5150 II High Pitch Squeal Issue

[Genius Gone Insane]

When I turn the presence knob past 3pm, I get a high pitch squeal, even at low volume. I retubed both the power and the preamp tubes with no change.

I sprayed contact cleaner everywhere, let it dry, and then tried again. After about 20 minutes of playing, a strange 15 second hiss happened. It sounded kind of like I was plugging/unplugging my guitar cord, but the pops were not that loud. After 15 seconds, all the sudden everything went normal. The high pitch squeal disappeared.

That happened 1 time. Since then the squeal is still there. Could this be a fuse issue? Anything else? Time to take it to an amp tech? Any good ones in the Bay Area?

 

[sprack]

Could be a preamp tube. Could be a filter cap. Get a spare tube and start swapping. If it's a cap that'll be a lot more work to find.

 

[guitarguru777]

PLug the cable in and out of your FX send and return a few times.

 

[TheWinterSnow]

Filter cap possibly, but the squealing that is dependent on high presence settings makes me believe that the reference feedback resistor went out which could have happened if you had a filter cap going out considering where that resistor is at.

 

[bryan_kilco]

Hmmmm listen to these guys....

When I had bad power tubes, I had a similar thing happen to me. At high gain/treble/volume, I'd get uncontrollable feedback (microphonic tube?) and now and then you'd hear the tube give power and then cut out again.....

 

[Genius Gone Insane]

What is a filter cap? Why do I still need to swap tubes? I already tried different tubes on the power and preamp tubes.

 

[sprack]

Sorry I missed that you said you replaced the tubes.

Filter caps are there to smooth out ripples and spikes in DC current coming off the PT. It helps clean up the power to components down/up-stream (depending on whether you subscribe to electron flow or current flow) as well as protect them from overage. When they go bad there's 3 mechanisms to spotting it; puffed up tops where the electrolytic is pushing out (this is the most extreme), checking with a DMM for low resistance or best an ESR meter.

When a cap goes bad it becomes an intermittent switch in the circuit with variable resistance/capacitance so it kind of becomes a crazy hi/lo filter...hence the squealing.

If you're gonna check the filter caps make sure they're drained, they're not like the big bottles on my JCM, but IIRC my 5152 has 2 350V_300mF caps which still hold a lot of juice.

 

[Genius Gone Insane]

How do you drain them? Just power off and leave plugged in?

 

[Omega_Void]

Could be an EMC issue (electromagnetic compatibility). Have you recently added anything electronic in the environment?

(Especially anything using a switchmode power supply. Any PSU which weighs less than you'd expect is probably switchmode, and they're often little bastards for RF.)

Have you tried the amp in a different room? Or even just moving it to different spots in the same room. That can help to determine if the problem is in the amp itself, or being picked up from an external source...

 

[Genius Gone Insane]

I bought this amp from another Sneap Forum member and I don't know if there were any mods to it. I tried plugging it in at a different location and the squeal is still there, so I don't think it's an external issue.