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Thread: Sunn 200S blows 3amp slo-blo after about an hour in service. Anyone know how to fix?

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    Sunn 200S blows 3amp slo-blo after about an hour in service. Anyone know how to fix?

    Sunn 200S blows fuses 3 amp slo-blo. Unit will go weeks without trashing a fuse, then after an hour of
    use it will blow a fuse. Have changed the 2 prong to grounded plug. Did the same thing with older tubes
    in it. Have replaced power tubes with matched Groove tubes. Works great for awhile, then....

    Anyone have a suggestion. Nothing inside seems burnt or out of place.
    Thinking of replacing the .0047MFD 600VDC cap across the line current input.
    Another website suggested the cap may be bad.

    Have not tried pulling tubes to see if the fuse blows without tubes. As I said,
    it seems to happen after about an hour in service and then not all the time.

    Thanks

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    Old Timer soundguruman's Avatar
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    What can happen is the bias filter capacitor will slowly short out.
    Also other electrolytic caps go bad with age.
    This is the 50uF cap at point D on the schematic.
    sunn_200s.pdf
    Sorry if this PDF does not attach correctly...
    actually there is two 50uF in the bias circuit.
    Replace both those with 47 uF 150 Volt.

    Then you have to set the bias for the tubes you are using.

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    If you have changed over to a 3 wire grounded plug, the .0047uF 600V cap across the line current input should be disconnected. It is not needed and can be a safety hazard.

    I worked on a Fender Bassman 100 that did this and it was a bad tube socket....it would eventually arc and short out and blow a fuse but it didn't happen consistently or right away.

    Greg

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    Supporting Member Jazz P Bass's Avatar
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    It sounds like 'nuisance tripping' to me.
    What has not been determined (or stated) is "how much current is the amp pulling from the mains, at idle & at full power?"
    Simply because the name plate states '3 Amp time delay fuse' does not mean that is what fuse is required.
    The 3 amp rating may have been fine at 117Vac line voltage (or right on the edge).

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    Line current.

    Quote Originally Posted by Jazz P Bass View Post
    It sounds like 'nuisance tripping' to me.
    What has not been determined (or stated) is "how much current is the amp pulling from the mains, at idle & at full power?"
    Simply because the name plate states '3 Amp time delay fuse' does not mean that is what fuse is required.
    The 3 amp rating may have been fine at 117Vac line voltage (or right on the edge).
    Howdy. The Sunn has blown a fuse about every few months. I replace the 2conductor line cord, because I was using the
    amp at a gig, and I got shocked when I touched the guitar cord. I suspected a bad ground somewhere. I replaced the
    cord and ground it to the chassis. This seemed to make everything ok. It just blew the last fuse last Thursday after
    typically an hour of being on (standby switched on). The the last time it blew a fuse was at home, so the line voltage
    could have been different at both locations. I haven't checked either, but I will. I suspected the line cap across the
    line current input, but apparently this is not needed, since it's grounded. The polarity switch may not be needed either.
    & since I replaced the cord, the polarity switch makes no difference in the sound or hum that is now gone.
    I have replacement filter caps for the unit and will replace the originals. I have done this on other Sunns I have and it
    makes a difference in the sound/dependability. I see no burnt cracked resistors, but I'll keep checking everything, until the issue
    is resolved.

    Thanks

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    Quote Originally Posted by soundguruman View Post
    What can happen is the bias filter capacitor will slowly short out.
    Also other electrolytic caps go bad with age.
    This is the 50uF cap at point D on the schematic.
    sunn_200s.pdf
    Sorry if this PDF does not attach correctly...
    actually there is two 50uF in the bias circuit.
    Replace both those with 47 uF 150 Volt.

    Then you have to set the bias for the tubes you are using.

    Howdy. The Sunn has blown a fuse about every few months. I replace the 2conductor line cord, because I was using the
    amp at a gig, and I got shocked when I touched the guitar cord. I suspected a bad ground somewhere. I replaced the
    cord and ground it to the chassis. This seemed to make everything ok. It just blew the last fuse last Thursday after
    typically an hour of being on (standby switched on). The the last time it blew a fuse was at home, so the line voltage
    could have been different at both locations. I haven't checked either, but I will. I suspected the line cap across the
    line current input, but apparently this is not needed, since it's grounded. The polarity switch may not be needed either.
    & since I replaced the cord, the polarity switch makes no difference in the sound or hum that is now gone.
    I have replacement filter caps for the unit and will replace the originals. I have done this on other Sunns I have and it
    makes a difference in the sound/dependability. I see no burnt cracked resistors, but I'll keep checking everything, until the issue
    is resolved.

    Thanks

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  7. #7
    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by hugnaba View Post
    It just blew the last fuse last Thursday after
    typically an hour of being on (standby switched on). .
    Was the amp just sitting there idling for an hour or where you playing it for an hour?
    If the fuse blew while playing, then that begs the question 'how loud?'.

    There is a distinct current draw difference between idling & maximum power.

    I am not saying that the fuse rating is the problem but it must be ruled out.

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    It was actually on, to amplifiy a keyboard, which was not being played. Volume was on about 3 if that and treble/bass was also level @ 3. There are high and a low boost switches, but those were not used.

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    Supporting Member Jazz P Bass's Avatar
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    Without any current draw measurements, we are still only guessing.

    But at those settings, the amp surely should not have been challenging the 3 amp fuse.

    Still, it would be helpfull to know what the amp is drawing at idle.

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  10. #10
    "Thermionic Apocalypse" -JT nickb's Avatar
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    If you got a shock with the two prong lead it could be an indication that that .047uF 600V in the ground switch is bad, as you suspect. It might also be the reason for the excess power draw. I would yank it out in a heartbeat.

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    Experience is something you get, just after you really needed it.

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