High pitched whine from amp - cable issue?

[heavenorlasvegas]

Hello all,
Recently my Peavey bass 400 has developed a high pitch feedback whine when I turn the gain all the way up..

The amp has four channels and it is only an issue with the one channel. It only happens when a cable is plugged in. A guitar does not have to be present for the issue to persist.

If I lower the gain slightly the noise goes away, if I roll off the highs with the treble switch the noise leaves.

The noise is actually coming from the amp itself, but can also be heard from the line and speaker output.

Thanks again sos.

Roger

[Wonks]

Assuming it’s the Mk 3 version, the amp has two channels. Each channel has a high and low (-6dB) sensitivity input.

There should also be an ‘automix’ input, where the bass signal feeds both channel at once.

With no cable plugged in, the input signals are shorted to ground, which is why you aren’t hearing the noise with nothing plugged in.

If it was a cable then you’d get noise on all the inputs.

So I suspect it’s down to something within the amp going wrong, possibly an op amp on that channels’ input circuitry. But it could also be a faulty capacitor. The amp is going to be pretty old so lots of components could now be out of spec.

But it may be something as simple as a poor ground connection on a jack or an internal ground wire. It’s worth checking all the nuts on the jacks and pots are tight and if you open up the amp, that the ground screws and nuts are all tight.

[heavenorlasvegas]

hi wonks,

thank you for this info. I have opened the peavey up and all of the jack connections look good.

does the fact that it only occurs when the gain is maxed, give you a hint as to weather its an opamp or capacitor issue? thanks again.

[Hugh Robjohns]

No, but it suggests a gain stage is becoming unstable. It will require a good electronics tech, an oscilloscope and a set of schematics to solve.

[ef37a]

https://shopz.off-69.cf/ProductDetail.a ... 0&pr=50.88

If that ^ is the amplifier in question it is a bit better than the rats nest you see in some but there are still wires running over PCBs. It could be that either someone has cut the ties to gain access or some wires have shifted naturally.

It might be worth adjusting their positions WITH AN INSULATED TOOL (we used to use a fat plastic Knitting needle) whilst monitoring the amp's output. But, don't leave it screaming for too long!

But mostly I agree with Hugh,this needs the ministrations of a good amp tech. Do not work on an open live chassis unless you are confident you can do so safely.

Also you cannot tell if a jack is shorting when closed by inspection, you need to meter it. Or plug in a short shorted cable. If that stops the whine you have a dodgy jack.

Dave.

[Wonks]

https://shopz.off-69.cf/ProductDetail.a ... 0&pr=50.88

If that ^ is the amplifier in question ...

I don't think so. I'm pretty sure it's a MkIII seies solid state bass head.



https://peavey.com/manuals/80301002.pdf

It's got an 'effects loop' in that the preamp out and power amp-in sockets are connected via a switching jack on the power amp in socket. If this hasn't been used, then the contacts may be corroded and need a spray of switch cleaner and repeated jack insertion. It wouldn't harm to do this, even if it's not the cause of the whine.

I'd also try switching the graphic EQ in and out to see if that has any effect, and also the compressor. Move the noisy channel's tone controls to see if that has any effect on the whine.

It seems to use NEC5532 and TL074 op amps in the main circuits.

[ef37a]

Fair enough, I just put "Peavey bass 400 internal photos" into Google and got that result. As you said it was an "old amp" I thought that might be it?

Good call on the FX jacks. OP can prove the jack switches by looping the jacks out with a guitar lead.

Might still be some disturbed or shronky cable routing?

"Parasitics" can be a real bugger to track down!

Dave.

[Wonks]

I think it was brought out in 1979 and made throughout the 80s. So still a pretty old bass amp.