Blowing lots of V1 AX7's

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by supergenius365, Jan 12, 2008.


  1. supergenius365

    supergenius365 Silver Supporting Member

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    The 12AX7's in the V1 of my 1984 Boogie MKIIC+ seem to last about 3 weeks. At that point, they get "fizzy", lose bass definition and decrease volume. Pop a new one in and the problem is gone.

    I only play in the bedroom maybe 30 minutes a day and once a week for 2 1/2 hours - mostly clean with a Strat or light overdrive, so I don't think I am wearing them out.

    I know there are many factors involved. Any insight is appreciated.

    Thanks,
    J
     
  2. Trout

    Trout Member

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    Do you have a complete schematic? the only ones I have seen do not have a V1 on them, most likely incomplete.
    http://www.schematicheaven.com/boogieamps/boogie_mkiic_plus.pdf

    Most certainly you should be getting more than a week or 2 on a tube, Try switching brands? Possibly a bad batch?

    Trout
     
  3. John Phillips

    John Phillips Member

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    I would take a guess that one of the cathode caps for V1 (one on V1B and two on V1A - one of which is used all the time, and an extra one added when you use the Bass Shift) has shorted, which will run the tube extremely hot and shorten its life to a matter of hours.

    Do you use the Bass Shift switch? Does it still work? Does it make an unusually loud pop which switching it? If it's either of these two caps it should be possible to identify which one from this.

    If it's the one on V1B, none of the stuff with the bass switch will be unusual though.

    Could possibly be a couple of other things, but that's my best guess.


    (The schematic currently on Schematic Heaven does show V1, BTW.)
     
  4. supergenius365

    supergenius365 Silver Supporting Member

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    Here is the inside:
    [​IMG]

    What am I looking for color wise?
     
  5. Trout

    Trout Member

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    Oh My,, Its over in the corner, I totally missed it there LOL

    :D
     
  6. supergenius365

    supergenius365 Silver Supporting Member

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    The Bass Shift does work. I use it occasionally, but not on all the time. No pop I can remember, but I will have to fire it up and check.

    Thanks
     
  7. John Phillips

    John Phillips Member

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    Difficult to see - possibly a small silver cylinder. If it is a dead cap, it's most likely to be an electrolytic - they do fail when they get old, and 23 years is beginning to get to the sort of time where they might. I think I can see one just to the left of the right-most two orange caps, but I'm not familiar enough with the layout of the amp to be sure. It will be relatively near V1, anyway.

    Mesa drew the schematic backwards, which doesn't help :).
     
  8. supergenius365

    supergenius365 Silver Supporting Member

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    Will taking the metal can off the tube extend their life at all? This is my only amp and need to time taking it in for repair.
     
  9. John Phillips

    John Phillips Member

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    No.
     
  10. Backstage Kent

    Backstage Kent Member

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    Not an electrolytic, but a tantalum, which Mesa has a mysterious fondness for using for cathode bypass caps. Chances are good you've got one shorted. Replace both V1 bypass caps just to be on the safe side and I bet your problem disappears.
     
  11. BBQLS1

    BBQLS1 Member

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    Would checking the voltages reveal anything?
     
  12. John Phillips

    John Phillips Member

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    Yes - if you measure the cathode voltages on V1 (pins 3 and 8). The correct voltages will be in the 1 to 2V range. If you've got a shorted cap it will read zero (or very close to it) on the one with the dead cap. You'll also probably find the plate voltage (pin 1 or 6) is unusually low.
     

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