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  1. #1
    Hot For Teacher danielleesale's Avatar
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    11.19.17 @ 03:14 PM
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    Default 5150 combo rhythm pre gain knob

    I've had the combo for years and now the first problem comes up. As I turn the rhythm pre gain knob up and down, the lead channel comes comes through as though I'm changing over to it.

    Is this just something I can clean or do I need a tech?

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    12.14.17 @ 12:39 PM
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    Do you mean the amp actually switches to the lead channel? You'll have to be clearer in describing your problem.

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    10.26.16 @ 03:37 PM
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    Quote Originally Posted by danielleesale View Post
    I've had the combo for years and now the first problem comes up. As I turn the rhythm pre gain knob up and down, the lead channel comes comes through as though I'm changing over to it.

    Is this just something I can clean or do I need a tech?
    Peavey 5150's, and the combo in particular, are known for having sketchy relay switching at times. 2 of the 3 5150 Combos I've owned over the years had that issue -- some days, it would switch to the rhythm channel all by itself, or switch to something 'in between' channels. Normally, if I sorta punched the top of the amp cabinet, it would switch completely to one channel or another, and then the problem would go away for a while, sometimes for weeks at a time.

    Try giving it a pound (on the tolex, not on the chassis) next time the problem comes up and see if that jolts it into staying in a channel, and then stops doing the switching by itself thing.

    Another thing to try is if you have a footswitch hooked up, turn the amp off, and unplug the footswitch from the back of the amp, and then re-insert the jack a bunch of times to make sure there isn't just some corrosion or junk on it.

    If you don't have a footswitch hooked up, take any 1/4" cable that you have laying around (like a guitar cable) and do the same thing to the jack, just to make sure it's not corroded and messing with your switching circuit.

    If none of that works, let us know.

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    Hot For Teacher danielleesale's Avatar
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    11.19.17 @ 03:14 PM
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    Alright, it's not literally (or literal as I think of it) switching channels, and it's not doing it randomly, or anything like that. I mean, I can set the rhythm knob to cleanish, and it'll stay there. But as I move it, the lead channel bleeds through, while the footswitch light, and the light on the amp, does not indicate that a channel switch is taking place.

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    10.26.16 @ 03:37 PM
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    Quote Originally Posted by danielleesale View Post
    Alright, it's not literally (or literal as I think of it) switching channels, and it's not doing it randomly, or anything like that. I mean, I can set the rhythm knob to cleanish, and it'll stay there. But as I move it, the lead channel bleeds through, while the footswitch light, and the light on the amp, does not indicate that a channel switch is taking place.
    I don't mean to sound condescending, but are you sure it's not just the gain of the rhythm channel kicking in as you turn up the gain on that channel? Maybe a scratchy pot is causing it to jump a bit of its travel so it's coming in harder than you're used to hearing it come up smoothly?

    Both channels do sound pretty similar...

    Or do you mean that you can clearly hear the lead channel just drastically cut in at full gain once the rhythm channel gain pot goes above a certain number?

    Do the master volumes on both channels enhance or reduce the bleed through? Like, if you set the rhythm channel's volume on 1, and the lead on 4, does the amp jump volume when you hear the lead channel bleed through as you turn up the rhythm channel gain knob?

    If you could take a quick little video or audio of the problem so I could hear it that might help.

    Or if you can tell that something definitely is messed up with it in person (meaning that it has really changed compared to all the rest of the time you've owned it) then you should probably just take it in to a tech. Channels bleeding into eachother based on a gain knob adjustment is an odd problem that will not be fixed with a simple tube swap, etc., and will likely require advanced troubleshooting of either the entire relay switching system in the amp, or possibly a short in the signal path.
    Last edited by mrjstudios; 08.03.12 at 10:34 PM.

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    11.25.17 @ 09:06 AM
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    It sounds to me like there could be an issue with the pot itself. Like the resistor plate is dickered.Just an example It's somewhat rare, but not unheard of. I had an old Techics hifi amp. It was a nicely built unit, but I damn near gave my self a heart attack a couple times. It started with the volume knob being a little scratchy, nothing more than that. Then one day in the morning I had the amp on, and went to turn up the volume a little, and it was like WHAM!!!!! Full fucking blast. I tuned it off, and after gaining my composure, I turned the pot rapidly back and forth, and braved turning it on again, and it was back to normal.......at least for a couple days, and again.........cardiac town!
    Reason I mention this is when I took it in for service, the tech said it was the pot itself, and the resistance plate had gotten either corroded, or loose, and it would go to least resistance with just a hair of movement if the conditions were right. The same can and does happen to a preamp pot. It happened on an old Laney combo I had, as well as on the treble pot. It's kinda flukey, but a real pain.

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    10.26.16 @ 03:37 PM
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    You're absolutely correct -- it could be just a bad preamp pot that is shorting to minimum resistance at a weird spot in its travel. That's actually not that uncommon at all - especially with the cheap pots that most companies use in their gear. I'm amazed at how great the 5150's are durability wise sometimes.... yes, it's a bulletproof amp that can be practically dropped, dragged behind your car, and still play a gig that night, but I think the pots are the weakest point by far. I'm somewhat surprised that over all the years that amp line has been going strong, you don't hear more about the cheap, mini-pots. I see 5150's all the time (including mine) with a few pot shafts and knobs busted off, and super scratchy pots.

    If the 5150 didn't have PCB mounted mini-pot construction in a tight space, I would have replaced every one of them in mine years ago! My favorite pot right now is the Bourns hi-turning-torque audio equipment line. I think it's called the PDA24 series. Super smooth, and hard to turn the knobs so they don't get bumped easily or jiggle around.

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    11.25.17 @ 09:06 AM
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    Quote Originally Posted by mrjstudios View Post
    You're absolutely correct -- it could be just a bad preamp pot that is shorting to minimum resistance at a weird spot in its travel. That's actually not that uncommon at all - especially with the cheap pots that most companies use in their gear. I'm amazed at how great the 5150's are durability wise sometimes.... yes, it's a bulletproof amp that can be practically dropped, dragged behind your car, and still play a gig that night, but I think the pots are the weakest point by far. I'm somewhat surprised that over all the years that amp line has been going strong, you don't hear more about the cheap, mini-pots. I see 5150's all the time (including mine) with a few pot shafts and knobs busted off, and super scratchy pots.

    If the 5150 didn't have PCB mounted mini-pot construction in a tight space, I would have replaced every one of them in mine years ago! My favorite pot right now is the Bourns hi-turning-torque audio equipment line. I think it's called the PDA24 series. Super smooth, and hard to turn the knobs so they don't get bumped easily or jiggle around.
    I totally agree MRJ.
    Actually when I first got my amp back in 91' or whenever it was, I was certain that the pots were a weak link, and I didn't hold much hope for them lasting all that long. it wasn't real common to find low friction pots on an amp of the 5150's caliber, so I was surprised. I know magazines made the same observations about them too saying that on an amp especially with the chickenhead (pointer) knobs, you'd think they would have at least used a high friction pot. it just feels more "quality" and positive to the user.
    So all these years on, I'm amazed that they have in fact stood up as well as they have. I've had the other issues that tend to be prevalent on the 5150.....The weird footswitch channel switching problems, the effects loop issue where it doesn't trip the internal relay every time, so it sounds muffled and weak, and the capacitor issue where the lead channel just didn't have any power beyond 2 on the post gain.(That actually happened within months of purchase, and was corrected very quickly, and it never happened again).
    Most of thos issues were easily remedied by a chassis cleaning, and coincidentally, since smoking was banned in most clubs, I never had those issues again, but dust can still be a problem if it gets into those electro-mechanical areas of the amp.
    I can't say with certainty that the resistance plate issue is what he's experiencing, but it does sound strangely similar. In a pre pot, it can definitely give the illusion of lead channel bleeding across due to the brief lag of the tubes being hit with higher input. They don't react as instantaneously as say a transistor clipping in a solid state amp. I don't see how there can be any bleed between channels at all, unless he's got a switch that's pooched. Still doesn't sound right....Think you'd get some sputtering if it was the switch.

 

 

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