I was playing and all of the sudden....

Discussion in 'The Workbench' started by matttornado, Nov 8, 2020.

  1. matttornado

    matttornado Well-Known Member

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    my volume dropped considerably and it sounded like i turned on a distortion box.

    haven’t had a chance to investigate too much but i checked my preamp tubes and they were all good. no fuses blown...

    hopefully it’ll be something somewhat obvious.

    amp is my 74 superlead
     
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  2. Sapient

    Sapient Silver Supporting Member Platinum Supporting Member

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    Did you check the power tubes?

    I'm not the guy really to solve this, but I did at least have that question.
     
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  3. matttornado

    matttornado Well-Known Member

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    not yet. i plan on it though.
     
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  4. Sapient

    Sapient Silver Supporting Member Platinum Supporting Member

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    A drop in power could easily be the ...power tubes. Worth checking out.
     
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  5. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Out of curiosity, how did you check the tubes?
     
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  6. MarshallDog

    MarshallDog Well-Known Member Gold Supporting Member

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    I would say there is a very good chance its the power tubes.
     
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  7. matttornado

    matttornado Well-Known Member

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    replaced each one with a known good one.
     
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  8. neikeel

    neikeel Well-Known Member

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    Have you tried the amp again?
    So I presume your output selector is tight in the sockets, what condition are your output tube sockets in? Any sign of burning or charring between pin 2 and 3 up top?
    How old are your output tubes?
    Suggest you do the above and retry the amp on light load etc.
    If this mode of failure is maintained when all of the above is ok you may have a dead or dying OT.
     
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  9. matttornado

    matttornado Well-Known Member

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    i regularly do maintenance and check those things but i’ll have to check those things again.
    it is the original OT and has taken the abuse of being cranked on 10 for gigs and practices for the past 20+ years.
    coincidentally, the amp been sounding weird lately, sort of like part of the signal is not going all they through. sounds thin and trebly when normally it’s thick and fat. checked EVERYTHING as far as that issue goes but i still need to reflow all of my latest solder joints.
     
  10. matttornado

    matttornado Well-Known Member

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    tube sockets are original. i replaced the impedance selector with a modern type many years ago.

    i will check out the power tubes, sockets and bias hopefully tonight.

    this amp been sounding funny for awhile now, sort of like not all of the signal is getting through. sounds thin and trebly when normally it’s fat and warm.

    i’m hoping it’s not the tubes and it’s a component failure and that this issue is also why my amp been sounding generally off prior to this new problem.
     
  11. _Steve

    _Steve Well-Known Member

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    I had a similar problem and it turned out to be my Bass pot. Might be worth checking that too.
     
  12. matttornado

    matttornado Well-Known Member

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    thanks for suggesting.

    I replaced bass mid and treble back in the summer.
     
  13. matttornado

    matttornado Well-Known Member

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    i checked power tubes and bias supply. all good there. Next up I will measure preamp voltages and do a chop stick test and listen as i work toward the input.
    Maybe i’ll find something? lol
     
  14. matttornado

    matttornado Well-Known Member

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    checked all of the tube voltages and they are fine. tubes all checked good. speakers / cab good. speaker cable good.

    next up a “click text” to see if if i can spot the weak spot.

    I read that an output tranny can be tested by, “measuring the B+ voltage while playing a power chord. If the voltage drops below 100 volts, it’s bad.”
    I did that and the B+ stayed above 475vdc. I have no clue if that test made any sense or not, i did it out of curiosity.
     
  15. neikeel

    neikeel Well-Known Member

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    After all your tests does it still sound bad?
     
  16. 2L man

    2L man Member

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    I don't know if that is good test but you should see AC voltage on power tube anodes using a multimeter when playing low notes.

    If your amp is built to a circuit board you should check tube socket solders. Also potentiometer and jack solders to circuit board. Tube socket solders to circuit board can eventually detach because pins expand and shrink when tubes heat and cool and also changing tubes cause mechanical stress. Take out as much solder as possible using vacuum tool or using solder soak wire using additional flux and then resolder using a solder which still has lead in it.

    Esa
     
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  17. neikeel

    neikeel Well-Known Member

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    It’s a 1959 so all hardware is chassis mounted.
    Difficulty if fault is intermittent.
     
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  18. matttornado

    matttornado Well-Known Member

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    I was playing when it happened and it sou
    yep . hardly any volume and lots of splatty distortion.
     
  19. neikeel

    neikeel Well-Known Member

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    Well carefully check all your solder joints and then run a voltage check of all your valves and report back.
    Do you know anyone with a scope?
     
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  20. GIBSON67

    GIBSON67 Well-Known Member

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    I know you checked the power tubes, how about the preamp tubes?
     

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