Amp squeal when guitar too close.

[J M Fahey]

MAYBE some wire inside , which might be a signal carrying one, a ground one or one connecting, say, a tube load resistor to some decoupling cap a few inches away is acting as a small antenna and radiating an audio modulated signal field around.

If some wiring inside the guitar itself or a section of the cable is parallel to this suspected antenna, they will couple and probably squeal.

All 3 suspects, no matter what function we use them for, share the property of being a conductor through some current is passing.

That, like it or not, will create some (audio modulated) magnetic field around.

Nice theory, let's get some practical use: I'd leave the guitar somewhere "squealy" and move some wiring inside the chasssis with a chopstick, to see whether something changes or not.

Sometimes body position influences electromagnetic fields, here's proof :

Mr.bean - Buys a T.V - YouTube

well, that was for fun,but back to the amp, we already know that guitar position influences squealing, let's try transformer position (unbolt it and turn it around, without unsoldering wires) aand test moving wires here and there, á la Ken Fischer/Trainwreck

And alsp MB,with their trick wires which go nowhere.

[Alan0354]

Yes. It has power scaling, the choke is on the screen grid and PI side. But I have a RC of 2.2K with 40uF on the preamp side in place of a choke. The first stage has a RC of 10K and 20uF just for the first tube alone. The second stage has it's own 5K and 20uF. The third and last stage has it's own 10K and 20uF. I did parallel another 20uF with all the caps and has absolutely no effect on the squealing.

[Alan0354]

MAYBE some wire inside , which might be a signal carrying one, a ground one or one connecting, say, a tube load resistor to some decoupling cap a few inches away is acting as a small antenna and radiating an audio modulated signal field around.

If some wiring inside the guitar itself or a section of the cable is parallel to this suspected antenna, they will couple and probably squeal.

As you can see, the decoupling cap is point to point with the plate resistor on the first stage, the other are like 2 inches max. You can see in the picture that the decoupling caps follow each stage along the signal path.

The guitar is the same for testing the other amps, so the guitar is not the problem.

I have to start poking the wires in a little bit and report back.

[Mark Black]

Please do try another guitar cord as well (if you haven't already). Just a thought...

[Alan0354]

1) I loosen the primary and secondary wiring of the OT and move it around, it did nothing.

2) I use the chopstick to poke around the wiring inside, nothing.

3) I hold the guitar above the chassis with pickup facing the circuits, the squeal stop. I think the thick chassis shielded the OT completely from the guitar when the guitar is on top of the chassis.

4) I sat down with the guitar like the original position I was testing before. I took a big socket wrench that is steel ( magnetic material), I wave it in all different position. The only time the squealing frequency goes lower was when the head of the wrench touch the laminate of the OT. I tried the wrench in front of all the tubes, inside the chassis, touching the chassis, non have any effect on the squealing, only when I move the wrench very close to the OT then the frequency goes lower, I can hear the pitch dropped.

5) I tried a second guitar, same squealing. So the guitar is not a problem. Besides, both guitar works perfect to start with.

I am really thinking it's the OT. I am going to try another OT by just hook up the wires to make the amp work and move around.

If it's the OT, turning the OT 90 deg is not going to work, I walk to the side of the chassis ( 90 deg) it still squealed. So turning the OT is not going to solve the problem. I might have to take the Vibrolux OT out of the Bassman and try it ( bummer, that's a working amp).

[Alan0354]

Just tried changing to a new guitar cord, same thing, cord is not the problem.


Guys, keep making suggestions. Anything I can try before changing the OT!!! That's a real bummer.

Thanks

[Leo_Gnardo]

Alan you've maid the claim that it's not the guitar, but I'll claim it is. Pickups to be specific. I think you may find some relief if you hold a wax dipping party for them. If there are metal covers, do your final dip with the covers ON.

Sing along with Kenny Loggins: 'dip dip dip dip dip dip dip dip I'm ALL RIGHT!' Who knew it was guitar advice? Try it, you've tried just about everything else.

[Jazz P Bass]

Have you investgated the preamp tubes for microphonics.

I has a Hartke HA3500 in that squealed when you got too close.

Bad preamp tube.

[gui_tarzan]

I took a big socket wrench that is steel ( magnetic material), I wave it in all different position. The only time the squealing frequency goes lower was when the head of the wrench touch the laminate of the OT.

When you touch the wrench to the OT you're creating a ground loop between it and the input section of the amp by going through your body->guitar->amp input. I read all the posts and I'm still suspicious of the quality of the ground from the preamp section to the chassis. Where are your input jacks and preamp grounded and have you tried grounding it to a different area of the chassis?

[Jazz P Bass]

Have we explored the fact that it may be a microphonic preamp tube?

[tedmich]

I'd set up amp and guitar and get a squeal going (sorry neighbors...) and chopstick the leads, moving each signal lead in turn to change right angle wire orientations, push wires closer to ground plane (amp chassis) pull two wires apart etc and then try power leads. Trouble shooting a PTP rats nest usually requires this, its slow and a pain but you will find a wire that affects squeal, then try to minimize it.

[Alan0354]

Alan you've maid the claim that it's not the guitar, but I'll claim it is. Pickups to be specific. I think you may find some relief if you hold a wax dipping party for them. If there are metal covers, do your final dip with the covers ON.

Hi Leo. Both Guitar have no problem with the other two amps that have very similar circuit of this one. The pickups are all D Allen's custom pickup that are wax potted. As I said before, my amps has switchable 4 stages gain. The other two is not sensitive even with the 4th stage kicked in until I cranked both preamp volume to max. This one squeal with only 3 gain stage, don't even have to kick in the 4th stages. Big difference.

Thanks

[Alan0354]

Have you investgated the preamp tubes for microphonics.

I has a Hartke HA3500 in that squealed when you got too close.

Bad preamp tube.

I change all the tubes including the power tubes before I even posted. That's what the first thing I suspected.

Thanks

[Alan0354]

When you touch the wrench to the OT you're creating a ground loop between it and the input section of the amp by going through your body->guitar->amp input. I read all the posts and I'm still suspicious of the quality of the ground from the preamp section to the chassis. Where are your input jacks and preamp grounded and have you tried grounding it to a different area of the chassis?

Hi Gui_Tarzan, As shown in the picture, the ground of the input jack is isolated from the front panel and tied to the ground lug to the chassis at the extreme left. The cathode resistor and bypass cap are tied to the same point PTP. The ground of the 20uF decouple cap is tied to the same point as seen in the picture. So the whole first stage is single point grounded completely isolated from the rest of the circuits. I did try grounding the 20uF cap some place else also. The resistor before the 20uF cap is 10K, I think I have a very good filter and ground going. this practice is done on the following stages. As you can see in the picture, I have 20uF caps following each stages.

I use Chassis as ground plane, All the high current grounds of the power tubes and PS filtering caps are one point grounded on the top right side where you see all the big green wires go to a single screw. This to ensure the large current path goes to that point, no large current flow into the chassis. I use the same grounding design in the other two amps that have no problem.

Last night, I tried moving the guitar to all different position, the only position that the squeal stopped was the guitar on top of the chassis and pickup pointing straight to the circuits from above. I hold the amp sitting on the side and hold the guitar with pickup facing the bottom of the chassis facing the OT, it squeal just as bad.

I did use a chopstick to poke and move wires around, have no effect what so ever. When I use the wrench, I don't have to touch the OT, just very close and I could hear the effect.

Thanks

[Alan0354]

I'd set up amp and guitar and get a squeal going (sorry neighbors...) and chopstick the leads, moving each signal lead in turn to change right angle wire orientations, push wires closer to ground plane (amp chassis) pull two wires apart etc and then try power leads. Trouble shooting a PTP rats nest usually requires this, its slow and a pain but you will find a wire that affects squeal, then try to minimize it.

Thanks, I did that twice already. No effect. I even freed the wires from the OT and move that, no effect.