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

[prongster]

Hello there - first post here.

I have some significant hum in the reverb circuit of my first AB763 DR build (this is my 3rd build in total), that I can't seem to track down and eliminate. At this point it's got me pretty much stumped and I can use some help. Here's what I know so far:

-the amp has a noticeable hum at rest, but within what I've heard in other amps. It's not what I would call quiet, but the amp is certainly useable as is. Sounds like 120Hz to me, but I suspect it's 60Hz in origin.

-the hum gets unacceptably louder with the reverb pot, ie it's tied to the pot. The tuner on my phone says this hum is 180Hz, and it's noticeably a different pitch than the baseline hum. (This is why I suspect the 60 Hz origin - it's clearly a harmonic of the base hum.)

-neither the vibrato or normal channel controls have any effect on the hum.

-disconnecting the reverb pan stops the hum

-turning off the reverb with the foot switch stops the hum (does this mean it's origin is before v4?)

-removing v3 does NOT stop the hum

-removing v4 does stop the hum

-I've tried known good tubes in both v3 & v4 with no change

-moving the reverb tank away from the chassis does not affect the hum

-I swapped in a known good tank and it still hummed. Connected this tank to another amp and it did not hum, so I think that rules out the tank as a possible source.

I'm just learning how to troubleshoot, so any help in isolating the problem would be appreciated. From what I've done so far it seems that the reverb transformer is now suspect, or a component tied to v3. I've also suspected a ground issue (because sounds like ground hum to me), but the continuity of all my ground connections checks out. At this point, I think I'm going to redo all the grounding and use a bus bar instead to soldering to the backs of the pots and hope that helps.

A little bit more info about the amp - it's got some mods:
-robrob normal channel reverb mod
-robrob lead channel mod
-3 way NFB mod
-slow tremolo mod
and I added the silverface extra bias filter cap and reverb filter cap hoping it would fix this, but no luck.

I've attached some pix:





Anything else?

Thanks!

[g1]

Still hums with footswitch disconnected?

[prongster]

Still hums with footswitch disconnected?

yes

[vintagekiki]

-disconnecting the reverb pan stops the hum

-turning off the reverb with the foot switch stops the hum (does this mean it's origin is before v4?)

-removing v4 does stop the hum

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.

[prongster]

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.

yes thanks - I’ve done this already, and it made a tiny difference but the hum remains. I tried other cables too with no luck.

[Helmholtz]

I assume your hum stops when you remove the reverb output cable.
But does it also stop with output cable connected but input cable removed?

[Enzo]

ANy chance it is acoustic? Pull the reverb pan out of the cab and place it as far from the amp as possible. ANy change?

[vintagekiki]

I’ve done this already, and it made a tiny difference but the hum remains.

I don't understand do you only swapped places with cables, or rotated reverb tank for 180 degrees into speaker cab.
Check inside reverb tank to see, whether is reverb tank chassis connection to any reverb tank jacks (gnd point).

[prongster]

ANy chance it is acoustic? Pull the reverb pan out of the cab and place it as far from the amp as possible. ANy change?

Yes I’ve done this and no luck. The reverb was prone to feeding back tho, which I seem to have mitigated by bending the end flanges of the tank out some. I don’t think it’s related to the Hum tho, as that remained unchanged.

At this point I can’t help but suspect a faulty component if anyone has any ideas? My inexperience makes a wiring error or ground loop more likely tho.

[prongster]

I don't understand do you only swapped places with cables, or rotated reverb tank for 180 degrees into speaker cab.
Check inside reverb tank to see, whether is reverb tank chassis connection to any reverb tank jacks (gnd point).

I rotated the tank 180 degrees AND swapped cables, with no real improvement.

can you explain further please about looking in the tank? I’ve checked the connections there and they look ok. Have have not put a meter on them tho. I did try a known good tank and got the same hum, so I think that rules out the tank itself as the cause.

[vintagekiki]

Question
Does reverb tank have hum or acoustic feedback?
For hum it is explained, for elimination acoustic feedback required reverb tank mount to elastic surface, similar as to Fender.
To eliminate speculation, temporarily connect AB763 DR reverb tank to another amp, or another reverb tank temporarily connect to the AB763 DR
1)
https://www.amplifiedparts.com/tech-articles/spring-reverb-tanks-explained-and-compared

[prongster]

I assume your hum stops when you remove the reverb output cable.
But does it also stop with output cable connected but input cable removed?

Yes it stops with the output cable removed. (I can still hear the baseline hum when I turn the reverb pot all the way up tho, meaning it's more noticeable than with the reverb pot at 0, but no where near as loud as the hum with the tank connected.)

With the input cable removed, the hum is somewhere in the middle. ie, quieter than with the tank completely attached, but not as quiet as with the tank disconnected. Does this suggest anything useful? [edited to add that this hum sounds kinda like when a cable is connected, but not plugged into a guitar, so I suspect that's why I hear some hum with the input unplugged.]

[Helmholtz]

With the input cable removed, the hum is somewhere in the middle. ie, quieter than with the tank completely attached, but not as quiet as with the tank disconnected. Does this suggest anything useful? [edited to add that this hum sounds kinda like when a cable is connected, but not plugged into a guitar, so I suspect that's why I hear some hum with the input unplugged.]

I'd say it shows that neither the reverb drive circuit (including the reverb transformer) nor an external ground loop are the (main) sources of your hum.

What happens if you connect the reverb output cable to the amp but not to the tank?

[vintagekiki]

Question
If volume pots on both channels on min, reverb tank connected, how audibly manifested when you rotate reverb pot from min to max.

[prongster]

I'd say it shows that neither the reverb drive circuit (including the reverb transformer) nor an external ground loop are the (main) sources of your hum.

What happens if you connect the reverb output cable to the amp but not to the tank?

Reverb cable connected to amp but not to tank hums, but in that unplugged cord way and sounds different than the other hum.

Listening again, the reverb cable input unplugged from amp hums about the same as the tank attached normally.

Can you explain please how you arrived at your conclusions about the hum (for my education)? Any suggestions for a next course of action to track it down and fix it?

I was thinking about redoing my heater wiring to see if that helps.