Assistance Using Scope to Troubleshoot

[Chuck H]

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.

[nosaj]

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

[nosaj]

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

[Perkinsman]

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.

[Chuck H]

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

[nosaj]

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

Guitar electronics half a volt of electricity Vs Amp Electronics hundreds of volts of Electricity.

That's like saying I'm a painter I can paint cabinets(and I do at work) but I ain't no Van Gogh....

nosaj

[Chuck H]

Guitar electronics half a volt of electricity Vs Amp Electronics hundreds of volts of Electricity.

That's like saying I'm a painter I can paint cabinets(and I do at work) but I ain't no Van Gogh....

nosaj

That's where I was going with my post. But often a guitarists tech also maintains their amps. That doesn't necessarily mean they KNOW about electronics either. Especially WRT modern amplification. Basic on stage problem troubleshooting, biasing and cleaning contacts is often all this position requires. That's a far cry from knowing what causes instability in an unknown circuit. But I put the caveat of the possibility that the guy could be an amp tech in the question because that also happens. Example...

It's interesting to me that no one credited or acknowledged my recognition of no 0DC reference at the downstream end of the channel switch. That should be addressed and I'd only trust this 'known guitarists tech' if he/she recognizes it as well. If they don't mention it or correct it I think Perkinsman should return here or find an actual "electronics tech" for advice on matters of amplifier repair and modification.

JM2C

[g1]

It's interesting to me that no one credited or acknowledged my recognition of no 0DC reference at the downstream end of the channel switch. That should be addressed and I'd only trust this 'known guitarists tech' if he/she recognizes it as well.

I just assumed they were using a MBB type switch there.

Seriously though, I did notice your valid statement with regard to the switch. Some of us may get a bit of 'tunnel vision' in terms of fault finding, which is why it's so good to have many eyes looking at problems as we do here.

[Chuck H]

I just assumed they were using a MBB type switch there.

Ah, Yes. That would make sense too. Like any commercial design I would still have a safety resistor there. But if it's not an immediate problem I suppose it doesn't need fixing.

[Helmholtz]

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.

Feedback requires gain. The more gain the more likely is (unwanted) oscillation. The "2203/2204" channel can be assumed to have way more gain than the other channel, so no surprise here. Actually I like the 2203 circuit for its (controllable) feedback capabilities. If it's not controllable but rather squeals it's likely due to microphonic PUs.

[Chuck H]

If it's not controllable but rather squeals it's likely due to microphonic PUs.

Yep. Like I suggested in post #9. Now we know an attenuator is being used as well so there's a possible reaction from that unit. Though the guitar was a Les Paul, which usually means humbuckers and they cancel external EMF's, but if it has p90's...??? Or it's instability. I think this is still a possibility because Perkinsman described being able to hear the feedback at low volume because the attenuator was being used. So all these possibilities should all be tested for if the described symptoms don't make it obvious to any local observer. In all immodesty I would know which of the three is causing the problem immediately had I been present for the test. I sincerely hope the guy taking charge now is qualified for the same confidence.

"mic drop"

[nosaj]

Yep. Like I suggested in post #9. Now we know an attenuator is being used as well so there's a possible reaction from that unit. Though the guitar was a Les Paul, which usually means humbuckers and they cancel external EMF's, but if it has p90's...??? Or it's instability. I think this is still a possibility because Perkinsman described being able to hear the feedback at low volume because the attenuator was being used. So all these possibilities should all be tested for if the described symptoms don't make it obvious to any local observer. In all immodesty I would know which of the three is causing the problem immediately had I been present for the test. I sincerely hope the guy taking charge now is qualified for the same confidence.

"mic drop"

It's a shame he had the tools to figure it out , just needed a dummy load. The sig Gen would've ruled out the guitar. Hopefully we will hear the final result of this.
nosaj