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Leslie 251 blows fuse

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  • Leslie 251 blows fuse

    Working on a 251 everything was up and running when she blew a fuse. Now upon startup she makes an instant 60 hz humm from the 15" speaker and pops it's fuse. This happens instantly before the tubes warm up. I was looking into cleaning the motors when this occurred. I've tried without the motors connected but it still blows the fuse. I examined the 6 pin connector plug and all tests out well. I thought about power caps but it worked fine until it blew, plus I'm not hearing the amp as it won't warm up. The humm is not real loud. I see the OP tranny has a center tap to ground. I can't figure what supplies the other half to make her humm. The rectifier tube looks good. I have had it up and running a minute with an oversize fuse but humm is present. I'm baffled.
    C3, Leslie 720, A100, E100, Have owned L100 and M3, http://soundcloud.com/twiggybush

  • #2
    How does a 251 amp have a rectifier tube???

    Never use an oversized fuse, not even for diagnostic purposes. I have a dish of 20A fuses on my bench, mostly out of amps with major damage as a result. Fuses blow for a reason, and you can cause collateral damage by trying to circumvent them.

    The fuse-blowing is probably caused by one of the usual suspects: An output tube with a short, a shorted filter capacitor, a shorted diode(s) in the power supply, or a bad power transformer. If it's not one of those, it's going to be something more obscure like one I worked on that had a short between the primary and secondary windings inside the output transformer.

    The motors are not fused at all in 251s, so disconnecting the motors wouldn't make any difference.
    I'm David. 'Dave' is someone else's name.

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    • #3
      You nailed it David. A shorted 6550. I'm unfamiliar with the amp and assumed the large odd tube to be a rectifier. The owner ran the over size fuse in my absence.
      I pulled the 6550's and the hum went away and reverb amp worked. In hind site I recall a tiny burn smell when she blew. The suspected tube smells smoky at the pins. Thanks for your help. Keith
      C3, Leslie 720, A100, E100, Have owned L100 and M3, http://soundcloud.com/twiggybush

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      • #4
        You may want to check components connected with said 6550's as a burn smell suggests something went with it or got stressed to the point of burning. The 150 ohm resistor would be a good place to start.
        Hammond B3 (55), B3 (70), B3 (72), B2 (51) conversion, A100 (61) chop, A100 (62), A105 (75), Northern BC (39) empty.
        Pile of Leslies of various flavours, Minimoog, ARP Odyssey, MaxiKorg, Hohner D6, Rhodes 54, Rhodes 73, Wurlitzer A200, Wurlitzer A203W

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        • #5
          Originally posted by nonreverb View Post
          You may want to check components connected with said 6550's as a burn smell suggests something went with it or got stressed to the point of burning. The 150 ohm resistor would be a good place to start.
          It depends on the nature of the short. I'd check the 150 Ohm resistor, but also the attached cathode bypass capacitor since some shorts might cause this cap's voltage rating to be exceeded. I've seen one cathode bypass that burst its seal from an output tube problem.
          I'm David. 'Dave' is someone else's name.

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          • #6
            Originally posted by David Anderson View Post
            It depends on the nature of the short. I'd check the 150 Ohm resistor, but also the attached cathode bypass capacitor since some shorts might cause this cap's voltage rating to be exceeded. I've seen one cathode bypass that burst its seal from an output tube problem.

            A burning smell justifies checking all related components around a blown power tube.
            Hammond B3 (55), B3 (70), B3 (72), B2 (51) conversion, A100 (61) chop, A100 (62), A105 (75), Northern BC (39) empty.
            Pile of Leslies of various flavours, Minimoog, ARP Odyssey, MaxiKorg, Hohner D6, Rhodes 54, Rhodes 73, Wurlitzer A200, Wurlitzer A203W

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            • #7
              Out of curiosity, what make/year of output tubes are you using?

              I saw some not-very-old JJ 6550s have a similar failure recently.

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