Hum in new AB763 DR build Reverb circuit - increases as reverb pot is turned up

[Helmholtz]

The reverb buzz doesn't sound normal indeed.

You may try the following:

- add a 10k grid stopper at the VA grid pin,
- wire a 2nF cap across the 220k grid leak resistor of V4A
- close/cover the open side of your tank with a grounded(!) metal foil or even better with a grounded steel plate.

I also suspect that the reverb return cable is not perfectly shielded (poor shield coverage).

If all that doesn't help and you're not able to post an actual schematic, I suggest to revert the amp to original Fender specs.
There's hardly anything to "improve" with a DR anyway.

[prongster]

The reverb buzz doesn't sound normal indeed.

You may try the following:

- add a 10k grid stopper at the VA grid pin,
- wire a 2nF cap across the 220k grid leak resistor of V4A
- close the open side of your tank with a grounded(!) metal foil or even better with a grounded steel plate.

I also suspect that the reverb return cable is not perfectly shielded (poor shield coverage).

If all that doesn't help and you're not able to post an actual schematic, I suggest to revert the amp to original Fender specs.
There's hardly anything to "improve" with a DR anyway.

How do I make sure the steel plate is grounded please?

And is there software or recommended way for producing and editing schematics?

Your input is much appreciated!

[vintagekiki]

I recorded some sounds of the amp noises ...

One mp4 is better than 100 words.
Now we have an idea of how the malfunction manifests itself.
The buzz (not hum) has nothing with a reverb tank. The buzz is constant and reminds, heater is no grounded, or some of tube have poor insulation between cathode and heater.

In troubleshooting, always start from output to input.
1. heater voltage
Check the AC voltage on both sides pilot light (blk & wht wires) according to chassis.
If it is 2 x 3.15 VAC it is ok and it means that PT has a middle terminal for heating.
If no AC voltage from both sides pilot light according to chassis, the heating is floating and this can be one of the causes of buzz.
In this case, from both sides pilot light according to chassis solder 2 x 100 ohm / 0.5W resistors
2. check tubes
Remove all preamp tubes so that only PI (12AT7), 2 x 6L6 and 5U4 rectifier remain in AB763.
Is there buzz?

1) Useful links

https://www.psaudio.com/ps-how/how-to-find-and-fix-hum/
How to find and fix hum in 3 easy steps

https://www.techhive.com/article/3063590/how-to-get-rid-of-hum-and-eliminate-other-noises-from-your-audio-and-video-systems.html
How to get rid of hum and eliminate other noises from your audio and video systems

https://www.sweetwater.com/sweetcare/articles/tube-amp-noise-and-sound-solutions/
Resolving Tube Amp Hum, Buzz, or Hiss

https://www.calltoptech.com/hearing-an-electrical-hum-or-buzz/
Hearing an Electrical Hum or Buzz? Here’s What to Do

[vintagekiki]

And is there software or recommended way for producing and editing schematics?

see post #24

[prongster]

The reverb buzz doesn't sound normal indeed.

You may try the following:

- add a 10k grid stopper at the VA grid pin,
- wire a 2nF cap across the 220k grid leak resistor of V4A
- close/cover the open side of your tank with a grounded(!) metal foil or even better with a grounded steel plate.

I also suspect that the reverb return cable is not perfectly shielded (poor shield coverage).

If all that doesn't help and you're not able to post an actual schematic, I suggest to revert the amp to original Fender specs.
There's hardly anything to "improve" with a DR anyway.

The 10k grid stopper didn't work unfortunately, and there's already a 2nF cap across the 220k grid leak on V4A. I'll need to track down some sheet metal or good foil before I can try additional shielding. I read somewhere mu metal might be good for shielding - anybody have any experience with that?

And I could definitely use some tips on how to ground the shielding properly. After that I guess I'll make a proper schematic, and if I can't manage that I'll convert it to a stock AB763.


Any thoughts on splitting the cathodes on V4 and grounding V4A with its 220k grid leak? I read a post where someone said that's cured a lot of buzzy reverbs for him.


There was a moment when I first hooked up the 10k grid stopper that I turned the amp on and there was no buzz. Then I realized there wasn't any reverb either and my hopes were dashed...

[loudthud]

Just wondering. The schematics I'm looking at all seem to have a 2.2K ( red, red, red) resistor as the cathode resistor for the 12AT7 reverb drive tube. In the pictures posted in the first post of this thread, it looks like that resistor is 110 Ohms, ( brown, brown, brown). Sometimes between the lighting, the digital camera and the LCD screen, colors can change and you can't read the color codes on resistors correctly. What's in there ?

[vintagekiki]

https://robrobinette.com/Build_an_AB763_Amp.htm
Build an AB763 Blackface Amplifier

[Greg_L]

I'm afraid I don't know anyone with an AB763 DR, but I have a hard time believing they all exhibit the noise coming out of the reverb that I linked above.

They don't. They wouldn't be as revered as they are if they hummed and buzzed uncontrollably.

I'm no expert but there are a few things you need to understand...1) Amps shouldn't "hum" or "buzz". That is a problem somewhere. Maybe you have DC getting somewhere it shouldn't or have poor grounding. 2) Amps will hiss when turned way up. White noise can be normal when an amp is turned up and idling.

Is this a combo? Could the speaker magnet be interfering with the reverb tank? Also again, I respectfully suggest you remove the mods and put the amp as stock. There's nothing wrong with mods, but you've created variables that can affect other things. Get it working as stock and then re-introduce your mods one at a time.

[vintagekiki]

^^^^^^^^^^^^^^^^
Voice of reason

" I respectfully suggest you remove the mods and put the amp as stock. "
" Get it working as stock and then re-introduce your mods one at a time. "

[Helmholtz]

How do I make sure the steel plate is grounded

You already verified that the shell is grounded, so just make sure the plate/foil is in electrical contact with the shell (Ohmmeter).

[Chuck H]

How do I make sure the steel plate is grounded please?

On the output side jack board, where the top tabs are already soldered, you can use one of those tabs as a connection point for a bottom plate and that will give you continuity with the reverb pan case.

If the metal plate you choose isn't solder-able you can make a mechanical connection at the plate end with a screw.

[vintagekiki]

Reduce experience and follow a little of what is happening on the topic.
prongster AB763 DR buzzing with and without reverb tank.
1)
post #74

https://app.box.com/s/qopps6wuvcdvi9wd292rphur2jxph5nj
DR at idle, all gain knobs down
DR no tank, rev on

[prongster]

Just wondering. The schematics I'm looking at all seem to have a 2.2K ( red, red, red) resistor as the cathode resistor for the 12AT7 reverb drive tube. In the pictures posted in the first post of this thread, it looks like that resistor is 110 Ohms, ( brown, brown, brown). Sometimes between the lighting, the digital camera and the LCD screen, colors can change and you can't read the color codes on resistors correctly. What's in there ?

It's red, red wine, I mean red. So red, red, red. Thanks for checking.

[Greg_L]

It's red, red wine, I mean red. So red, red, red. Thanks for checking.

Lol I see what you did there.

[prongster]

They don't. They wouldn't be as revered as they are if they hummed and buzzed uncontrollably.

I'm no expert but there are a few things you need to understand...1) Amps shouldn't "hum" or "buzz". That is a problem somewhere. Maybe you have DC getting somewhere it shouldn't or have poor grounding. 2) Amps will hiss when turned way up. White noise can be normal when an amp is turned up and idling.

Is this a combo? Could the speaker magnet be interfering with the reverb tank? Also again, I respectfully suggest you remove the mods and put the amp as stock. There's nothing wrong with mods, but you've created variables that can affect other things. Get it working as stock and then re-introduce your mods one at a time.

It is a combo, but it's still buzzing even when I remove the tank and place it 6ft (length of cord) away from both the chassis and combo cab (with speaker).

It's never been stock, but I can replace the mod parts with the stock parts and layout if that will make things easier. It might take awhile tho. I'm going to trying shielding the tank on all sides first.

[Helmholtz]

I might also help to use short but hopefully better shielded reverb cables. Especially a long, poorly shielded return cable will collect hum.