Fuse value on SSE....and blown output transformer - diyAudio
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Old 23rd June 2015, 07:56 PM   #1
Pooch is offline Pooch  United States
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Default Fuse value on SSE....and blown output transformer

Here's my tale of woe... I fired up my SSE (Edcor XPWR035-120 power, Edcor GXSE15-8-5K ouputs) after about 2 months of inactivity. I left the room and came back to a blown fuse, heated output transformer to the point of smelling. Probably should have watched it warm up, but I've never had a problem with it firing up before. So I pulled the fuse and found I'd put a 5 amp slo-blo in there. Not sure why, but I'd ordered them for the build. Seems to me 3 amp would be what I should use, but 5 amp it was. So I took it apart and found C22 was blown. Replaced both C22 and C12, started it up and got red plate on the 5AR4. I then pulled the tubes, fuse didn't blow. Put rectifier in, got red plate. Disconnected outputs (T3-pri, T2-pri) and no red plate. Hooked outputs up, one at a time, to locate which output was the problem, found it. I then jumpered the good output to the bad outputs terminals, to see if there was also a board problem...and no red plate on the rectifier tube.
I have two questions, was the fuse value too high and should I be looking for additional problems? I'll order a new output transformer from Edcor.
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Old 23rd June 2015, 08:32 PM   #2
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Yes, and I would go looking for the root of the problem. C12 and C22 are feedback caps and not sure that would be the ultimate cause. I'd also look for a bad power tube. I run a 2 amp sb in mine and that's probably too high.
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Old 23rd June 2015, 11:06 PM   #3
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A 5 amp fuse is too high.

An amp built for 120 volt power should use a 2 amp fuse maximum. I used a 1.6 amp slo blo in my builds but they aren't common. I tried a 1.5 amp slo blo but Florida's random power surges would pop a fuse every month or so in the summer lightning season.

An amp built for 240 volt power should use a 1 amp fuse.

There are three possibilities that could have fried the OPT.

The transformer could have developed an internal short which placed the primary directly across the B+ cooking its guts. If any wire on the OPT measures a very low resistance (less than 10 ohms) to the transformer case, this could be the initial failure. An internal short on a new production OPT is rare, but it does happen. It is also possible that one of the transformer wires was pinched or cut where it transitions through the chassis causing the short that fried the OPT.

You say that C22 was blown. Was it shorted, measures a low resistance between the leads? If so this could be the primary failure mechanism. Yes, C22 is a feedback capacitor, but it is also the cathode bypass capacitor in a cathode biased amp. If the cap shorted out it would kill the bias on the output tube causing it to run away and draw extreme current, cooking the OPT. A new capacitor of decent quality that survived for the first few weeks, will rarely short out if it was installed in the correct direction. A bad cap will fail in the first few days. A counterfeit cap, may fail at any time since they are often surplus caps that may have a different voltage rating than what is on the shrink wrap.

C22, was blown....usually this is caused by a gassy output tube that goes into runaway. The tube starts to draw excessive current which causes more gas, which causes more current often accompanied with a blue glow, and more current. The excessive current causes the cathode voltage to rise, eventually to a point above the voltage rating, causing it to fail, often by spilling it's guts. This removes the bias from the already running away output tube causing the tube to short out, or arc. This is unfortunately too common on new production tubes, and more common after a long idle period. Usually this results in a blown fuse.

In any case the output tube that is associated with the shorted cap should not be trusted.
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Old 23rd June 2015, 11:37 PM   #4
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Thanks for the quick replies. I tried running it with no feedback and the rectifier plates did not glow red, so I tried it and got normal output on one channel, limited on the other. Switched speakers, to see if that was an issue and it wasn't. I did touch the speaker connections while doing this and got a zap up my my elbow! Something definitely fried in that output transformer. I checked the tubes on my tube tester before I started changing the caps and they were OK. I'll check again. I checked the speakers too, and they survived whatever that output was. I'm sure glad I grounded everything.
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Old 23rd June 2015, 11:54 PM   #5
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I forgot to mention, the cap ( C22) lost it's guts, when it originally failed. It was a 1500uf 50v Panasonic, 105 deg C. I replaced it and C12 with Panasonic 1500uf 63V, 105 deg C.
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Old 25th June 2015, 07:17 PM   #6
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If you got disconnected the ground from the speaker side of the OPT and the amp ran, but you got zapped by touching the speaker wires, then there is a short between the primary and secondary of the OPT.

If you can confirm this with an ohm meter, Edcor should replace the OPT if it is not too old.

I have seen such a short in a vintage transformer with paper insulation that was 50 years old and kept in Florida's humidity. This should NOT happen in a new OPT.
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Old 28th June 2015, 06:10 AM   #7
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Thanks, George, I'll check that out with Edcor. I just ordered a replacement. Any chance the cap spilling it's guts, is the cause of OPT failure? Maybe it's the other way around.
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