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

[prongster]

Use your Ohmmeter to find out where ground connection is interrupted.
Both amp jack grounds should have continuity to amp chassis and cable shields.
At least one of the tank jack grounds should have continuity to the tank shell.
AT least one of the cable shields should be connected at both ends.

Continuity here means a resistance reading below 1R.

All of these are true with less than 1R, disregard my screwy continuity results.

[prongster]

Your tank/pan seems to be wired correctly. Only the output jack - (ground) should be connected to the reverb chassis. Continuity other than zero could be transducer resistance as Helmholtz noted.

Here's a diagram of the MOD reverb jack board. You should have no solder on the split pads for the input jack and a soldered connection on those pads on the output jack.

Yes, this is how my tank looks, which is consistent with my finding that swapping in other tank produced the same hum, and this tank functioned normally in the other amp. Seems to me it's safe to say that the tank is not the problem.

What next?

[Helmholtz]

It could still be magnetic hum picked up by the output transducer from a nearby hum source.

To test, short the reverb output transducer leads inside the shell.

[vintagekiki]

It could still be magnetic hum picked up by the output transducer from a nearby hum source.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[prongster]

It could still be magnetic hum picked up by the output transducer from a nearby hum source.

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?

[vintagekiki]

Output coil (~ 600 ohm) from reverb tank is located below the main transformer and collects hum from PT (TR1).
Rotate reverb tank 180 degrees so that input coil (~ 10 ohm) from reverb tank is below the PT (TR1)

Assumption that cables from reverb tank to the AB763 DR are shielded.

Rotate reverb tank for 180 degrees into speaker cab

Is there some problem for upload real schematic and wiring layout of your AB763 DR?

[prongster]

Rotate reverb tank for 180 degrees into speaker cab

yes, I did that and no real change.

[vintagekiki]

We may be strenuous in our questions, but all participants in topic try to help you.
I'm interested what happens if disconnect your reverb tank together with connecting cables from your AB763 DR and in set connect to another amp.

[Helmholtz]

Does it make a difference to the hum level if you touch/grab the tank shell?

[prongster]

Does it make a difference to the hum level if you touch/grab the tank shell?

Nope. Makes no difference.

[prongster]

We may be strenuous in our questions, but all participants in topic try to help you.
I'm interested what happens if disconnect your reverb tank together with connecting cables from your AB763 DR and in set connect to another amp.

If you see my earlier posts, I've connected this tank to another amp and it worked with no hum there. Likewise, I connected the other reverb tank to this amp and it exhibited the same hum.

Everyone's help is certainly very much appreciated! Doing my best to answer all the questions.

[g1]

Did you try the shorting of the output transducer from post #48 ?

[vintagekiki]

Is there some problem for upload real schematic and wiring layout of your AB763 DR?

Please you for your and our good.

[prongster]

Did you try the shorting of the output transducer from post #48 ?

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

[vintagekiki]

Is cable for reverb tank, cinch cable or audio (shield) cinch cable?

[prongster]

I seem to have maxed out my upload bandwidth - the forum won't let me upload anything. And I don't have a schematic for this amp. It's an AB763 with these mods tacked on:

-robrob normal channel reverb mod
-robrob lead channel mod
-3 way NFB mod
-slow tremolo mod

https://robrobinette.com/AB763_Modif...nel_Reverb_Mod

I pretty much followed his layouts, except I added an extra eyelet to accommodate the added 220k resistor before it joins the vibrato channel going into the reverb. There's a jumper under the board that takes the signal from the 220k resistor to where it joins the vibrato channel's signal right before the 500pF cap.

I hope this helps.

[vintagekiki]

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