Major noise issue with AB763 build

[pompeiisneaks]

For the PI, I'd guess the difference is just due to imbalance between the sides.  Basically, one side is conducting more than the other and dropping voltage.  If you were to look at the output you'd likely see the waveform smaller on one half of the PI.  There are many means of balancing the PI, some adjust the dropping resistor on one side from say 100k to 82k, etc.  I don't know this particular amp super well, but  looking at it, it definitely has differing voltages on the schematic, but only by 5v.  You may have a weak 12AT7 in that spot I guess yes.  If you've got a spare, swapping wouldn't hurt.  You could also try swapping the higher voltage one with the 100k to a 82k so that it doesn't resist the current flow as much and helps balance out the PI.

~Phil

[Apexelectric]

I'll probably just get another with balanced triodes before I start swapping resistor values. There is an 82k and 100k currently installed.

Any issues with the imbalance on the reverb recovery stage before the PI? Or should I get a balanced triode for that as well?

[Apexelectric]

Anyone have any ideas on how I can keep the MIX pot from injecting too much noise into the signal when turned up?

One of my noise issues that I was able to remedy earlier was the path of the two shielded cables for V1. Above board proved much better than below.

Could this be a similar issue on the reverb?

[pompeiisneaks]

The reverb itself uses shielded cables for the rca connections, I think this part of the circuit can't hurt with shielding, but a lot of that depends on the layout itself.  It also depends on the kind of noise.  If you posted some pictures of the build, many here can spot problems just from that.  What kind of noise comes from the MIX pot exactly?  is it scratchy noises?  If so you've got DC on the pot.  Have you chopsticked around the amp to see if there is a specific area making noise?  also you can just move wires around with the chopstick to see if that initiates any noise?

~Phil

[Apexelectric]

The mix pot, as it's turned up, introduces more Hum into the output. Not scratchy, just seems to amplify the inherent noise in the circuit. More like a ground Hum. Been through all the wiring with a chopstick and couldnt get it to change by moving any of the wiring. Rechecked all the grounds. Moved them around in a couple configurations with no change.  Some of the reverb wires between the tubes, board and the controls pass through a hole in the board and either go under the board to the tube socket or terminate on the back of the board. Was wondering if this might be an issue.

[bnwitt]

You've got a lot of flying wires in that chassis and they're all running along together.  That's a great opportunity for cross talk.  I'd say lead dress is at least part of your issue.  I try to keep my leads as short as possible and run them down to and then along the chassis bottom where possible keeping flying leads to a minimum.  See the original fender AB763 photo and the 2 channel Hoffman DR photo I attached.  You'll notice I have even run my shielded input wires under the board straight to the tube pins instead of over the top.
Barry

[bnwitt]

After looking at your two board layout, I'd say that one board with the bias circuit taking up a smaller footprint in the chassis would have allowed some of the main board components to shift an align better with the controls and tubes thus shortening the wires between the board components and those devices.  I know it's a little late in the game for that but for future reference.

[Apexelectric]

I have a feeling I will be redoing the chassis layout and redoing the bias board before all is said and done. I have another blank chassis coming. With that being said I'll try to lay it out so that the leads are a bit shorter. I did twist the wires to each of the respective pots after this picture with no real improvement.  I did however have better luck with the shielded wire to and from V1 above board for noise rejection. It was originally under the board. Would shielded wire be an option for the reverb pot wires? It's much smaller gauge wire so I question whether is adequate for this application.

I also changed out the heater wiring to shielded 20ga control wire. One run for the 6L6s and one for the Pre's. No real improvement there.

When I went through it with the chopstick after relocating the shielded wires, nothing I did improved the Hum.

Thanks for all the advice!

[ac427v]

The preamp tubes are very close together. So the white signal wire connecting the reverb tank return jack to the recovery stage valve is close to several very high voltage sources of hum. Is there any way to move the white lead farther away from all the blue wires without putting is closer to the heater wiring?
Craig

[Apexelectric]

I think the tube sockets might be too close to the reverb jacks. I lenghthened the white wire going from the reverb return to the reverb recovery grid to give me some routing options and that seemed to help. Thanks! I'm guessing that if there was a little more distance between the tube sockets and the back panel of the chassis it would further improve the interference issue on the reverb stage.

Getting there!

Tone is pretty good but a little loose on the low end. Need to tighten up the lower frequencies. First five or six notes on the low E string seems to be the only ones farting out. Would also like to coax a bit more volume from it.