More amp trouble - second SOS!

Discussion in 'Amps & Cabs' started by RVA, Nov 14, 2015.

  1. RVA

    RVA Well-Known Member

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    After replacing the tube and the fuse on my Peavey Triumph 120, I started getting a hum when the amp is fully engaged (not in standby).

    After watching this video on different causes of tube hum,



    I checked and found that the hum is not responsive to volume changes. I pulled out the chassis and swapped out each 12ax7 with a spare. I used the same 12ax7 to briefly check the 12at7 spot since I did not have a spare 12at7. I pulled 2 power tubes, check, replaced them, and then pulled the other 2 power tubes. None of this had any effect on the hum.

    I did notice that the noise level of the hum increased briefly from cold to warm, but leveled off quite quickly.

    Does anyone have any ideas at this point, or is it time for a tech?
     
  2. donepearce

    donepearce Well-Known Member

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    Things you can check? Well, it is not responsive to the volume control, so it is happening after that. Look for broken connections (obviously) and check the large electrolytic cap (That silver tube between the two transformers). Check whether the plastic insultation underneath is bulging. Otherwise, having removed it from its normal home, have you disturbed the grounding in any conceivable way?

    On the other hand, if you are not technically pretty competent, I shouldn't be leading you into the guts of the amp. There are seriously high voltages there that genuinely could kill you. Take it to a tech.
     
  3. RVA

    RVA Well-Known Member

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    I think you may be right. I see nothing visually wrong to me , so it may be time for someone with better eyes!
    Thanks
     
  4. syscokid

    syscokid Well-Known Member

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    "After replacing the tube and the fuse on my Peavey Triumph 120..."
    What exactly happened here?
     
  5. Voxman

    Voxman Moderator Staff Member

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    Check the coupling caps in the phase inverter for DC leakage current, ground points must have no resistance at all also check the balancing resistors and ground connection to the heater supply, if the amp has them. Then we need to know if the noise is 60 cycles or 120 cycles as we need to check the power rail, ie. the main first can/ or group of caps, and all the value of dropping resistors and filter caps leakage downstream from there to the first preamp tube , but for a neophyte that might be a bit much since your hands are very near the highest potential of the amp , the center tap of the output tranny ... BE WARNED, if you slip, touch or bridge where you shouldn't you might not live to tell about it or do some serious arc wielding!
     
  6. RVA

    RVA Well-Known Member

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    I was playing and "pop" - amp went dead. A 6L6 was shattered and the fuse was blown. A new fuse and a matched set brought it back in tip-top shape...for a week. Now the hum!
     
  7. RVA

    RVA Well-Known Member

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    Um...60 cycle..I think. After that you lose me! While I do not mind living dangerously, I may be over my head. Feel like stopping by?!
     
  8. Voxman

    Voxman Moderator Staff Member

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    Listen to the you tube vid again , and when they play the two examples of a 60 cycle hum and a 120 cycle hum try to copy the pitches on your guitar thou you might have to retune a bit and memorise the pitches.... hint; the "A' string is 440 n see which on you hear
     
  9. RVA

    RVA Well-Known Member

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    Ok, will do, thank you. I did make my assumtion that it was 60 cycle because the hum is not responsive to volume changes. Is that inconclusive?
     
  10. syscokid

    syscokid Well-Known Member

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    I bet the amp sounded the best right before it blew... Ok, I'm being a smart-arse.
    Burning flesh will smell really bad. The heart doesn't care for an extra 400 to 500 volts either.
    Even with the amp off and cooled down, can you smell an electrical burn?
     
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  11. Voxman

    Voxman Moderator Staff Member

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    That means the issue could/should be after the volume control and sounds like a power Can / cap perhaps loosing it's insulation resistance so if it's easier I tack solder in replacements I have as spares and see if I've isolated the suspect(s)
     
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  12. Voxman

    Voxman Moderator Staff Member

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    Oh and discharge the caps before you touch em with a alligator clipped cord to a 10 ohm 2 watt resistor, another alligator clip from the resistor to ground then measure the voltage to em but do disconnect the connections BEFORE you repower up the amp .. another way I addressed that same problem was the BIAS cap, watch the polarity of the original and put in another of the same value
     
  13. RVA

    RVA Well-Known Member

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    No, unfortunately you are dead-on accurate!

    OK, thank you. I really appreciate the advice. I may have more questions after I digest this and do some research. Here are some gut shots I took when I changed the tubes (before this problem).

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
  14. Voxman

    Voxman Moderator Staff Member

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    One thing I'm 100% sure of .... the answer to the problem is hidden in plain sight and in front of our eyes! It always is ...
     
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  15. RVA

    RVA Well-Known Member

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    Based on a comparison to the video, it is definitely a 60 cycle hum.
     
  16. Voxman

    Voxman Moderator Staff Member

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    How are we making out on this?
     
  17. RVA

    RVA Well-Known Member

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    Thanks for asking. I rechecked the tubes - no luck with such simple solutions. I am trying to learn how to test the caps. I thought a multi meter with the ground on the chassis and the positive to test the solder points was the answer, but syscokid pointed out in another thread that a least 1 lead would have to be disconnected to properly test, so I have to go back to the learning drawing board. Also, I am not sure what readings I am looking for when I do learn how t properly test it.
     
  18. ScottMarlowe

    ScottMarlowe Well-Known Member

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    To properly test caps you need to test both capacitance and ESR (Equivalent Series Resistance) which takes a special ESR meter. You don't have to desolder caps to test ESR.

    I wonder if you've got something like a cold or dodgy solder joint on one of your tube sockets.
     
  19. RVA

    RVA Well-Known Member

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    Thanks for the info. It is quite frustrating as there is likely a simple answer that is beyond my ability to diagnose. The local amp guru wants $95 per hour in labor, which I just cannot justify.

    The problem appeared suddenly and while playing if that means anything.
     
  20. Dave_Death

    Dave_Death Well-Known Member

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    It blew a fuse too. I am wondering how a tube or a cap might have caused that. Was it the main fuse, or is it is one of the other fuses?

    I see you posted this on the Peavey forum. I was going to suggest doing that.

    http://peavey.com/forum/viewtopic.php?t=41614
     

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