Tone stack and volume not working on my AB763

[deuce42]

Hi guys

Can someone shed some light on where I might be going so wrong?

My single channel AB763 build works except the volume control and tone stack controls don't have any effect. I just can't understand and have checked the wiring on the pots and board over again about 10 times now.

If I disconnect pin 7 of the first preamp tube (i.e. the middle pin of the volume pot to the preamp tube) I still get the same clean guitar signal passing through. Its as though the tone stack+volume control simply are not in the circuit. I've also pulled one leg of the 250pf tone cap out of the board to test if breaking the tone stack circuit will remove the guitar signal coming through to the rest of the circuit and the signal still passes fine.

I can attach the AB763 scheme here if you'd like, although I'm guessing you guys almost know it off by heart.

So my question is, where should I be looking in the circuit to identify why the tone stack and volume control are not working at all? I had thought it might be grounding but the last pin on the middle control is definitely connected to ground and I've continuity tested the entire tone stack and volume circuit.

Any starting ideas of where to look for would be great. Thanks very much

[Reeltarded]

Instead of a schem toss up a single decent overall shot then a pic from the rear towards the controls. You have an atypical thing going on so it would be nice to look.

You were right to suspect gnd first.. nothing jumps out at me except.. that and .. you are missing a ground, has to be.

[Stevem]

What dc voltage do you have on pin 3 and 8 of that preamp tube?
Do you have the controls wired up backward?

[robrob]

Check the ground connection(s) to the volume and tone pots. Lift the ground and they will be ineffective. If you are using a ground bus make sure and get your bus wire hot before you start to solder a wire to it.

If the grounds look good I'd still re-flow the connections as the symptoms are perfect for a loss of ground to the tone stack and volume pot. Do they share a ground?

[JazzGuitarGimp]

I would start by tracing the signal wire from the input jack to see where the other end is connected. It sounds like the input is wired into the the amp somewhere downstream of the tone stack / volume control.

[Reeltarded]

Pleas note what you find. This loud clean sound might be just what the guy trying to cut bass on the amp we never saw was up in the tech section before I clawed my brain out.

[deuce42]

Hi guys - oh how I wish I could post a photo but my iPhone is broken and being repaired and I have no other camera!!

This thing is making me nuts. I'm eternally grateful for your patience with helping me.

So I note the following:

1) Hot input from the input socket goes to a 68k resistor and the other end of the resistor goes straight to pin 2 of the first preamp tube.

2) Hot input from input socket also goes directly to a 1m resistor going to ground bus wire.

3) Pin 3 of the first preamp tube measures 2.6vdc.

4) Pin 8 of the first preamp tube measures 2.1vdc.

5) I have both cap/resistor pairs that come off pin 3 and 8 going to the same ground bus wire.

6) I've continuity tested connections for the volume pot and middle pot pins that hit the ground bus wire. I'm definitely getting a beep from my meter when I touch these pot pins and the ground bus wire.

7) I've re-checked the wiring order of all pots to make sure none are wired backwards.

I've also tried to test that the hot input going to the 68k and then to pin 2 is not hitting any grids or other pins or connections of any other preamp tube, therefore sending guitar signal to another part of the amp later than the volume/tone stack.

Yet despite the above, I'm still getting nice clean pristine fender guitar sounds even with the volume control all the way off and moreover the volume and tone knobs do nothing when turned to either extremity.

I just can't find any other grounds on the preamp schematic that are not being currently grounded or are missing.

I'm am just thrown!

Please put me out of my misery

[JazzGuitarGimp]

This is a solly question, I know - but have you measured for continuity between the bus wire and chassis?

[deuce42]

I reckon nothing is silly:) Yes bus wire and chassis check in with perfect continuity

[robrob]

Take a look at the schematic. Since the signal is getting through the tone stack and volume pot it MUST be their grounds. Take an alligator clip and clip it from the chassis to the volume pot #1 ground pin and give that a try.

[img1568]http://www.thevintagesound.com/ffg/sche ... _schem.gif[/img]

[deuce42]

Yes I understand but unfortunately even alligator clipping the chassis to the pin of the volume pot doesn't rectify the problem

[xtian]

Does your problem persist with BOTH volume controls at zero?

You'll notice that V2 has a shared cathode resistor and bypass cap. That's one sneaky way signal can couple between triodes.

[deuce42]

Ah yes I see the point of the double triode possibility but in this case I've only made a single channel AB763. I accept there's maybe possiblity for something like this occurring elsewhere in the circuit I just can't for the life of me see where.

[JazzGuitarGimp]

This question has been asked, and I know you've answered it by saying you have checked the wiring, but have you actually turned the volume and tone stack knobs all the way up to see if that cuts the signal (volume / tone knobs wired backwards)?

[deuce42]

Yep rotated all knobs over and over again whilst strumming with absolutely no affect on the signal whatsoever. It's like they are just dummy knobs but never connected.