Hum from Reverb Recovery AB763 Twin Reverb (Resolved!)

[MoparWade]

Trying to track down a hum from what I believe must be my reverb recovery circuit. Pulling valves 2 and 3 has no affect but once 4 is pulled the hum goes away. I have no reverb tank or foot switch currently hooked up. The following are the numbers I measured at the pins for valve 4 with valves 1-3 out;

1: 281 VDC
2: 0 (But this is very sensitive to anything near the wire running to reverb return to tank output, even a finger approaching)
3: 2.17 VDC
4: 3.26 VAC
5: 3.26 VAC
6: 274 VDC 
7: 0
8: 2.25 VDC
9: 3.27 VAC

I have tried swapping tube around with no change. Sounds like a heater hum but the heater wiring is very clean so I doubt that is the cause. Any ideas are greatly appreciated. Also checked the B+ to make sure it was clean for AC by the "B" node and it is.

[shooter]

this is very sensitive to anything near the wire

Is it a shielded wire?

[sluckey]

Is this a stock Fender TR? Does the hum disappear if you set the reverb pot to zero? Does the hum disappear if you ground the return jack?

[MoparWade]

Sorry I should have been more clear. No it's not a stock Fender. It's a Hoffman AB763 build. Grounding the jack has no affect. Pot set to zero or max has no affect either other than I can hear a bit of a hiss at max which I'm sure is to be expected. It's the type of hiss you hear when the an amp is free of hum.

[sluckey]

Pot set to zero or max has no affect

That means the hum is coming from V4-B. Put a ground clip lead on V4 pin 7. Does that kill the hum?

[MoparWade]

Indeed grounding the grid of V4-B eliminates the hum but why I have yet to determine. Disconnecting the B+ from the plate of V2 made no difference, disconnecting the B+ from V2-B made no difference either. Thought maybe there was some leakage but that would have meant the B+ was not very clean DC anyway. Could it be internal leakage from the cathode heater? Maybe I'll try tacking on some more capacitance.

[sluckey]

I would concentrate on V4-B grid circuit. If you connect a 220K from grid to ground, does the hum go away?

Is hum the only sound the amp makes? Does it sound good except for the hum?

[MoparWade]

Connecting a 220k with jumpers seems to make it worse. The amp sounds great otherwise. Normal channels are perfectly clean. Reverb channels have great sound but the hum is quite bad, not as bad as a direct short to the heater circuit but more than just a quiet hum.

[MoparWade]

Additional note: Grounding at the location circled in red reduces the hum but does not eliminate.

[sluckey]

Follow the wire on V4-7 back to the board. It connects to a 470K and a 220K. Ground the bottom side of that 220K. Any joy?

[MoparWade]

By bottom side do you mean the side that connects to ground? If so no change when connected to a confirmed ground. If I connect to the side that connects to the grid then yes the hum goes away.

[MoparWade]

Update: I disconnected the 10p cap and 3.3M resistor from the grid side. The hum was still there. With that still disconnected I removed the wiring to the pin for V4-7 and no surprise it fell quiet. I thought maybe the jumper running under the board from the 220k to the 10p/3.3M combo might have shorted to something but I cannot find continuity to anything other than itself. It would seem that the issue must be somewhere with the 220k and 470k resistors, which doesn't really make sense to me.

[MoparWade]

Still no luck. Only way to shut it up is ground the grid or disconnect the wire from V4-7. The valve is incredibly microphonic, any valve I stick in there, but fine in V1. That makes no sense to me or I'm frustrated and missing something obvious.

[PRR]

> valve is incredibly microphonic, any valve I stick in

Not the valve. Tap smaller, or just replace all the stuff on that socket, possibly down to the socket.

The motor in your Mopar rattles. Different engines, same rattle. So not the engine. Did you use the same air cleaner? Motor-mounts? Hood ornament?

[MoparWade]

Replaced socket and all components, no change. Did find that grounding the wiper of the reverb pot shuts it up too. Perhaps I'll try putting a shielded cable in from wiper to V4-7.

[MoparWade]

RG174 was a bust too but that's definitely where the hum is coming from. I have my filter caps grounded to the buss bar that runs across the back of my pots. Could this be my issue?

[HotBluePlates]

Is the 220kΩ at the Reverb Return jack actually mounted/grounded at the jack as drawn?

What happens if you lift its grounded end off the chassis, and instead run a wire to the grounded end of V4 pin 8's cathode resistor?

[MoparWade]

Yes, it is connected as drawn. Lifting the grounded end and connecting to V4-8 on the grounded side of the cathode resistors makes no change. The only two ways I've found to silence it is grounding the reverb pot wiper or removing the wire from the wiper to the 470k resistor. Further, disconnecting the wire between the .0033 cap and the reverb pot does nothing either which leaves me with grounding only. The grounding must be flawed but it isn't affecting the normal channel so it has to be somewhat isolated.

[ac427v]

I have my filter caps grounded to the buss bar that runs across the back of my pots. Could this be my issue?

Maybe...Which filter caps are grounded to the buss? The first two filter sections must be grounded far from the buss chassis connection--usually with the center tap of the power transformer.
--Craig

[2deaf]

So you can set the pot. to zero and still get hum, but grounding the wiper directly stops the hum?  Sort of makes it seem like the pot. is involved somehow either directly or by the way it is hooked up, doesn't it?

[MoparWade]

Which filter caps are grounded to the buss?

All of them are.

[sluckey]

Ground the first and second cap (plate and screen) to a PT bolt. Also connect the PT center tap to this same bolt. Any better?
 

[MoparWade]

Ground the first and second cap (plate and screen) to a PT bolt. Also connect the PT center tap to this same bolt. Any better?

That did it! No more hum! Huge thanks to everyone for their help. I learned a ton diagnosing this so hopefully the next build will go smoother. Pretty excited and I couldn't have done it without all of you. I'll post some pics once I get the cabinet built. Thanks again!!

[Niksevig119]

Ground the first and second cap (plate and screen) to a PT bolt. Also connect the PT center tap to this same bolt. Any better?

Wow. This is exactly what is going on with an AB763 kit in my amp   Im a novice to amp repair, but have already grounded the PT center tap to a stud with a soldered eyelet. Can you point me in the direction of what caps to attatch to the stud there?  Thanks for any help!

[shooter]

Can you point me in the direction of what caps

see reply #21

you want to keep the big power, less filtered stuff away from the sensitive stuff, preamp, inputs etc.

[Niksevig119]

Can you point me in the direction of what caps

see reply #21

you want to keep the big power, less filtered stuff away from the sensitive stuff, preamp, inputs etc.

Oh ok. Is this because the op had the filters tied into the ground bar?

[shooter]

Yup, PA power typically has higher ripple (AC), not such a problem because signal levels are way above the ripple, but preamp signals can be less than ripple volts on the PA filters.