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Peavey 5150 II killing speaker cables

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  • Peavey 5150 II killing speaker cables

    First off, thanks for any help.

    I recently decided to "upgrade" my 5150 II that I purchased new a long time ago. This amp was completely stock. I ordered new JJ high gain tubes (pre and power tubes), and a bias test adapter from a reputable online vendor.

    First, I did the bias mod by replacing the 12k ohm resistor with a 6.8k ohm resistor on the bias adjustment circuit board. Next, I replaced the tubes and cleaned the sockets with electrical contact cleaner as I went. Finally, I biased the power tubes to 36 mA using the adapter.

    I had a very noticeable cloudy noise in both channels (clean/lead) of the amp even when the guitar volume was turned off. I diagnosed the problem to a pre amp tube that both channel signals travel through. I switched tubes (these tubes were alike) around and got it to not make the cloudy noise anymore. Next, I re-biased and tried it out.

    It sounded great for about 30-60 minutes, but then the distortion started to kinda of crackle and then cut out with ~90% drop in volume. I troubleshot some more (not changing anything, but I did try a different cab and had to change the impedance selector on the back of the amp to accommodate), then I tried a different speaker cable and it worked. I played for about 30-60 minutes and the same scenario happened... crackling into volume loss.

    The tubes all seem to look fine. My gut says something is up with the impedance selector, but I'm not sure. This just happened and I'm immediately posting here. Any suggestions would be GREATLY appreciated.

  • #2
    Tube amps don't kill proper speaker cables (might be different if instrument cable is used instead). Analyse the defective cables and find the failure cause.
    Also clean jack(s), cable plugs and impedance selector contacts.
    Mind that a defective speaker cable or bad output or cab contacts cable can damage your amp/OT.
    - Own Opinions Only -

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    • #3
      What kind of speaker cab? For whatever reason this thread made me think of a Peavey 412 stereo cab I got that had a burned out stereo/mono switch.

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      • #4
        Originally posted by glebert View Post
        What kind of speaker cab? For whatever reason this thread made me think of a Peavey 412 stereo cab I got that had a burned out stereo/mono switch.

        It's a Harley Benton 2x12 and it does have a mono/stereo switch. I just checked the selector switch operation with a meter (good thinking), but it seems to be working right.

        I also performed all the steps in the previous reply... I pulled the impedance selector switch apart, cleaned the parts, and reassembled. I also cleaned all relevant ports/jacks with electrical contact cleaner.
        Last edited by Burnam; 12-14-2019, 10:35 PM.

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        • #5
          I also performed all the steps in the previous reply... I pulled the impedance selector switch apart, cleaned the parts, and reassembled. I also cleaned all relevant ports/jacks with electrical contact cleaner.
          Still the same problem?

          What are the cable defects?
          - Own Opinions Only -

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          • #6
            Plug a patch cable between FX Send and FX Return. Does that fix it? If it does, you need to clean the return jack.

            Also, you must measure the resistance of your speaker cable before you may announce it is damaged. Measure tip to tip, sleeve to sleeve, and tip to sleeve while making sure you are not touching them with your hands.
            It's weird, because it WAS working fine.....

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            • #7
              If you haven't verified a fault in either speaker cable, then it is probably just a coincidence that the fault occurred at the same time you swapped cables.
              You will have powered down to change them and that could be responsible for the sound coming back. You have not really verified whether it is a speaker cable or not (at least as described here).
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


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              • #8
                Originally posted by Helmholtz View Post
                Still the same problem?

                What are the cable defects?
                Oh yeah! I just went and checked both cables. The cables looks structurally sound and they have continuity through both legs, and are open across legs. Correct me if I'm wrong, but that shows the cables are fine. I guess I just assumed it was a bad cable when the 2nd one worked after the first crackle and volume drop. I had waited some time before trying with the other cable.

                Now I really have no idea what's going on.

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                • #9
                  Originally posted by Randall View Post
                  Plug a patch cable between FX Send and FX Return. Does that fix it? If it does, you need to clean the return jack.

                  Also, you must measure the resistance of your speaker cable before you may announce it is damaged. Measure tip to tip, sleeve to sleeve, and tip to sleeve while making sure you are not touching them with your hands.
                  I will try this later tonight.

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                  • #10
                    If the problem is with the FX jacks, it could easily be triggered just from the clunk of patching in a new cable, as well as the amp warming up and causing the metal to expand. See the thread about Dreaded switching jack Problem at the top of the Maintenance and Repair category.
                    It's weird, because it WAS working fine.....

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                    • #11
                      So I cleaned the jacks on the amp again and the problem persists. Low volume on all channels, and I can hear pre amp effects on the signal.

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                      • #12
                        OK, approach it like it just happened, rather than trying to connect it with something we did.

                        Isolate the problem. Plug the guitar into the FX return, sound clear and strong, or still weak?

                        And also, plug into the input and play, and run a cord from FX send to the input of some other amp. IS the sound at the other amp clean and strong, or does it suffer the weakness?

                        Lots of things can cause weak sound, but one of my favorite suspects in these is the mute JFET. Q7. Up top right on the schematic. Try removing it.
                        Education is what you're left with after you have forgotten what you have learned.

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                        • #13
                          Originally posted by Enzo View Post
                          OK, approach it like it just happened, rather than trying to connect it with something we did.

                          Isolate the problem. Plug the guitar into the FX return, sound clear and strong, or still weak?

                          And also, plug into the input and play, and run a cord from FX send to the input of some other amp. IS the sound at the other amp clean and strong, or does it suffer the weakness?

                          Lots of things can cause weak sound, but one of my favorite suspects in these is the mute JFET. Q7. Up top right on the schematic. Try removing it.
                          I just tried guitar in through the FX return and it's the same problem. Low volume, doesn't sound clear, post amp knobs do not add gain the signal.

                          Thanks again for the help.

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                          • #14
                            You may have a damaged pad(s) on the board from too much "tube rolling". This happens. Try gently rotating the tubes in a small circular motion and listen for crackles and noise. If there is none then I think you may need to clean the tube sockets again. Too much tube rolling can be a problem for board mounted sockets because the pads and traces become damaged. And even chassis mount sockets WRT introduction of oxide and grime.
                            "Take two placebos, works twice as well." Enzo

                            "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                            "If you're not interested in opinions and the experience of others, why even start a thread?
                            You can't just expect consent." Helmholtz

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                            • #15
                              Also, you mentioned cleaning the speaker jacks on the amp but didn't say if you examined the solder connections.
                              It's possible unplugging/plugging the speaker cable temporarily restored an intermittent solder connection on the jack.
                              Originally posted by Enzo
                              I have a sign in my shop that says, "Never think up reasons not to check something."


                              Comment

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