Audible Oscillation... on one ohmage setting??

[Mr_bibbles]

Has anyone ever come across something like this? A Peavey 5150 II produces a tea-kettle noise on only the 4 ohm setting. I have it hooked into an attenuator, and have never experienced anything like this with the same setup before, and have also confirmed it happens on any speaker cab. Other than the low-medium whistle, the amplifier is working fine, and on the other two settings (8,16) are noticeably absent of oscillation.

Has anyone ever experienced something like this? Again, besides the whistling noise, everything seems to be functioning fine, and the only variable that changes it is the ohmage selector.

I have thoroughly cleaned and metered out the ohm switch, it seems fine from all apparent tests.

So far the Output transformer has passed all the basic meter tests, but I will be performing advanced voltage tests on it to see what I can find.

Thanks!

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Jon Snell]

There is an awful lot of twidling going on in the negative feedback loop.
Do any of the controls affect the whistle?
Both presence controls and resonance controls can easily be adjusted to place the NFB loop into possitive feedback, resulting in oscillation and whistling. Judging by how these work, I would think the whistle is to be expected under certain conditions.

[Chuck H]

The 5150 is a VERY high gain amp. Is the attenuator you're using employing an active load or a purely resistive load? I ask because I have had some trouble with active loads and their inductors reacting with single coil pickups. In fact I had to create a solution for just this affect in my own attenuator circuit. And certainly specific ohm setting on the amp and it's overall gain level can be a part of the equation.

In other words... It may be the attenuator and your guitar reacting rather than the amp itself having much to do with it besides supplying enough gain.

[The Dude]

OK, I have to ask. When you switch the amp to 4 ohms are you matching the attenuator to that load? What attenuator are you using? Do you have the problem if you don't use the attenuator?

[oc disorder]

Certainly sounds like a weird one.. only thing I can recall was an oscillation in a combo which I cured by grounding the speaker frames. Well was a quick cure but I never did find the cause.
I would suggest making sure all grounds are good , if the attenuator has a metal case check that is grounded then try putting atten. and all cables from speaker away from the amp as far as is practicable.
As Chuck H mentioned it's a high gain amp... a prime candidate for weird oscillations !

[Mr_bibbles]

Easy to miss, but I wrote above that It happens on my other test cabinet and on the customers rig as well:

"I have it hooked into an attenuator, and have never experienced anything like this with the same setup before, and have also confirmed it happens on any speaker cab"

I will check the resonance and presence controls, I did not even think of that!

[Helmholtz]

Easy to miss, but I wrote above that It happens on my other test cabinet and on the customers rig as well:

"I have it hooked into an attenuator, and have never experienced anything like this with the same setup before, and have also confirmed it happens on any speaker cab"

I will check the resonance and presence controls, I did not even think of that!

Not clear if there is always the attenuator between amp and speaker. Why not be a bit more explicit?
Resonance and presence control settings should influence the oscillation but are not likely to be the root cause.

[Mr_bibbles]

Not clear if there is always the attenuator between amp and speaker. Why not be a bit more explicit?

Sorry, I honestly don't even think about it, I have a weber 100 watt attenuator on my bench with an old fender speaker hooked into it permanently. I probably shouldn't have mentioned it at all as I think I am providing confirmation bias for something that is very normal for me

I also was thinking that something in the resonance or presence circuit might be faulty; a ground or a drifted component might interact with the NFB in an unintended way....

[Helmholtz]

Sorry, I honestly don't even think about it, I have a weber 100 watt attenuator on my bench with an old fender speaker hooked into it permanently. I probably shouldn't have mentioned it at all as I think I am providing confirmation bias for something that is very normal for me ..

Ok, I tried.
Won't bother you with more questions.

[Mr_bibbles]

Ok, I tried.
Won't bother you with more questions.

Hey Heimholtz,

Sorry if my comment came off as dismissive or if my wording was confusing, I only meant that since it's happening with or without the attenuator (on several cabs) it seems safe to discount it as the problem. You have always given me great advice and succinct answers, I always try and listen sincerely. Thanks,

matt

[Helmholtz]

since it's happening with or without the attenuator (on several cabs) it seems safe to discount it as the problem.

Exactly that is what I was trying to find out as it was never clearly stated before.
(But maybe the problem is my language barrier.)

[Chuck H]

Ok... So to sum up:

Amp oscillates at certain settings that accentuate the HF BUT ONLY when the amp is set for 4 ohms.

The mention of the attenuator is moot because this happens with or without an attenuator.

This does not seem to be cabinet specific because you have tried the amp with different cabs/attenuator/etc. with the same result. The 4 ohm impedance setting being the only common denominator.

Is this correct???!!!

I'll second The Dudes question and ask if the impedance setting is being matched to the correct load IN ALL CIRCUMSTANCES???!!! Otherwise we're back in random territory. If we can know what the actual circumstances are in all failed experiences we might be able to determine something. Without a very clear and accurate representation of the circumstances or answers regarding there is NO way to move forward and, therefor, no possibility of diagnosis. There is no "weird" or bizarre thing happening that can't be diagnosed. It is entirely dependent on accurate reporting and answering succinctly and entirely any questions asked by those trying to help.

[Greg Robinson]

The few times I've encountered unstable feedback/oscillation in the Peavey 5150/6505 series it was resolved by cleaning the FX loop jacks. Why exactly this point outside of the NFB loop should cause instability I do not know, but it certainly won't hurt anything else, and may solve the issue if you're lucky.

[eschertron]

The few times I've encountered unstable feedback/oscillation in the Peavey 5150/6505 series it was resolved by cleaning the FX loop jacks. Why exactly this point outside of the NFB loop should cause instability I do not know, but it certainly won't hurt anything else, and may solve the issue if you're lucky.

The dirty FX loop connection could create a high impedance point that increases EM radiation inside the chassis. Just a hypothesis to possibly explain what you observed. No experience to back that up...

[Mr_bibbles]

The dirty FX loop connection could create a high impedance point that increases EM radiation inside the chassis. Just a hypothesis to possibly explain what you observed. No experience to back that up...

Thanks Greg and Eschertron, it took me forever but it was a 2200uf Supply cap going intermittently bad! I don't know how it was transferring to that sympton, but after replacing the cap the amplifier has functioned well with no squealing at good volumes. Again, sorry to everyone about the attenuator confusion, I'll just leave it out of my description in the future.