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Thread: Assistance Using Scope to Troubleshoot

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    Assistance Using Scope to Troubleshoot

    I’m starting my first troubleshooting of a homemade amp head using a scope & signal generator & will be asking a lot of stupid questions of the experts here so please indulge me. The amp has switchable dual preamps and the complaint is feedback at the 4-5 volume level that doesn't decrease when volume is turned down on the JCM800 preamp side. My plan was to send a signal thru the amp & try to isolate the problem. I’m going to post pics of the schematic and my gear, Techtronix 465 scope & HP 204b oscillator & cables. First question is where on the amp to input the signal & what settings to use on the scope.
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    Last edited by Perkinsman; 12-05-2019 at 03:31 AM.

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    Bent Member Chuck H's Avatar
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    Put 100mV @ 400Hz to 1000Hz into the input. Amp controls set as you would when playing through it and the problem occurs.

    You'll be probing along the signal chain starting from the input and moving toward the output.

    Setting the scope is another matter and depends on where in the signal chain you're probing. I'd suggest a 10:1 probe and just set the division so you can see the wave clearly enough. The right division setting for each node will be different. Perhaps after the distortion is identified a more finite setting for accurate measurements will be recommended. But for now you just need to "see" the wave form.

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    Bent Member Chuck H's Avatar
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    Oh, and...

    You really should hang a resistor from the downstream side of that preamp switch to ground. The arrangement as it is leaves the grid without bias. Only for a moment, I know, but it's bad practice. It makes more noise and if anything happens to the switch the tube could be damaged.

    2.2M would drop signal about 1dB at that grid. Which is no great shakes.

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

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Perkinsman View Post
    I’m starting my first troubleshooting a homemade amp head using a scope & signal generator & will be asking a lot of stupid questions of the experts here so please indulge me. The amp has switchable dual preamps and the complaint is distortion at high volume on one of the two. My plan was to send a signal thru the amp & try to isolate the problem. I’m going to post pics of the schematic and my gear, Techtronix 465 scope & HP 204b oscillator & cables. First question is where on the amp to input the signal & what settings to use on the scope.
    I would suggest hooking the scope up to the signal generator and then playing with the knobs to get a feel for how things should operate.
    For one you won't be learning how it operates near high voltages and 2 your still learning.

    I got a handle on mine that way . The kids really love it when I hook a microphone up to the scope.
    Jack Darr guitar amp bookhttps://nartlof.com.br/livros/JackDarr1973.pdfSee page 47 for general test points.

    nosaj

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    Quote Originally Posted by Chuck H View Post
    Put 100mV @ 400Hz to 1000Hz into the input. Amp controls set as you would when playing through it and the problem occurs.

    You'll be probing along the signal chain starting from the input and moving toward the output.

    Setting the scope is another matter and depends on where in the signal chain you're probing. I'd suggest a 10:1 probe and just set the division so you can see the wave clearly enough. The right division setting for each node will be different. Perhaps after the distortion is identified a more finite setting for accurate measurements will be recommended. But for now you just need to "see" the wave form.

    I edited my intial post to read feedback at low volume, not distortion. I'm going to start in on this tomorrow, thanks Chuck.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Perkinsman View Post
    I edited my intial post to read feedback at low volume, not distortion. I'm going to start in on this tomorrow, thanks Chuck.
    That IS an entirely different matter. Fortunately the advice is still the same.

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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Quote Originally Posted by Chuck H View Post
    That IS an entirely different matter. Fortunately the advice is still the same.
    Sure is....so what are the most common reasons for feedback in a circuit like this?

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    Preamp tubes themselves are probably the most common cause of feedback. While it is occurring hold them with a rag (or something to prevent burns) one at a time to see if anything affects the feedback/squeal/microphony.

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    Bent Member Chuck H's Avatar
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    This^^^^^^^^^^^^^^^^^^^^^^^^^

    is the first go to. Especially if the amp was known to work correctly at some point.

    If the amp has always had a "feedback" problem then I might suspect parasitic oscillation due to layout and/or lead routing. This is where you'd want to have a scope.

    EDIT: This amp doesn't have a master volume so it's going to be loud when turned up. Other things it could be might include microphonic guitar pickups and if an attenuator is being use some have inductors that can emit enough of a field to interact with the guitar when playing too close to the unit. Too close being relative. One attenuator I made had to be modified because I couldn't get within about eight feet of it.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    The two connections that usually generate more problems (feedback, bad sound...) in the 2204 circuit are the input (to V2A grid) and the output (from potentiometer to V2B grid). It ´s advisable to use shielded wire in them.

    In that design, another problem will be the different final volume on each channel. If you only have one master volume you can assign it to 2204 channel exclusively. That way you can find a balance between both. It can be done with a relay or switch synchronized with the previous one.

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    Supporting Member The Dude's Avatar
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    Agree and I'll add. Often tubes are microphonic in one position and not another. In other words and for example, a tube that might be microphonic in V1 position may be just fine in the V2 position. I often find that simply swapping tubes from one spot to another will solve a microphony problem.

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    Im going to try & simulate the problem first. Im using an 8 ohm dummy load & have the amp switched to 8 ohm. Since this is my virgin scope&signal gen project, I need to start slowly on this. If I hookup my signal gen black lead to chassis, where does my red lead go? Told u i was gonna ask stupis questions. Also, remember that this a dual preamp so why would the preamp tubes cause a feedback problem on the 2204 (jcm) side & not the 1987 (jmp) side? I'm thinking seńor Vecino is on to something with the wiring shielding. I've never used shielded wiring, what type do I need?
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    Last edited by Perkinsman; 12-06-2019 at 12:52 AM.

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    The easiest way to apply signal would be to just get a 1/4" plug and connect your leads to the plug. That also opens the shorting part of the input jack.

    On the preamp tubes: If it were not preamp tubes and were in the power amp, it's likely both sides would feedback, so it is indeed likely a preamp problem. If it's a microphonic issue, you should be able to tap lightly on the tubes and see if one of them is causing the feedback/oscillation. That would be a simple first check.

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    I get sound at both the middle & one leg of jcm volume control. Is there a preference?

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    Bent Member Chuck H's Avatar
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    The project might be getting ahead of itself. You may not even need to hook up the scope. As you already deduced, the first thing to do is instigate the problem. After that you should tap on the tubes and see if any of them sound particularly microphonic. A thunk or ping is usually ok. A crunchy sound or if the tube wants to break into a longer noise like a shriek or whistle would be a clear indication of a microphonic tube. If it's not a microphonic tube Then...

    You should discover what controls affect the problem. What makes it worse and what makes it less or stop. It's probable that Pedro's shielded lead solution is all that's needed and it could be indicated by this information.

    As to where to put the red lead, are you ok with transposing the schematic? That is, if I were to mark places on the schematic could you locate them on the board? Making marks on the board would be harder for me because then "I" would have to trace the circuit from the one photo angle.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Perkinsman View Post
    I get sound at both the middle & one leg of jcm volume control. Is there a preference?
    I don't understand this question. Are you saying that when you inject signal you get signal voltage at these locations? And what would be the criteria for preference? That information is a little arbitrary to the process just now. But that should be normal unless the volume knob is set to zero. In that case there should be no signal on the middle leg and the left leg (looking at the pot from the back).

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    Let me start with the easiest stuff. I'm not injecting a signal yet, just plugging in a low wattage hi-fi speaker I use for testing small guitar amps for sound. I powered up & turned up the master & jcm volume to 2 so i could hear the amp thru the speaker. I tapped each preamp tube & each sounded like a dull thud except v2, which didn't make any shrieks but it just sounded different, maybe more "plinky" & more "live" if that makes sense.

    As far as the shielding on the two locations Pedro noted, if you look at the white wires, they are different, theyre thicker with what looks like plastic shielding inside. Is that shielded wire that was used?

    Also, i can't turn up too loud to simulate the feedback cuz I'll probably blow the speaker or disturb neighbors so i need to use the scope. I hooked up to a jack as suggested & have signal in now. Where do I hook up the scope? Black to chassis, probe set on x1, x10 or REF? Then what?

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    Supporting Member The Dude's Avatar
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    The shield isn't plastic. It's actually a conductive shield around the center conductor. That said, I looked at your pic, and those wires are indeed shielded. You can see the shields soldered to ground.

    If you can't turn the amp up to replicate the feedback, you're not going to see it on the scope. There's nothing yet to see except normal signal. Do you have a dummy load? If not, you're going to at least need to turn up the amp loud enough to get it to feedback.

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    Bent Member Chuck H's Avatar
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    This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

    You're going to need to replicate the problem if you're going to solve for it. If you need to talk with the amps owner to discover the exact circumstances then that's what you must do. This is step one.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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    Supporting Member The Dude's Avatar
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    Normally, I wouldn't suggest this simply because the problem only appears to be on one channel of the amp. But, since it's a new build and gain structure is different on each channel (and since it's a very simple test), try temporarily unhooking the NFB wire from the speaker jack. It's the wire that goes to the 47K resistor. See if the feedback is still there. If that does take care of the problem, you'll need to reverse your OT primary wires.

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    Quote Originally Posted by The Dude View Post
    Normally, I wouldn't suggest this simply because the problem only appears to be on one channel of the amp. But, since it's a new build and gain structure is different on each channel (and since it's a very simple test), try temporarily unhooking the NFB wire from the speaker jack. It's the wire that goes to the 47K resistor. See if the feedback is still there. If that does take care of the problem, you'll need to reverse your OT primary wires.
    Ok, I'll do that but first I need to be able to hookup the scope and duplicate the feedback. I have an 8 ohm 50W dummy load attached but not sure how to hookup the scope. Ground clip to chassis and x10 probe but what next?

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    Lifetime Member Enzo's Avatar
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    Don't be afraid of the scope. Really all it is is a voltmeter on a screen, a visual voltmeter. As the voltage you are probing changes, the line moves up and down. So you use it like a volt meter. Ground your probe and use the hot probe to touch points of interest in the circuit. Instead of reading the voltage as a number, you see it as a graph.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Enzo View Post
    Don't be afraid of the scope. Really all it is is a voltmeter on a screen, a visual voltmeter. As the voltage you are probing changes, the line moves up and down. So you use it like a volt meter. Ground your probe and use the hot probe to touch points of interest in the circuit. Instead of reading the voltage as a number, you see it as a graph.
    This is a good analogy. With the caveat that the scope could be damaged by high voltage at it's input. That's why the 10x probe is recommended. You'll want to keep your test points limited to the DC decoupled AC signal signal path for now I think.

    And, as I mentioned, you're not even at a place in the effort where a scope is helpful yet. There are still possibilities to eliminate that don't require a scope. The first thing you need to do is recreate the problem the amp owner is experiencing, but at your bench instead of their playing space. This is the first step. Setting up the scope and THEN recreating the problem is sort of like getting your car repainted in case it get's scratched up in the future.?.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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

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    Quote Originally Posted by Enzo View Post
    Don't be afraid of the scope. Really all it is is a voltmeter on a screen, a visual voltmeter. As the voltage you are probing changes, the line moves up and down. So you use it like a volt meter. Ground your probe and use the hot probe to touch points of interest in the circuit. Instead of reading the voltage as a number, you see it as a graph.
    To be honest, I’m not sure what oscillation even looks like on a scope, I’ve only used it to see AC sine waves & that was awhile ago. Do I set the master & channel volumes at the point where the feedback occurs? Will that damage anything if I leave it feedbacking for awhile as I probe? Do I just start at the input & follow the signal path to the speaker out? Are there any places I shouldn’t touch with the probe?

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    You're up against a bit of a wall here. You don't know what oscillation looks like on a scope, but there is also the problem that the oscillation may be strictly acoustic feedback that will only occur with a speaker, not with a load. (as has been pointed out above)
    So there is only a chance it will occur, and if it does, you may not recognize it.

    A couple things may help you out. First, like nosaj said, scope the output of your generator to get a feel for what your basic sine wave looks and acts like.
    Now at least you will know if the amp is producing something else.
    Another thing would be to run the amp into a load, but also monitor it with your cheap speaker through a volume control in parallel with the load. Then you will at least be able to hear if it's feeding back or not.

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    Bent Member Chuck H's Avatar
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    Perkinsman,

    You seem bent on using the scope for solving this problem no matter what. You're trying to solve the problem of "diagnosing this amps failure with a scope" rather than trying to diagnose the problem the amp has!!! What I'm trying to tell you is that the scope isn't part of the troubleshooting shooting process for this sort of amp problem yet. Why did you ask for advice about the amp problem if you're going to ignore it in favor of this strange path you seem to be on? (not rhetorical)

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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    [QUOTE Another thing would be to run the amp into a load, but also monitor it with your cheap speaker through a volume control in parallel with the load. Then you will at least be able to hear if it's feeding back or not.[/QUOTE]


    I'd love to resolve this at the bench without using the scope...but I don't know how to replicate the feedback problem without playing thru a large enough speaker (which I don't have) to handle this amp & even if I could find one, I don't want to disturb my very pregnant neighbor....everyone is recommending that I use a scope.... so aside from using a scope, a dummy load, I'm not sure what else to do.
    What are examples of the type of load you're suggesting?....and how would I monitor the feedback with the little speaker without blowing it?

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    To me it is not clear up to now what kind of "feedback" you have. Could be acoustical (meaning feedback between speaker and PU or a microphonic tube) or purely electrical. How does it manifest/sound? Do you need to connect a guitar? How do controls' settings influence the oscillation? Have you changed input tubes to exclude microphonic tubes? Can you hear it with a dummy load and a speaker (with a 100 Ohm series resistor) in parallel?

    An oscilloscope is a powerful tool to trace oscillation but requires experience. Amp oscillation may show in different disguises.

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    Last edited by Helmholtz; 12-06-2019 at 11:36 PM.
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    Bent Member Chuck H's Avatar
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    If the amp is unstable (parasitic oscillation) your little speaker should be sufficient to reproduce the problem and you usually don't need to plug in a guitar since these typically do not depend on volume or even input signal to actuate. Please get description of the problem and what typically causes it from the amp owner and report back.

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    Supporting Member The Dude's Avatar
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    I'm starting to wonder if there is even a problem. Playing a guitar in close proximity to a cranked amp is often going to cause feedback. Is that what we have here? Is the feedback only when you stop playing the guitar, or is it constant once it starts? If you move further away from the amp, does the feedback stop?

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  31. #31
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by The Dude View Post
    I'm starting to wonder if there is even a problem. Playing a guitar in close proximity to a cranked amp is often going to cause feedback. Is that what we have here? Is the feedback only when you stop playing the guitar, or is it constant once it starts? If you move further away from the amp, does the feedback stop?
    This is what I was talking about way back in post #9. We don't even have a description of the problem. Just a second hand account that "there is a feedback problem" and that's all we know. I think Perkinsman is hoping that there's a way to use the scope like your local auto repair shop uses a diagnostic machine for some things. Just plug it in and it tells you what the problem is. But of course it's not like that.

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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  32. #32
    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Perkinsman View Post
    To be honest, I’m not sure what oscillation even looks like on a scope, I’ve only used it to see AC sine waves & that was awhile ago. Do I set the master & channel volumes at the point where the feedback occurs? Will that damage anything if I leave it feedbacking for awhile as I probe? Do I just start at the input & follow the signal path to the speaker out? Are there any places I shouldn’t touch with the probe?
    Jack Darr's book covers some of what signals look like on a scope also. Do yourself a favor and read the book. It may be old but it is a wonderful resource written in easy to read language even for non technical types.
    nosaj

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by The Dude View Post
    I'm starting to wonder if there is even a problem. Playing a guitar in close proximity to a cranked amp is often going to cause feedback. Is that what we have here? Is the feedback only when you stop playing the guitar, or is it constant once it starts? If you move further away from the amp, does the feedback stop?
    Couple of other factors here that are possibilities, type of guitar pickups(single coil, active or what?) Overloaded inputs are easy to feedback.
    nosaj

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  34. #34
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    The owner dropped by and brought an attenuator & hi wattage speaker so we were able to hear the feedback at low volume. We put the master at mid and the JCM started feeding back at 4 or 5. No feedback on the other channel at any volume. It would also feedback when we raised the tone controls at higher volumes. No feedback on the other channel at any level of the tone controls. None of the tubes emitted microphonic noises. Same Les Paul used on both channels. We're lucky enough to have a high profile guitarist's tech nearby so we decided it's best left to an expert. Thanks for the assistance and especially for the tip on Jack Darr's book.

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  35. #35
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Perkinsman View Post
    We're lucky enough to have a high profile guitarist's tech nearby so we decided it's best left to an expert.
    Just to be clear, this guy knows about amplifier electronics? Not always the case. Confidence and vaguely related credentials aside,.. We are here if it comes up

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

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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