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Noisy Peavey 5150 II

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  • Noisy Peavey 5150 II

    Yeah I know, they are *Always* noisy, but this one is more than usual.
    After it warms up for a few minutes it makes a rather large "SSSHHHHHHH" sound when you put it on standby and makes a quite large "SSSSHHHHHHHHHHHH" sound when you flip it on (off of standby). It clears upafter 3 seconds. It did not do it at all during the first few minutes. The owner says it cuts out eventually when it gets really hot - but it never did that for me (maybe we didn't have it on long enough).

    I have eliminated the tubes as an issue (subbed them all out with new and same issue). It is not a loud "pop" sound either - I have had that when a standby switch is starting to go out - this ain't it.

    Ideas? Anyone seen this? It would be easier if it would do it all the time instead of for a brief few seconds. At least we can recreate it. But it has to warm up for a few minutes - it does not do it at first.

    Thanks in advance!

  • #2
    Isolate the problem.

    If you can reliably keep making the symptom by flipping standby off and on, then trace it to the source.

    First, do any of the controls affect this noise at all? Does it make a difference what channel you are in? Do you have the foot switch so you can access all the features and channels? Any control that affects the noise is AFTER its source.

    If the two POST controls to zero kills the noise, then it is in the preamp. If not, then power amp.

    Plug somethihng into the effects return jack. Either remove the foot switch or make sure the FS is set for loop ON. Does the noise remain? Plugging into the return disconnects the preamp out and loop send buffer stage. If the noise remains, it comes from the power amp.

    Om fact the preamp out jack is one more stage later - just before the phase splitter. Noisy there?

    ANother trick is to pull tubes. The small tubes are in this order across the rear of the amp - V1,2,5,3,4. There is really only one path through this amp. The channels are really only gain switching, and signal attenuation changes. So pull V1. Still noisy? Then continue down the signal path. Pull V2. Noisy? Then pull V5. Noisy? Those three are the preamp. If pulling one kills the noise, then the source of the noise was at or before the [particular tube.

    If those had no effect, replace them in their places. But keep going. Pull V3. NOisy? If so, then pull phase splitter V4. Noisy? Only thing after that is the 6L6s.

    Let's say you narrowed it down to the power amp - things after the effects loop. Scope the noise, what does it look like? Is it on the grids of the powerr tubes? What does the bias voltage do at those grids?

    It could be something like one of the relays got sticky. Tap on each one to see if it makes a difference.

    Once you find out where the problem lies, them it is far easier to cure it.

    Isolate the problem.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      The last time I saw this it was a cold solder joint on the power tubes. Might want to try touching them up a tad. Could be a tube socket or any thing above Enzo mentioned. I agree that isolating it is cutting your work at least in half if not more.
      KB

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      • #4
        Now that I look, I see you have a 5150-2, while my remarks were made with a plain 5150 in mind. The V numbers will not be exactly the same, but the concepts still apply.
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          Thanks for the good suggestions. I talked to a bud with a 5150 (not the same amp, another guy) and he says HIS does the same thing. It might be common with the amp and I just didn't notice it before.

          The customer took the amp back for a practice or gig, but I should have it back later (possibly today). Maybe I should call peavey too - but I work a regular full time job too and don't have the time during the day.

          I will have to dig into it a bit to find out and report back. Anyone with a 5150 or 5150 II please try what I said: run your amp for about 30 minutes (power is on, standy by is set so the amp plays) and then flip the standby then flip it back so it plays again. Does it go "SHHHH" really quite loudly? Is it just a quirk with the amp or a real problem.

          Thorny

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          • #6
            I don't think it is a huge problem, or a problem with your amp in particular. They just should have arranged the standby switch better in those things. looking at the schem... it is just in a bonehead place in the PSU rail.

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            • #7
              It mutes the amp, it stops most current through the power tubes by removing the screen voltage. That is what a standby switch should do.

              other standbys do things like open the cathode legs, some short the two sides of the phase inverter output together. However they do it, the job of the circuit is to silence the amp while leaving it warm.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                The guy never broght the amp back - so I suppose it isn't bad enough to worry about. If he does I will report back.

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