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Old 2nd April 2011, 02:34 AM   #1
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Default Motorola SE console amp blown fuse

working in dim light with 46 year old eyes I inadvertently switched one of the el84 output tubes with the ez81 tube in this
former console amp. When I powered it up I blew an inline fuse off the PT.
What are the odds that I did serious damage to the any of the transformers.
I don't have any 3A slow blow fuses to soldier in at the moment and I am sick over this blunder. Any thoughts would be greatly appreciated.
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Old 2nd April 2011, 03:47 AM   #2
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I wouldn't be too worried just yet. The fuse probably did it's job and saved the power transformer. It takes time with a heavy overload to cook a transformer. If there was no heating or burnt smell from the tranny, it's most likely ok. Looking at the pinout for both tubes, the EL84 became a diode across half the PT secondary as it warmed up and that's what caused to fuse to go. Be glad you had the proper size fuse in place. Were it too big, you would have cooked things. We've all done stuff like that too, so don't feel pregnant.
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Old 2nd April 2011, 04:04 AM   #3
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Thanks for the info and the kind words. Serves me right trying to wire this up before my wife realized I bought another piece of audio equipment!
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Old 3rd April 2011, 12:11 AM   #4
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Can you indulge me one more question? What slo blo should I put back in there? Is a fast blo better? Should I put in a fuse holder to save time over re-soldering in the event that I have to do this again? Ok, that's actually three questions.

Thanks,

Jason
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Old 3rd April 2011, 01:42 AM   #5
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A thru-hole fuse holder is always a convenient modification. I've even done it to wall-warts were space permitted. The value of the fuse needed is dependant on the number of tubes and thus the full load current. Since I don't know exactly what you have, I'll guesstimate based on years of servicing. For a pair of EL84s plus a couple more small tubes, 1 to 2 amps should do it. For four EL84s and a few more small tubes, 2 to 3 amps at most. Fast blow types unless you have turn on serge problems. (Fast verses slow is designers choice. Both types are used successfully.)
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Old 3rd April 2011, 05:27 PM   #6
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Thanks again for your input. I am going to go ahead with the fuse holder and cross my fingers that I didn't trash the tubes or worse. I didn't want to invest a lot of money into this. The plan was to buy a cheap working el84 get a sense of a SE Amp for my compression drivers and later consider something better like Decaware etc. Anyway,

Thanks again,

Jason
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Old 9th April 2011, 02:45 AM   #7
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Finally got the fuse soldered back in and all looked good but when I got it running the volume was very low. I am using a Dynaco Pas2 preamp and running compression drivers that should be pretty efficient. I am wondering if I cooked the rectifier tube which is an EZ81. It glows but quite red in color. Would this be the first thing to check or should I check the output transformers?


Thanks,

Jason
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