Squeal! on a 12xxx build!

[vh junkie]

Squeal comes in if presence or volume are to hi. Both Bright and Normal channel have 820/.68uf on the cathodes (looking to jumper eventually). Squeals on the bright, not on the normal. New mixer cap, no help. No treble cap on the volume pot(s).
NOS Eries in all filter positions. 82K on the FB (8 ohm tap). Ideas! Have scope, will debug!

I had left out the NFB wire at the top and the 47pf cap across the PI when these pics were taken!

Preamp:


Power:

[axeman]

Are the leads to the input jack long? or are the leads coming of the Bright and Normal channel crossing the input jack leads? and doble check the grounding to the Presence pot. If that doesn't work take a got shot and post it.

Art

[6burgh]

Not sure if it helps, but I had a nasty squeal in my 12xxx when I dimed everything with the amp actually sitting on the 4x12 cabinet. Move some wires around a bit with a chopstick and then made sure the head cab shielding was making good contact with the chassis... problem fixed. Does it squeal with no guitar plugged in or is it just while playing?

[vh junkie]

Thanks guys!
Edited OP with pics and summary of parts added after pics were taken.

[axeman]

First of all, the build looks fucking incredible. How about the resistor that goes above the diode next to the bias cap? everything else looks good

[6burgh]

Yeah, that amp looks shitty . Beautiful. It's clean, but have you tried moving some of the input wires around gently and double checked good connection with your head cab shield? I actually added just a tad of duct tape (like the real metal tape for sealing aluminum duct work) and it helped. But yeah, where's your B+ dropper?

[Omn]

NFB added after the pictures were taken... OK, that side should be in order. But what about the wire from the Presence pot?

[vh junkie]

NFB added after the pictures were taken... OK, that side should be in order. But what about the wire from the Presence pot?

That was what I mean't the purple wire from the top of the NFB resistor to the presence pot was added later, along with the 47pf cap across the PI.

Bias resistor (top left of main board) was sized/added later. The bias circuit looks a little odd because some of the parts are moved down to the pots (between v4 and v5) that are part of the bias balance circuit.

I haven't really tried shielding the chassis completely yet. The squeal seems a bit to intense for that to be the answer. I will try it though.

[Omn]

Goody - good.
Pre-amp tubes? If you change the position of 1 and 2 - still the squeal?
Squeal was there before and after the changes done (NFB and PI)?

At the bright channel pot; I have a cap there...

[emmjaydubya]

Really nice build!

So what's the story on the bias balance circuit with the two pots? Looks interesting...

[SteadyEddie]

Is the head box shielded?

[mightymike]

Regarding the sheilding for the cab. Go to Ace Hardware. They have the exact shit used in the old Marshall head boxes. Just cut to length, and staple in.

My first thought when hearing about the squealing was NFB, Input Jack wires....... The sheilded head cab helps, but it shouldn't squeal without it.

[neikeel]

Could you post pics how it looks underneath now - pretty please?

[toner]

It shouldn't be the cause of squealing, but I would suggest lifting the V1, V2 and V3 green grid wires up away from the chassis and heater wires to reduce potential hum noise.

[JimiJames]

Had that similar prob on my 12 in the beginning... Try Larry's ground scheme. Graveyard quiet.
From what you have there, should be easy to do.

What laydown & voltage did you go with ?

Nice build