer over long periods of time, but usually the distortion would become noticeable long before damaging the transformer.
If the amp had the solid state rectifier option, even if turned off or not connected, it is possible that a diode could have shorted due to a power surge. This would have ordinarily blown the fuse instantaneously, or caused the amp to quit working. If the amp was left on even for a few minutes in this condition, it would fry the power transformer.
I suppose that a bad (shorted) rectifier tube could do the same thing. I have never seen a shorted 5AR4 (GZ34) but shorted 5Y3's and 5U4's do happen if the filament breaks and touches the plate (rare, but I have seen a couple).
A blown (shorted) C1 or C2 could do it too, but this is also uncommon in an amp that has worked fine for two years.
If a single (or a few) turn of wire in a transformer were to short out due to a bad spot in its insulation, or a kink in the wire when wound, it would get very hot. The localized heat will eventually damage the insulation of neighboring wire causing more shorted turns and more heat, leading to eventual thermal runaway and a fried transformer.
An overload condition on one or more windings will cause a lesser amount of excessive heat, but spread over a larger number of turns, causing a heat buildup deep in the winding. The HV winding is worse case, followed by the primary itself, because there are more turns of thinner wire, usually buried closer to the transformer core. The internal temperature deep inside these windings could get high enough to cause an insulation failure leading to one or more shorted turns and eventual thermal runaway.
This used to be fairly common in consumer grade equipment from the 40's through 60's where the wire insulation was enamel and the winding insulation was paper. Modern transformers use high temperature materials that can withstand some serious heat.
It is possible for a bad (gassy) output tube to draw excessive current and slowly cook the transformer over long periods of time, but usually the distortion would become noticeable long before damaging the transformer.
If the amp had the solid state rectifier option, even if turned off or not connected, it is possible that a diode could have shorted due to a power surge. This would have ordinarily blown the fuse instantaneously, or caused the amp to quit working. If the amp was left on even for a few minutes in this condition, it would fry the power transformer.
I suppose that a bad (shorted) rectifier tube could do the same thing. I have never seen a shorted 5AR4 (GZ34) but shorted 5Y3's and 5U4's do happen if the filament breaks and touches the plate (rare, but I have seen a couple).
A blown (shorted) C1 or C2 could do it too, but this is also uncommon in an amp that has worked fine for two years.
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