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Weird amp problem or not?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by kingsxman, Jan 9, 2005.

  1. kingsxman

    kingsxman Silver Supporting Member

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    History: bought a JCM 800 amp that was blowing fuses when I got it. Replaced power tubes. Great for 2 gigs. THen started blowing again after running hard for a gig. Took to tech who found no problem and replaced with different power tubes. (I have had a little reliability issue with the Groove tubes I was putting in).

    First gig with new tubes last night and amp was running good..but then I got a crackling noise when I adjusted volume pot on guitar. I thought the pot on my guitar was bad as that is exactly what it sounded like. Crackling and just noise when I turend the pot. Grabbed other guitar...same thing. This kept up for about a set. Last set problem went away.

    I'm wondering if there is still something wrong with the amp or if this could have been a weird grounding issue.
     
  2. Blue Strat

    Blue Strat Member

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    One big question is whether or not you or your tech biased the new power tubes. If not, this is HIGHLY recommended.
     
  3. kingsxman

    kingsxman Silver Supporting Member

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    Mike, yes the tubes were biased.
     
  4. John Phillips

    John Phillips Member

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    The crackling volume pot is almost certainly caused by a very tiny DC leak from the first preamp tube grid. Change the first tube - the one furthest from the power tubes. You can swap it with one of the others if you don't have a spare.

    (That is, assuming you do have the guitar connected directly to the amp - if not it could be a DC leak from a pedal, it will do the same thing.)
     
  5. kingsxman

    kingsxman Silver Supporting Member

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    Ahhh, I do NOT have the guitar directly into the amp. I have it going through pedals....and I did notice that one pedal (only one on battery) was "probably" losing its battery. (Visual sounds H20 pelal). Maybe thats whats going on........
     
  6. kingsxman

    kingsxman Silver Supporting Member

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    John, re-read your post. Hmmm...the first tube farthest from the power tubes....thats V1 right? If so, I hate to say it..but its a new GT 12ax7m.
     
  7. John Phillips

    John Phillips Member

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    If any of the pedals are buffered, it's not the amp - the buffer will block any DC from the amp completely. If they're all 'true' bypass types, it could still be the amp, but only if it does it when all the pedals are turned off.

    Newness of a tube does not guarantee perfect operation, unfortunately...
     
  8. kingsxman

    kingsxman Silver Supporting Member

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    John, I doubt that all pedals are true bypass.
    In line with guitar: early 70's crybaby that I HAVE done true bypass on, Boss TU-2, Blues Driver, big box rat. In the effects loop I have a H20 pedal and a Boss EQ.

    My comment on the GT 12ax7m was meant more as a note that they have apparently had some issues.
     
  9. John Phillips

    John Phillips Member

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    Yes, I've heard plenty of people say thay've had problems with them. I've never tried one yet, but I'm not very inclined to at the moment.

    But, since you're running two Boss pedals (buffered) between the guitar and amp, it's not the amp that's causing the scratchy-pot problem. It should be whichever pedal comes first after the guitar, not counting 'true' bypass ones that are turned off at the time.
     
  10. kingsxman

    kingsxman Silver Supporting Member

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    Well, that would be my Boss TU-Tuner then....which hasnt given me a stich of issues before.

    I'll have to do some troubleshooting if the issue arises again. Would you say that it potentially could be a pedal in the effects loop or not?
     
  11. John Phillips

    John Phillips Member

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    No, definitely not.

    What's almost certainly the cause is a very slight DC voltage being fed into the guitar. It's this that makes the pot sound scratchy (I won't bore you with why! :)).

    Whatever is doing it must be the first device that the guitar is coupled to, excluding any 'true' bypass switched devices which are totally out of the signal path when off.

    Any buffered device will totally block any leak from further down the chain - so in this case, the fault can't be with the Blues Driver, the amp or any pedal in the loop (which are themselves buffered by the amp), since they all come after the TU-2 which is a buffered pedal. But it's probably the cause of the trouble too. I think I've come across this with a Boss pedal before, but I can't remember...

    If it is the problem, you can very easily stop it by swapping the TU-2 and BD-2 around - then the buffer in the BD-2 will block the leakage.
     
  12. kingsxman

    kingsxman Silver Supporting Member

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    Thanks John. Great advice. I'll give that a shot.
     
  13. kingsxman

    kingsxman Silver Supporting Member

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    Well, no issues this weekend.

    Hopefully its gone.
     
  14. muddy

    muddy Member

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    ****, i'm having a similar prob, but no scratchy guitar vol pot. i do get spurious crackling occasionally that'll lead to a blown fuse, & sometimes it blows without any noise. i thought, at first, that it might have been a grounding issue in the amp (smf 15watter), but i can't see anything suspicious looking inside. it's really starting to upset me, because it's gotten much more frequent lately. i thought that maybe it had something to do with the power in this building, as a lot of the wiring is ancient. i do use pedals, both buffered & true bypass; i even have a bs-2 buffer in the chain. i'd considered maybe putting in a new phase inverter, though i don't really know that it' be the culprit. i've been considering getting a good ac cable, and/or an ac power conditioner. what am i looking for? i'm not getting noise from the amp's volume pot from the clean channel; the v1 is one of those 10,000m mullards (60's), not a tube you would normally think of as being problematic. any ideas?


    ml
     
  15. kingsxman

    kingsxman Silver Supporting Member

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    WHen I was blowing fuses it was becuase of the power tubes. Have you tried another set in there?
     
  16. muddy

    muddy Member

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    and what if it's not the power tubes? also, how much, if at all, does the brand and/or type of preamp valve affect the tone? should i be blowin' a nos tube for this spot? or would a triode balanced jj do an equal job?


    ml
     
  17. muddy

    muddy Member

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  18. muddy

    muddy Member

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    DAMN!!! it looks like it was that ****in' expensive mullard in the v1!!


    ml
     
  19. kingsxman

    kingsxman Silver Supporting Member

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    Thats interesting. I didnt think preamp tubes woudl cause you to blow a fuse.
     
  20. John Phillips

    John Phillips Member

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    Extremely unlikely. It would have to be a filament short of some kind, which are very rare. The plate circuit can't draw enough current to blow a fuse.

    The #1 culprit for this sort of thing is a power tube, or occasionally a rectifier tube.

    If you watch the back of the amp, you might even see arcing inside one of these tubes just before the fuse blows.
     

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