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

[Chuck H]

I had some trouble with my most recent build regarding hum in the reverb. In the past I've always been able to find a quiet spot for the tank. Even if it only meant scootching it an inch to the right or left. Flipping it 180 degrees AND scoothing it. Or a combination of these options. I've also had enough trouble with grounding on reverb circuits to have worked out a pretty good understanding of problems that come up in that respect. But on this last build, with all things mounted in a typical manner, the only position for the tank that would stop it from picking up hum from the PT was to mount it bottom side flat on the rear baffle!?! I even experimented with pulling an old, broken tank apart and using the transducer as a humbucker coil. But only mounting the tank sideways up would stop the hum. So that's how I mounted it. I had to stabilize that baffle so it would vibrate less and add some insulation wrap on the reverb tank to otherwise stabilize it, but that's what worked so that's what I did. I still intend to order the right tank for that mounting position. Provided such an option is still available? But the amp is working fine and the only side effects is a greater likelihood of reverb spring "crashing" if the amp is moved while it's turned on. So that's my story. If the hum is indeed a PT related hum transfer then don't fail to explore ANY option for tank position. I don't know why it never came up like this before, I only know it absolutely did in this case. PT in this amp mounted normally. Tank flat to the bottom of the cabinet in any orientation or position = HUMMMMM.... Tank mounted side up on the baffle, nice and quiet. Probably some difference in who/how the tanks are being made? Because of the mediocre tone and difficulty in stabilization I've been having trouble with implementing reverb circuits since Accutronics sold to east Asia. JM2C.

EDIT: I failed to take into account that you mentioned removing the tank from the amp didn't help. In this case I'll agree with VK that it's likely a ground loop that needs to be hunted down. I'm leaving this post though because it may help someone researching reverb hum in the future reading this thread.

[vintagekiki]

Can you explain please how you arrived at your conclusions about the hum?

It can. Read your post #1 well, starting with "I have some significant hum in the reverb circuit", "the hum gets unacceptably louder with the reverb", etc.
Personally, I think in your case problem is with ground loop .
Compare grounding on pots and jacks of any AB763 with grounding your AB763. You have continuous wire on all pots and jaks of both channels.
vs
useful link
http://www.timeelect.com/jbl-twin.htm
http://www.voodooguitar.net/
https://www.vintage-guitar.de/detailsitem_2526_FENDER-Super-Reverb-Amp.html

[prongster]

You have continuous wire on all pots and jaks of both channels.

I don't dispute your suggestion that it is a ground loop - that's what I've suspected it is all along. The bus wire does run along the backs of all the pots, but it only connects to one jack on the vibrato channel - it doesn't connect to the jacks on the normal channel.

[vintagekiki]

The bus wire does run along the backs of all the pots.

Except jacks on normal channel, whole normal channel (tube V1, pots) is connected to vibrato channel. Compare grounding on Fender AB763 with diy AB763.

https://www.tdpri.com/threads/ab763-dr-build.1001105/
AB763 DR Build


1)
At Marshall amps bus wire does run along backs of all pots, but all jacks are insulated.

useful link
https://music-electronics-forum.com/forum/amplification/guitar-amps/theory-design/48388-is-there-any-benefit-in-a-grounding-bus-across-the-back-of-potentiometers
Is there any benefit in a grounding bus across the back of potentiometers?

[prongster]

I get your point - it's not like a ground bus on the back of the pots is not unheard of tho. The reason I did it on this amp is because I did it on a Princeton Reverb I built, and that amp is very quiet.

Hopefully I'll get a chance to redo the ground bus tomorrow with a bus wire, and that will fix it. (Must say after everything that I've tried, I'll be surprised if it does...)

[vintagekiki]

I did it on this amp is because I did it on a Princeton Reverb I built, and that amp is very quiet.

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)

[Greg_L]

I get your point - it's not like a ground bus on the back of the pots is not unheard of tho. The reason I did it on this amp is because I did it on a Princeton Reverb I built, and that amp is very quiet.

Hopefully I'll get a chance to redo the ground bus tomorrow with a bus wire, and that will fix it. (Must say after everything that I've tried, I'll be surprised if it does...)

I've built an AB763 that's dead quiet. I used a buss bar floating behind the pots and ran my grounds in stages with the part of the circuit that needs grounding. Think of it like a river that flows to one point far away from the power supply grounds. Now that amp had it's other little issues at first, but grounding hum was not one of them. I think you've got ground loops happening. First off, ditch all those mods and get the amp working stock as Leo intended. Rethink and clean up your circuit grounds. Get it quiet and functioning normally, and then put your mods back in one at a time.

[vintagekiki]

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

Redoing the ground bus with an insulated wire instead of bare won't change a thing. The ground bus should simply be removed and grounds for potentiometers should be run individually to wherever the filter for that circuit is grounded. Yes, Leo ran them straight to the chassis, but even that's not ideal and I never met a vintage Fender reverb amp that didn't hum at least a little too much. I never understood the point of the ground bus system anyway. The pots are IN the chassis so why ground the pot case??? It's not like there isn't a bunch of unshielded lead wire running from the pots anyway. Makes no sense and begs for ground loops.

[Helmholtz]

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?
.

Firstly, as the hum stops when you disconnect the reverb pan, I don't think your hum is caused by a ground loop inside the chassis.

There is the possibility of an external ground loop via the reverb cable shields, but that would be interrupted if only one of the cables is connected.

As it doesn't help to unplug the reverb input cable from the amp and the hum doesn't stop with V3 pulled, the reverb driver and reverb transformer are not involved.

It seems the hum is coming from the tank.

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

[vintagekiki]

Measure resistance between tank shell and chassis ... ...
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[Chuck H]

.., as the hum stops when you disconnect the reverb pan, I don't think your hum is caused by a ground loop inside the chassis.

Isn't it still possible that a redundant ground causing the loop IS inside the chassis? Though it's true enough that most ground loop problems with reverbs can be isolated to the pan itself.

[vintagekiki]

About Reverb tank

https://el34world.com/Forum/index.php?topic=23438.0
Reverb tank grounding

https://bmamps.com/v01/wp-content/uploads/2017/10/How-to-Reverb-Tanks.pdf
Replacing a Reverb Tank

https://www.thegearpage.net/board/index.php?threads/reverb-tank-grounding-question.2102482/
Reverb tank grounding question

https://www.diyaudio.com/forums/instruments-and-amps/308687-reverb-drivers.html
spring_reverb_tanks_explained_and_compared_1.pdf

Connector Grounding/Insulating
In order to suit any grounding scheme, reverb tank connectors come in all combinations of input and output insulated and non-insulated phono jacks (a.k.a. RCA jack). A non-insulated phono jack is one whose outer shell is grounded to the outer steel channel (chassis) of the reverb tank. Vintage Accutronics® specs recommend insulating both input and output connectors and grounding the tank chassis separately.
If the amplifier’s connection to the reverb tank’s phono jack shell is not at ground potential, it is important that the tank be chosen with an insulated connector at that connection point. If the amp’s connection to the reverb tank’s phono jack shell is at ground potential, either insulated or grounded connector may be used at that connection point.

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1)
https://dokumen.tips/documents/reverb-tank.html

[Helmholtz]

Isn't it still possible that a redundant ground causing the loop IS inside the chassis?

I don't see how.

The amp doesn't hum without the tank connected. So there are no signs of an internal ground loop problem.

Connecting only the reverb output can't close a ground loop, though it does produce the hum.

Leaves either hum induced in the reverb output transducer (could be from an external hum source) or a shielding problem.

[Chuck H]

I don't see how.

The amp doesn't hum without the tank connected. So there are no signs of an internal ground loop problem.

Connecting only the reverb output can't close a ground loop, though it does produce the hum.

Leaves either hum induced in the reverb output transducer (could be from an external hum source) or a shielding problem.

I was thinking about the grid bias/load/0V resistor at the recovery triode. But looking at the photos prongster has it grounded correctly right on the pedal jack. Not much room for a loop there.

But by the same logic, why should having the pan and connecting cable shields grounded at both ends cause a loop when the input, output and pedal jacks are is such close proximity? It's typically necessary to isolate the cable shields and pan ground to one jack or hum is probable. Maybe I'm not entirely getting it.?.

[Helmholtz]

It's typically necessary to isolate the cable shields and pan ground to one jack or hum is probable. Maybe I'm not entirely getting it.?.

Yes, that's the preferred arrangement as otherwise you get a large area induction loop acting as a magnetic antenna. Even a small (say 0.1R) resistance between the ground contacts of the reverb send and return jacks at the amp would allow for some hum voltage to develop.
Induction loops are low impedance voltage sources that can drive considerable currents.

I think the best wiring is shield connected at both ends with reverb input cable (as some power needs to be delivered to the low impedance input transducer)
and output cable shield connected only at amp side.
That would be with a tank that has both input and output jack grounds connected to the shell.

Or use a tank which has only the output jack ground connected to the shell with both cable shields connected at both ends.

A ground loop requires a closed conductor loop to allow for ground currents to circulate.

[Chuck H]

FWIW the MOD reverb pans have easy modify ground connection boards on the jacks. Really handy.