Amp squeal when guitar too close.

[Alan0354]

I power up my new build into the Marshall. Sounds good, everything works except if I have the guitar too close when in OD channel, it squeal. This is the third amp I build and I never have problem with squealing. I think I have the layout down pretty good. Particular I built 3 preamp into the Bassman, it's like I have 5 amps, this is the only one I have problem.

I change all the tubes out to make sure it's not the cheap tubes, it still squeal with the other tubes. Particular if I crank up the treble. The amp sounds right, that tells a lot about the layout already because from my experience, if the layout is bad, the sound changes too.

I put a 470pF on the plate of the second stage to ground, it helps. But I don't want to put any more as it will take away the chime. I don't even have any cap to ground in the Bassman and I never have problem. My KMD build don't have problem. I always thought when I have good layout, the squeal goes away. This one has to best layout because I actually planned the layout out ahead of time and is very similar to the KMD.

The main difference is the OT. I use the Classic Tone Vibrolux/Bandmaster/Pro OT. This one I use the Classic Tone Marshall Plexi 50 OT. Can this be the cause of the squealing? What other things I should look for?

Thanks

[J M Fahey]

Rotate on your feet 360 degrees, does squealing stop/increase at some position?

Unfasten your guitar strap, put guitar face up , towards the ceiling, then slowly rotate it until it points down, does squealing stop/increase at some position?

What guitar is it? What pickups does it use?

If you switch different pickups, what happens?

Set guitar volume and tone to 0 , does squealing stop?

Unplug guitar cable from amp, does squealing stop?

EDIT: what marshall version is it?

Schematic?

[Alan0354]

Thanks for the reply. Yes, the guitar position is everything, I have to get it close with pickup facing the amp to about 12" at the volume I play with. It is from the guitar to the amp, but definitely not through the speaker. I have A/B switch that I can switch to a speaker a good 8' away, still squeal when I get the guitar close to the amp chassis. So the guitar is definitely talking to the chassis.

I always work on amp on the table and test with guitar sitting down in front with the same guitar. This one just very easy to squeal. This is my own design, not a copy of Marshall, I only use the JCM900 chassis and cabinet, everything else are new. The circuit is very similar to the KMD build I already have it working already, so it's not like a completely new design, mainly it is a 40W EL34 vs the 6V6 20W design of the KMD. Also, I have the exact circuit build into the Bassman with EL34 and never have problem. Only difference is the Bassman still have the Vibrolux OT.

Can the OT make a difference, that's the only thing that is definitely different. It is not slightly squealing prone, it's a lot more sensitive. I have foot switchable to add the 4th gain stage, I never even have problem with the 4th stage kicked in with the others. This one squeal with only 3 gain stages. This is how bad it is.

Thanks

[g1]

I would think the OT could make a difference. Does the amp have NFB, if so post the NFB circuit. Actually, post the whole schematic.

[Loogie]

If you're looking for positive feedback you came to the right place.

[Alan0354]

Thanks G-One. The amp dose have feedback. It is nothing more than a 50K from the secondary back to the PI with a 5K at the tail to get a theorectical gain of 11 ( say 6 or 7).

I just did an experiment. The chassis is sitting on the table with the top open. I put first a two sided copper clad FR4 board over the top that grounded the copper, not helping a bit. I then put a thicker aluminum plate of about 1mm thick over the copper clad FR4, it did not help a bit. Then I put the thicker aluminum plate vertical in front of the amp chassis between the chassis and guitar, it help. The aluminum is not grounded, just there. This proof the interference is magnetic, not electric. This is because the magnetic field gets partially blocked by the thicker aluminum plate and the FR4 copper clad is too thin to do anything.


Newly Edited:

I further did experiment, I put a thick metal mixing bowl over the OT, it did not help, Metal bowl over the EL34, that did not help. I am very relieved, it does not seems to be the OT. Then I try using the aluminum plate to stand closer to the power amp side and the preamp side, The preamp side really help!!!

Strange, I would think the guitar should talk more to the later stage but it seems the preamp side is more sensitive. In fact, when I have the plate standing on the preamp side, it did not squeal even at 6". I am going to double check the wiring to make sure I don't have any loops. It is definitely magnetic coupling.

[Alan0354]

If you're looking for positive feedback you came to the right place.

[Alan0354]

I have added more filter caps on the gain stages, it did not help. When oscillating, I only see less than 0.4Vpp on the +V anywhere, it could well be the ground lead of the scope probe picking up. I don't think the oscillation is from the +V. I further using the plate to move around, seems like the more sensitive area is still on the tube and OT side, not the circuit side inside the chassis. This is getting bad, the worst kind.

I still think it's magnetic coupling, I can put the plate in front of the guitar pickup close up, it stop all squealing without touching anything, just holding in my hand. It really feels like it's the OT at this point. Other than different OT, the OT is turn 90 deg. Both this OT and the two in the other amps are mounted vertically. But the other two have it mount so the bobbin face is pointing to the side 90 deg. from the guitar. This one, the bobbin is facing towards the guitar.

[g1]

I'd still be interested in whether disconnecting the feedback loop makes a difference, with or without the plate.

[Alan0354]

I'd still be interested in whether disconnecting the feedback loop makes a difference, with or without the plate.

Sadly, no. Not a dent of a difference.

It's funny, I turn the FR4 clad board 45 deg so the it becomes diamond shape with corner pointing up to reduce the footprint of shielding. I move the board and the squeal change frequency when it is in front of the OT. Result is the same when I move from left to right or right to left.

I am so hoping it's the circuit or the ground wiring, but problem doesn't even seems to be inside the chassis. Particularly any ground routing in the chassis is in the horizontal plane, the guitar coil is pointing at the chassis, so the two planes are perpendicular, which gives the minimal coupling.

[Alan0354]

Any suggestion before I point to the OT?

I do have the original big OT from the Bassman and the OT from KMD, you think it's worth hooking it up outside and try before performing any surgery on the working amp?

[loudthud]

I'm trying to figure out an issue like this. A Strat oscillates with a 10 foot guitar cord but not with a 20 foot cord. Distance from amp to guitar about 3 feet.

Alan, check that all the metal parts of the OT are grounded.

[Alan0354]

I'm trying to figure out an issue like this. A Strat oscillates with a 10 foot guitar cord but not with a 20 foot cord. Distance from amp to guitar about 3 feet.

Alan, check that all the metal parts of the OT are grounded.

The bell housing is screwed directly onto the chassis with 4 screws very tight, I think so.

Is the E-I strips supposed to be all insulated? Then they are not.

How did you resolve the problem? I had this problem like this on a Michell amp in the late 70s. That was before I even know electronics. I ended up putting a lower resistance SS circuit as the first stage to solve the problem. But I don't want to do that now particularly I have two other amps with very similar circuit and absolutely don't have this problem even with the 4th gain stage switches in. This one squeal with only 3 stages. As I said, the layout is point to point and very similar to the KMD that has no problem. I have the tubes close to the control pots on the front panel to achieve almost point to point wiring, nothing like the conventional amps that have tubes way at the back and all connections are long.

Thanks

[g1]

Does this amp have a choke?

[Alan0354]

This is the wiring of the amp, I label the primary and secondary ( spelled wrong, don't know how to change). I also label the first, second & third, forth gain stages and PI in the picture.

It might look messy, but it is short wiring and layout is very logical from left to right, no cross over.