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

[prongster]

Princeton Reverb has only one (vibrato) channel.
Try. How audibly manifested AB763 DR when remove tube for normal channel (V1)

1)
If AB763 DR has still sound sensations, calm it first (fix it) without tube for normal channel (V1)
If diy Princeton Reverb is at hand, compare its wiring with wiring AB763 DR

2)
When wiring filter caps in cap box, pay attention to grounding caps 20u / 450V.
Filter caps in AB763 cap box have 2 independent ground points. Ground for power amp (70u) and ground for preamp and PI (20u)

Thanks for the replies everybody - I really appreciate it!

Pulling v1 has no effect on the hum - it's still there.

I believe my wiring on the filter caps to be correct - the first 2 caps are grounded to the power amp, the 3 on the left to the preamp ground bus.

[prongster]

Measure resistance between tank shell and chassis to make sure it's grounded.

I get nothing between the tank shell and the chassis. No continuity, no resistance.

As per my original post, I did connect the reverb tank from my Princeton to this amp (with the cables from the Princeton), and it hummed in the same manner. Likewise, I connected this tank to the Princeton (the the DR cables) and it performed w no hum in that amp.

[prongster]

So in hopes of improving my grounding scheme, I made a couple of changes last night:

-I put a ground bus bar across the RCA jack grounds, as per the Hoffman grounding scheme

-I cleaned up the wiring around my power amp ground on the PT bolt, and moved my power tube grounds from the chassis to the PT bolt

Hmm...tried to upload pix, but it says I've exceeded the limit.

Also, I hope to redo the preamp bus bar today - get it off the backs of the jacks, and have it connect to the chassis at only 1 point. Even if it doesn't fix the hum, it will be a good exercise in cleaning up the grounding.

[Helmholtz]

I get nothing between the tank shell and the chassis. No continuity, no resistance.

That might be the reason for your hum problem.
The tank shell must be grounded.

At least one of your reverb cables should provide continuity between amp chassis and tank shell.

[prongster]

That might be the reason for your hum problem.
The tank shell must be grounded.

At least one of your reverb cables should provide continuity between amp chassis and tank shell.

ok, how do I make that happen?

[Helmholtz]

ok, how do I make that happen?

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.

[g1]

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.

Classic Fender tank is 4AB3C1B. Output jack grounded to shell. So loss of tank shell ground seems to mean bad shield of output cable. Or output jack at amp chassis has been insulated.

[Helmholtz]

A simple test shows if an ungrounded shell is the problem: See if touching the shell increases the hum.

I never understood why reverb units are not shielded from all sides.

[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.

OK this is weird - not only do I not get continuity from the tank shell to the amp jack grounds, I don't get any continuity when I place both probes on the tank shell itself. The shell doesn't seem to be conductive?

I do get continuity thru to the little PCB on the inside of tank where the jack is. That doesn't seem to be connected to the tank shell tho.

This tank is a MOD 4AB3C1B btw.

[Helmholtz]

You may need to scratch through the finish to contact the metal.

[prongster]

You may need to scratch through the finish to contact the metal.

That did it - I have continuity all the way from the amp jack grounds to the tank shell. There's also continuity on both tank jack sleeves to the tank shell. Is only the output supposed to be grounded to the shell?

And in double checking that, I seem to have continuity between both the sleeve AND the tip to the shell...and continuity between the tip and sleeve themselves. That doesn't seem right??

[vintagekiki]

I cleaned up the wiring around my power amp ground on the PT bolt, and moved my power tube grounds from the chassis to the PT bolt
Even if it doesn't fix the hum, it will be a good exercise in cleaning up the grounding.

If believed attached files ( post #1 ) since amp is diy, design looks perfect, I don't understand why cleaning grounding, except ...
If you are deciding for cleaning grounding, clean contacts between all jacks and chassis.

It would be helpful upload real schematic and wiring layout of your AB763 DR.

[Helmholtz]

Continuity is vague.
Use your Ohmmeter and see if you get a close to zero Ohm reading. Cable shield testing is best done when disconnected from amp and tank.
Otherwise you might be seeing the low DCR of a transducer or the reverb transformer secondary.

[Chuck H]

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.

[vintagekiki]

https://reverb.com/uk/item/3016903-accutronics-4ab3c1b-reverb-tank-made-in-u-s-a
Accutronics 4AB3C1B Reverb Tank


Grounded output jack


Isolated input jack