Tone stack and volume not working on my AB763

[sunnydaze]

In the normal two channel version, the signals from each channel run through separate 220K resistors before going to the phase inverter. If you made a mistake on this part, you could be sending a signal directly to the phase inverter and bypassing the volume and tone stack. Try disconnecting the input to the tone stack/volume control and checking if you still have volume. If so, then for sure the signal is going directly to the PI from the preamp.

Hope this helps.

Thanks
Mike

[robrob]

If it's not the external ground wires then your pots' must be bad. I'd test them and replace them. The pots are variable voltage dividers and they're not functioning as dividers at all--no signal is being shunted to ground.

[deuce42]

In the normal two channel version, the signals from each channel run through separate 220K resistors before going to the phase inverter. If you made a mistake on this part, you could be sending a signal directly to the phase inverter and bypassing the volume and tone stack. Try disconnecting the input to the tone stack/volume control and checking if you still have volume. If so, then for sure the signal is going directly to the PI from the preamp.

Hope this helps.

Thanks
Mike

Ok so I disconnected the 100k slope resistor of the tone stack and the 250pf tone stack cap from the 100k resistor feeding the tube plate and just as you say, the amp is still playing signal.

So this 220k dividers must be where the problem is as you say - i.e. feeding straight into the phase inverter. The way I connected it was to refer to the 2 channel ab763 schematic and send one of the 220k resistors to ground and the other to the 0,1uf cap going to the 100k tube plate resistor. I thought this would be correct. Is this where I have screwed up?

If this is not the problem then perhaps as RobRob notes I should try replacing the pots. I did try and measure the change in resistance from the pots as I rotated them and did seem to get a reading but perhaps my volume pot is the culprit if my 220k divider technique has not been wrong.

I'm really appreciative of all your help guys. Honestly am:)

[bcook]

It sounds like you've got your plate resistor to tube socket connections reversed on the first tube, but really, a picture is worth a thousand words.

[robrob]

Ok so I disconnected the 100k slope resistor of the tone stack and the 250pf tone stack cap from the 100k resistor feeding the tube plate and just as you say, the amp is still playing signal.

No, your pots are most likely ok, you have a major wiring mistake.

The two 220k resistors just before the phase inverter are mixing resistors for the two channels which keeps the two channels from interacting. If you have only one channel you can delete both 220k resistors. If you deleted tremolo you can connect the .047 and .001 caps directly together and delete everything else.

[sunnydaze]

A couple of good pics and we can probably sort this out pretty quick.

At least you know where the problem isn't.


Mike

[deuce42]

It sounds like you've got your plate resistor to tube socket connections reversed on the first tube.

And there you have it!!! What a stupid mistake I made. Reversed the order of pins 1 and 6 and hey presto, volume and tone stack perfect.

Thanks so much guys for all your assistance. I am so grateful indeed.

Cheers:)

[robrob]

Cool, glad you got it working. Nice catch bcook.

[sunnydaze]

Cool, glad you got it working. Nice catch bcook.

+1

Glad you got it sorted out.

How's it sound?

Thanks
Mike

[bcook]

Cheers:)

Beer up!

[deuce42]

Sounds great - maybe a slight 120hz hum which could be reduced but it's not something that's too bad.

I had separated the grounds into two areas:

1 ) Preamp grounds (bus wire and two points on board) and the filter cap in the power supply feeding the preamp all connected to input socket which touches chassis,
2) The other grounds including the isolated from chassis output socket wire going to the power transformer bolt. Thought this would be enough.

About to embark up reading the 1 million threads herein related to grounding issues to tighten it up. The noise isn't too bad though. I've heard worse noise in other peoples builds.

You guys are very generous with your time and patience:)

[Teleguy61]

In the Fender style builds, it usually works out pretty well to have the PI
power supply node grounded at the input, with the preamp tubes, not with the output section.
Grounding it with the output section can induce some hum.