A question for the Techs

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Dickie Fredericks, Mar 5, 2006.

  1. Dickie Fredericks

    Dickie Fredericks Member

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    Could a bad tube cause a mains fuse to go? Im wondering cause I was at practice tonight and the amp just stopped working. Has a blown mains fuse. I also notice that one of the Yellow Jackets has a slight bulge in it as well.

    I of course tried to use the fuse from the unused fuse holder of the unused tube sockets but to no avail as I guess all the fuses have to be good.

    I recon I can get fuses for this thing at Radio Shack? or am I gonna have to order them from Mojotone?

    Dickie
     
  2. VaughnC

    VaughnC Supporting Member

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    Yes, a bad tube, or dozens of other things could cause a fuse to blow. I've never seen an amp with an "unused fuse holder"....perhaps the 2nd fuse is for the high voltage circuit only?

    Anyway, Radio Shack generally has the common fuses. You just need to get the proper amp rating AND type (fast or slow blowing). Tube amps typically use the slow blowing type to allow for the brief surge of current when the amp is first turned on.
     
  3. Dickie Fredericks

    Dickie Fredericks Member

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    No what I meant was: This amp has 4 tube sockets. Im only using 2 tubes in this amp. This amp has a fuse for tubes 1/4 and 2/3. Since I had no tubes in 1/4 I was gonna try to use that fuse to see if the amp would come on. It didnt cause Im sure you have to have all the fuses in place for the amp to work. Thanks for the reply VaughnC
     
  4. RussB

    RussB low rent hobbyist Silver Supporting Member

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    Dickie,

    Stop being so mysterious, and tell us what amp you're talkin about!
     
  5. VintageJon

    VintageJon Member

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    The Prime Suspect in a "blown fuse" is a failed tube. Yellow Jackets are known to fail above 450V or so. If it has a bulge it's probably bad.

    Note that you spoke of mains fuse and unused fuse so I suspect this is a modern Marshall...

    Please Confirm,
    Jon
     
  6. John Phillips

    John Phillips Member

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    If it's a JCM900 with the separate tube fuses you can definitely run the amp with one fuse out - that's the easiest way of turning off two of the power tubes, if you want to. You don't have to have all of the fuses in.

    However, using the 'spare' fuse in the mains fuseholder will blow it immediately since it's totally the wrong value - 500mA instead of 2A (UK) or 4A (US).
     
  7. Dickie Fredericks

    Dickie Fredericks Member

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    Yeah John nailed it. JCM 900 the same one weve been discussing the last few months John. Russ that was a funny post. Almost sig worthy LOL.


    Flash forward to today. hehe

    I put a new fuse in the mains. 4 amp. Installed 2 known good 5881's. No power up. No light no nothing. Im now gonna suspect that I have a blown transformer?

    And Im gonna blame it on the Yellow Jackets. No reason for that buldge to be there. Im thinking the bad YJ is the cause of the issue at hand.

    Yeah I can get a direct replacement from Mojotone and since Ive had experience with wiring and solder, could probably do the replacement myself. However, do any of you guys know of a good tech in the Charlotte area? Or maybe Columbia,SC? or Florence SC?

    Or even more interesting... would anyone wanna help me out via phone or PM in getting it done? See I live in the freakin woods. Charlotte is an hour and a half away and so is Columbia (yeah I live right in between but how else are ya gonna afford 15 acres?)

    Id really be up for doing it myself. Ive changed out transformers before in Air Conditioning systems. I have decent soldering skills. Anyone up for it?

    Maybe its not the tranny? Ill let you guys be the judge. As always, I appreciate your wealth of knowledge.

    Dickie
    formerly known as Marshall 4102
     
  8. Dickie Fredericks

    Dickie Fredericks Member

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    Checked Mojotone and the replecement tranny is $125.00. Doesnt seem to hurt too bad there. Im gonna call em in a few for specifics. I think Id like to replace it with a like tranny. Dagnall.
     
  9. brad347

    brad347 Member

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    wait just a minute, it might not necessarily be the transformer. Do you have a meter? If so, there are tests you can do on your tranny to see if it is indeed bad.
     
  10. Dickie Fredericks

    Dickie Fredericks Member

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    Well I have a meter. Not digital but it does read voltage and ohms.You know the 2 probe kind. Up to 500 volts.
     
  11. Dickie Fredericks

    Dickie Fredericks Member

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    FWIW I called MF and got an RA number and FedEx label to send the YJ's back for a replacement.

    OK Its 9 PM here and I gotta get up at 4am for work. Ill check back here tomorrow at 6pm ET. Until then, let me know what tests to do on the tranny and Ill try em while Im online here at The Gear Page with ya...

    Dickie
     
  12. Dickie Fredericks

    Dickie Fredericks Member

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    The chassis is out of the cab. Time to track down where the problem is in this beast...
     
  13. brad347

    brad347 Member

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  14. Dickie Fredericks

    Dickie Fredericks Member

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    Yeah I started reading the page you gave me. Tried #1. Hit every lead coming off both transformers and they all peg the needle at 0 ohms. Since the primary should be under 1k ohms Im assuming that so far we have a good transformer. Im gonna get the schematic out tomorrow so I can see exactly which ones are the primaries and stuff. Probably a good idea to know where to put the meter.

    Dickie
     
  15. Dickie Fredericks

    Dickie Fredericks Member

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    Hhhm,m tried that thing with no tubes in and put the leads on the primary (red) and the center tap (I think its a centertap) which goes to the standby switch and it pegged the meter. 500 volts. Interesting. This was when I had the switch off then turned it on. Meter jumped and pegged there.
     

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