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Help with my Peavey 5150 amp...

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  • #1

    Help with my Peavey 5150 amp...

    I want to know from another 5150 head owner... (signature script model.. not the EVH block letter) if when you turn the amp on you hear a constant low steady hum. Sounds like a transformer hum. Its low but I can hear it. It doesn't change whether in the clean or lead channels. It doesn't matter if an instrument cable is plugged into it or not. I am thinking maybe a filter cap or caps? What about power tubes? The tubes that are in it ar GT's and they look OLD. Really old.
    Could old tubes cause this hum? I am thinking no but I am not sure about that one. The amp sounds fantastic but that hum is worrying me. I just got the amp used from a guy so I know NOTHING about the history of it. It was REALLY dirty when I got it.
    Thanks!
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  • #2
    A hum through the speakers, or a physical humming coming from the amp? If the former, is the hum still there with the master volume turned all the way down?

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    • #3
      Does it happen when in Standby? There is a very low audible hum from my 6505+ even in standby. I bought it new and it's only audible if I'm in a quiet room. You wouldn't even be able to hear it over a person talking.

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      • #4
        Originally posted by Electric View Post
        Does it happen when in Standby? There is a very low audible hum from my 6505+ even in standby. I bought it new and it's only audible if I'm in a quiet room. You wouldn't even be able to hear it over a person talking.
        its quiet on:standby.

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        • #5
          Like a 60 cycle hum?
          I had that problem on my 72 Sound City head even after new caps and filter caps.
          The tech did get the hum out. it was a weak resistor in the tone stack IIRC.
          Really? well screw Mark Twain.

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          • #6
            Originally posted by metalhobo View Post
            A hum through the speakers, or a physical humming coming from the amp? If the former, is the hum still there with the master volume turned all the way down?
            It hums through the speakers. If I turn the master volumes down it still hums exactly the same. The best way to describe the hum is it sounds like a dead short in a circuit breaker right before it pops. Its not loud but its a steady low hum.

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            • #7
              Originally posted by straycat View Post
              Like a 60 cycle hum?
              I had that problem on my 72 Sound City head even after new caps and filter caps.
              The tech did get the hum out. it was a weak resistor in the tone stack IIRC.
              Its not a 60 cycle hum which is more like a buzz. This hum is very clear and and low. If I turn the amp way up I can't hear it anymore because the hiss is louder than the hum. Its definately something wrong and concerning. It sounds like a transformer hum.

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              • #8
                I listened to a bunch of samples of amplifier hum on Youtube and my 5150 definately ISN'T 60 cycle hum. Its 120 cycle hum. Could that be a bad preamp tube?

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                • #9
                  If it hums through the speakers with the master volume potted down then that would likely indicate either a problem with the power amp section or the power supply rather than in the preamp. Swapping the power tubes seems like a good starting point.

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                  • #10
                    Fixed it. I looked it up online. For future reference for all my JCF brothers.... a 60 cycle hum sounds like a bad ground. A 120 cycle hum sounds like a bad transformer. Mine had the 120 cycle hum. From what I read online.. that usually points to a bad PREAMP tube. Thats right a preamp tube. What happens is this... and excuse me if this is not 100 percent correct... the preamp tubes are split in two. If one of the plates inside them goes bad it gets too close to the other plate and the tube becomes a capacitor and will generate a 120 cycle low frequency hum. I pulled all 5 tubes, replaced them with 5 tubes that I know are good and the amp is completely silent. No more hum of any kind.

                    I love the internet for this kind of stuff!

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                    • #11
                      Glad you found the problem. Thanks for the info.
                      Really? well screw Mark Twain.

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                      • #12
                        Originally posted by jgcable View Post
                        Fixed it. I looked it up online. For future reference for all my JCF brothers.... a 60 cycle hum sounds like a bad ground. A 120 cycle hum sounds like a bad transformer. Mine had the 120 cycle hum. From what I read online.. that usually points to a bad PREAMP tube. Thats right a preamp tube. What happens is this... and excuse me if this is not 100 percent correct... the preamp tubes are split in two. If one of the plates inside them goes bad it gets too close to the other plate and the tube becomes a capacitor and will generate a 120 cycle low frequency hum. I pulled all 5 tubes, replaced them with 5 tubes that I know are good and the amp is completely silent. No more hum of any kind.

                        I love the internet for this kind of stuff!
                        Glad you got it sorted out quickly! Do you know if it was an actual preamp tube or the phase inverter tube?

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