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

[prongster]

If I understood correctly the attached images in post #1 is not images your AB763 DR, images was downloaded from somewhere?

No that is not correct. The images from post #1 are 100% my AB763 DR, the amp we're talking about. I'm not sure how you got the impression otherwise.

[vintagekiki]

I'm not sure how you got the impression otherwise.

By to pictures and your description, your amp is perfectly, but doesn't work as it should.
You mention cleaning grounding, which is illogical for new amp.
Since everything was processed through 60 posts, only thing was fault somewhere in wiring, or some component or tubes was changed value.

[loudthud]

My money is on B+ hum on the drive side. Anything strange with the choke or the filter cap for the screen node and it's ground ? If you hook a small speaker to the output of the reverb drive transformer, can you hear the hum ?

[vintagekiki]

If you hook a small speaker to the output of the reverb drive transformer, can you hear the hum ?

Well done. That didn't occur to me.
Maybe should set all pots to min, disconnect reverb tank, and with audio (shield) cinch-cinch cable connect both reverb jacks on amp chassis.
Carefully amplify reverb pot and monitor what is happening, is there any noise, etc.

[Helmholtz]

My money is on B+ hum on the drive side. Anything strange with the choke or the filter cap for the screen node and it's ground ? If you hook a small speaker to the output of the reverb drive transformer, can you hear the hum ?

As the hum stops with reverb input disconnected it can't be the drive side.

[Helmholtz]

I've moved the tank all over the place, including 6 feet away from the chassis and cabinet. The hum loses a little low frequency content when I do that, but it's still there. I've also used the amp in another physical location with the same result. Does that rule out magnetic hum as a source?

Could still be a remote but strong magnetic hum source. Do you also have strong guitar PU hum?

[Helmholtz]

No, I had missed this, but I did it just now, and it seems to stop most of the hum. What does that mean?

Means that there is a hum signal across the output transducer terminals which seems to confirm induced magnetic hum.

Do you have a footswitch connected?
If yes, remove it and see if it makes a difference.

[vintagekiki]

First off, ditch all those mods and get the amp working stock as Leo intended.

It may be best remove all mods and reset the AB763 DR to factory settings.
When the amp is brought into mint condition, then can be include desired modifications step by step.

[prongster]

Means that there is a hum signal across the output transducer terminals which seems to confirm induced magnetic hum.

Do you have a footswitch connected?
If yes, remove it and see if it makes a difference.

The foot switch has been disconnected all this time.

The hum follows the amp around, so it's been in 3 different rooms in my house, and the different part of town where my band practices, and hummed everywhere it goes. Could there be a component in the amp generating the crazy mechanical hum? The choke? (I read another thread where the choke was the culprit). Remember tho, I moved the reverb tank 6 ft away from the amp and it still hummed.

And I should mention that when I turn the amp on, an audible 60Hz hum comes from it before it warms up enough to put out any sound. I can't tell if that's coming from the PT or another component tho.

Feels like we're getting closer at least!

[Helmholtz]

Could there be a component in the amp generating the crazy mechanical hum?

How mechanical hum?

And I thought the hum stops when you disconnect the tank?

[vintagekiki]

Could there be a component in the amp generating the crazy mechanical hum?

Exist . PT if not sufficiently tighten, or if is not correctly positioned in relation to the OT or RT.

[prongster]

Could still be a remote but strong magnetic hum source. Do you also have strong guitar PU hum?

There is a very strong pickup hum, the gets quite noticeably louder as I move my single coil guitar nearer to the chassis of the amp.

And I repeated some tests to make sure I'm reporting accurately:

-First of all the noise coming from the amp as I turn the reverb up seems to consist of a low hum, and a higher freq buzz.

-If I unplug both reverb cables from the amp and turn the reverb all the way up I hear the low freq hum, but not the buzz.

-At idle, all knobs down there is a little bit of hum coming from the amp, a little more than I would hope, but not unexpected from a tube amp. It sounds related to the hum that comes off the amp when I first turn it on.

-With the reverb all the way down, if I turn the vibrato channel up to 10 there's a little hum, but again not unexpected.

-With the reverb tank connected normally, a combination hum/buzz becomes pretty audible about 3-4, and quite loud when turned up to 10.

-Unplugging both reverb cables, the hum is still present when turning the reverb knob up, but the buzz component is gone.

-With the reverb out cable plugged back into the amp, the buzz has rejoined the hum.

-With only reverb in plugged in, it's the same as no reverb cables plugged in.

-With reverb in cable unplugged from the tank, both hum and buzz are present.

-With reverb out cable unplugged from the tank, it hums loudly, but in that "cable plugged into my amp w no guitar connected" way.


Apologies if some of this is different than my earlier reporting, just trying to make sure we're working with accurate info here.

[vintagekiki]

And I repeated some tests to make sure I'm reporting accurately ... ...

Judging by the detailed description your post #72 your diy AB763 behaves absolutely normally.
I don't understand what you expect from your AB763. Don't forget it's a guitar amp, not a HiFi amp.
At the end borrow from some colleague his Fender AB763 amp and under the same conditions (one on one) compare with your diy AB763 amp.
Inform us about the result.

[prongster]

I recorded some sounds of the amp noises that will hopefully clarify what I'm trying to say. I can't upload them here, so hopefully you can listen to them at this link:

https://app.box.com/s/qopps6wuvcdvi9wd292rphur2jxph5nj

[prongster]

Judging by the detailed description your post #72 your diy AB763 behaves absolutely normally.
I don't understand what you expect from your AB763. Don't forget it's a guitar amp, not a HiFi amp.
At the end borrow from some colleague his Fender AB763 amp and under the same conditions (one on one) compare with your diy AB763 amp.
Inform us about the result.

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.