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Slash sig/Jubilee tube fails

Discussion in 'The Workbench' started by Dan Elder, May 17, 2021.

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  1. Dan Elder

    Dan Elder New Member

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    HI, 1st time posting.
    I have a 90's era Slash signature jubilee. Back in Oct '20 I took it to a gig and the HT fuse blew. One tube dead (forget which one) Replaced with another matched quad. Biased to 37mA, burned in for a few hours in basement. Took it to a gig last week, sound-checked, left on for 45 min. Went to tune up (did not have on standby) and the mains fuse blew. Got home and replaced mains. turned on but V3 tube arced . I replaced the HT fuse. the amp did start working but that V3 tube had an off the chart current reading. Changed sockets, issue followed the tube.
    Got a replacement tube to match. Amp is working, sounding fine, all tubes at 37ma average. I've read the plate load resistor could be the issue. Is that something that could work ,then not work/cause issues? or if it is the resistors would it cause issues in a particular spot (I.E. V3) every time. I know that "they don't make tubes like they used too". Just seems odd that I've blown 2 tubes in a short time. Again, can't rememebr which tube blew on the 1st round. Just looking for opinion on if I should try and find a shop to give it a once over, or if there's anything that can check since its working. Appreciate any assistance, thanks
     
  2. Chris-in-LA

    Chris-in-LA Well-Known Member

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    I would measure those resistors and see if they are good.

    Might be tubes. But make sure amp is set for the right speaker load.

    You’re not running an attenuator are you?
     
  3. pedecamp

    pedecamp Well-Known Member

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    What brand tube, if Tungsol they have a reputation for blowing in V3, try a JJ there. :yesway:
     
  4. Dan Elder

    Dan Elder New Member

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    No attenuater. 16ohm load (2 8ohms in series in a 1936
    cab.) set correctly
    1st blown tube was EH, this round is TheTubeStore.com's preferred series.
     
    Last edited: May 17, 2021
  5. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Just to be clear, are we talking about power tubes or preamp tubes? V3 is usually the phase inverter but you are changing out quads of power tubes. I’m confused. What was the “off the chart reading?”
     
    Pete Farrington likes this.
  6. Pete Farrington

    Pete Farrington Well-Known Member

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    https://drtube.com/schematics/marshall/2555amp.gif
    V3 is the LTP phase splitter, not a particularly demanding application, cathode sits at maybe 100VDC. Unlike the DCCF in V2 of the earlier tweed Bassman based preamps, there doesn’t look to be any potential for a start up current surge or inter electrode arcing.

    But as you’re having trouble, for the short term at least, perhaps consider a solution used there, ie only use vintage or Chinese manufactured ECC83 in that socket.
    Identifying the root cause of the issue, and hopefully getting it resolved, would of course be best.

    But from the context it seems more likely that the valve nomenclature has been reversed, and an output valve / socket is being referred to. If so, and the issue is obvious and persistent, then it would be best to take it to a competent tech. As the grid stopper, valve socket etc may be the issue.
     
    Last edited: May 18, 2021
  7. Dan Elder

    Dan Elder New Member

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    ya upon further review...Marshall calls pre-amp tubes V1-3, and the power tubes V4-7, so in that context it would be the V6 tube that had the issue. I never got an actual reading but when the multi meter was set to AMPs
    it just read OL
    I've since received a replacement tube. Replaced. Opened chassis, saw no sings of blown components. Played on good volume. Burned in for about 8 hours, No issues. hopefully was just a rash of bad tubes.
     
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