Presence Squeal on 68' Plexi clone (SOLVED)

sdn25

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Hey guys, I have this 50w Ceriatone Plexi clone that I built that just had its OT replaced. The amp puts out a high pitched squeal when the presence knob is turned past about 5. Volume and treble does not affect the squeal, only the presence knob. The squeal is especially bad when I have the bright cap engaged. its a 4n7 BC. I have included comprehensive pictures of the inside of the amp, hope they can help resolve the issue. Thanks in advance!
 

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Browneyesound

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With an OT replacement, possibly the negative feedback has been reversed?
 

Browneyesound

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Also, are you trying the amp with the guts open?

If the amp is installed in the cab with the shielding (I assume it has some kind of metal plate for shielding) that could help a lot.
 

sdn25

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With an OT replacement, possibly the negative feedback has been reversed?
I have eliminated that possibility, as I had it installed reversed before, had an uncontrollable fart like positive feedback, this sounds more like parasitic oscillation.
 

sdn25

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Also, are you trying the amp with the guts open?

If the amp is installed in the cab with the shielding (I assume it has some kind of metal plate for shielding) that could help a lot.
Yes the chassis is open, have a head cabinet on the way from Ceriatone. but im pretty sure there's more to it than just a head cabinet, although im sure that the cabinet will help but this seems too severe of a squeal.
 

Browneyesound

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Yes the chassis is open, have a head cabinet on the way from Ceriatone. but im pretty sure there's more to it than just a head cabinet, although im sure that the cabinet will help but this seems too severe of a squeal.
Don’t underestimate the amp shielding…been there done that with a mystery oscillation that appeared while working on the amp, only to go away magically after putting the amp back in the head cabinet.
 

junk notes

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Nice clone, laydown tranny!

Yes, shielding. Having it in a shielded head cab will help. You could temporarily use roofing flashing on some plywood that you can cut to size, and see if that would remedy the issue, while you wait for your head cab to come in.


If the issue persists, there are several other areas. Your lead dress, wires going to the V1, V2, V3 preamp tube sockets, look alright; possibly rerouting the neg fb wire coming off the Presence. Usually a purple wire, but you are using green there.

(Just checking:yesway:, on the safety , careful of lethal voltages, and the draining of the electrolytics. and using a chopstick/ wooden dowel, other free hand in pocket.)
 

XTRXTR

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Just in case use a different guitar with it, some old guitars don't have wax potted pickups. Also even new guitars with wax potted pickups can loosen up if the guitar was subjected to direct sun, or over heated in a truck or storage for a while.
 

william vogel

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You need to make sure the NFB wire from the impedance selector is routed away from other wires except possibly grounds. I see it next to the secondary wires from the output transformer, it’s okay there. If it runs next to the preamp wires or power tube wires, you should move it. In all the pictures, you didn’t include the NFB wiring. Here’s a picture of my chassis’s. My NFB wire is purple.
3AD12B66-9B18-4713-95B7-D4E25679EF12.jpeg
 

XTRXTR

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Are your pots 'ground buss' grounded to the chassis at the presence pot? Hard to tell from the picture. Where is it grounded to chassis? Should be at the inputs. The pic at the presence pot ground seems to show the solder may need to be re-flowed.
 

sdn25

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Are your pots 'ground buss' grounded to the chassis at the presence pot? Hard to tell from the picture. Where is it grounded to chassis? Should be at the inputs. The pic at the presence pot ground seems to show the solder may need to be re-flowed.
The buss bar is grounded at the chassis through a wire going to a star ground.
 

sdn25

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I will send the updated pictures
 
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South Park

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It sounds like a ground loop off of the presence pot reroute the ground
 

sdn25

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here are the pictures of the NFB areas.
 

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sdn25

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update: I moved some of the pre amp wires away from each other and re routed the ground on the presence pot and its significantly helped, now it starts to squeal only when the volume and presence is above 8.

however when I touch the chassis of the amp the squeal significantly reduces and has a higher pitch.

When I let chords ring out, after about 2 seconds a really fizzy high end overtone comes through similar to how it would sound if the squeal was trying to fight for the higher frequency. Does this narrow it down?
 

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neikeel

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It’s hard to tell from pics as I see different versions. Ideally your v1 plate wires (red) should be soldered to the turret underneath (not top like I see in pics) then run down to the chassis and straight to socket. Grid wires are to be kept away from these.
Also I cannot see grid stoppers on pin5 of your output tubes. (4k7 or 5k6)
 

sdn25

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It’s hard to tell from pics as I see different versions. Ideally your v1 plate wires (red) should be soldered to the turret underneath (not top like I see in pics) then run down to the chassis and straight to socket. Grid wires are to be kept away from these.
Also I cannot see grid stoppers on pin5 of your output tubes. (4k7 or 5k6)
I have grid stoppers (5k6), they are covered with black insulation to prevent any shorting.
 

Pete Farrington

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Volume and treble does not affect the squeal, only the presence knob. The squeal is especially bad when I have the bright cap engaged

now it starts to squeal only when the volume and presence is above 8.
So before, the setting of the volume controls (either or both channels?) didn’t make a difference, now they do?

when I touch the chassis of the amp the squeal significantly reduces and has a higher pitch.

When I let chords ring out, after about 2 seconds a really fizzy high end overtone comes through
How close are your pickups to the amp?
Need a couple of metres separation to reduce electrical coupling.
Putting some metal screening that’s electrically connected to the chassis over the open amp chassis would be illuminating.
 

sdn25

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So before, the setting of the volume controls (either or both channels?) didn’t make a difference, now they do?
Yes.
How close are your pickups to the amp?
im sitting far enough id say, because I did a test where I put a strap and moved around my room to see if it became any better. there was a point where if I got too close it would squeal really bad with the guitar volume turned up, this squeal im talking about is not that squeal.
 

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