Reverb Hum on Super Reverb

[PlateVoltage]

I have a Super Reverb clone AB763 that was built by a reputable amp builder here in the U. S. Their work is phenomenal and so is the amp. I do like to play surf music and often have the reverb of my amps set to 10. With this amp once I get past 5 it starts humming and increases as I turn it up to 10.  I’ve searched the forum and the internet over for possible fixes to the issue but none have worked yet.

This is what I’ve tried:

Swapping V3 and V4 with different new tubes.

Swapping out the new reverb pan with another known working pan.

Swapped out the rca cables.

Wrapped the reverb pan with about an inch of foil and put it back in the bag to attempt shielding it.

Rotated the reverb pan, tried mounting it vertically and upside down.

Tightened jacks on the face plate of the amp.

Sprayed Deoxit on rca connections.

Covering the reverb pan and bag in the cab with blankets.

Adjusted the tension of the screws holding the bag and pan to the cab.

None of this has worked at all. I am somewhat familiar with the inside of the amp and tube amps in general. Especially the high voltages and how to drain it. I can also solder well.

If anyone could help me fix this I would greatly appreciate it.

Thanks

 

[tubeswell]

Is the source of the ‘hum’ microphonic? (When it starts making the noise, put your finger on the back of the reverb pan - does that stop the noise?)

[sluckey]

Do you have a schematic and layout for your clone? Can you provide some hi rez pics? How long ago was the amp built? Did it always have this hum? Have you discussed it with the builder?

[Leevi]

This issue is very often related to grounding on the reverb recovery and even the reverb foot switch grounding.

In many cases a good way to prevent the reverb hum is to use an isolated RCA connector(s)and ground everything i,e, 220K resistor and ground lug of the RCA connector to the same point as the cathode of the recovery stage.
/Leevi

[PlateVoltage]

Do you have a schematic and layout for your clone? Can you provide some hi rez pics? How long ago was the amp built? Did it always have this hum? Have you discussed it with the builder?

I don’t have the schematic they used but they said they built it exactly to a Ab763 Super Reverb circuit. Except for three prong plug addition and a mid control for channel 1. The amp is three months old so it’s brand new. It has done the humming since I’ve had it. I did talk to the builder about it and they offered some of the fixes I tried. They said they’re more than happy to look at it for free I would just have to cover shipping the amp to them and getting it back to me. So I’m trying to avoid those costs by doing it myself. They did however build the amp only and shipped it to me. I bought a cab from Ted Weber along with 4x10s from them as well. The pan is a new Accutronics I bought. I assembled the complete amp once it all arrived.

Let me know if that photo works.

[PlateVoltage]

Is the source of the ‘hum’ microphonic? (When it starts making the noise, put your finger on the back of the reverb pan - does that stop the noise?)

No that doesn’t stop the hum.

[PlateVoltage]

This issue is very often related to grounding on the reverb recovery and even the reverb foot switch grounding.

In many cases a good way to prevent the reverb hum is to use an isolated RCA connector(s)and ground everything i,e, 220K resistor and ground lug of the RCA connector to the same point as the cathode of the recovery stage.
/Leevi

I wish there was a way to see this on a drawing. Because you’re talking about running the ground of the rca input and ouput to pin 8 (cathode) of v4 (reverb recovery)?

[Leevi]

Hopefully this explains better, follow the thick black lines.
/Leevi

[tubeswell]

Is the source of the ‘hum’ microphonic? (When it starts making the noise, put your finger on the back of the reverb pan - does that stop the noise?)

No that doesn’t stop the hum.

Then the noise source must be from unwanted coupling in the reverb recovery signal. A few possibilities spring to mind (in no particular order).


1) Check that the pan output cable shield is grounding.
2) try moving the leads around the reverb recovery triode tube socket and the leads around the wires to the level pot.
3) Any lengthy wires attached to the reverb level pot might need to be replaced with shielded cable
4) try adding a 68k grid stopper between the lug on the RCA Jack for the ‘pan return’ and the grid pin of the reverb recovery triode socket.
5) try adding ferrite beads to each end of the pan return cable
6) swap the reverb recovery tube (in case it’s ‘hummy’)

[mresistor]

I might add to check the rca jacks on the chassis to make sure they are mounted securely and grounded.  I would also take the black wire from the reverb transformer and ground it to a bolt such as one on a tube socket or one of the transformers mounting bolts close to where it enters through the hole into the chassis interior. IOW not grounded where the reverb send return jacks are located on the back panel.  I think I had a noise problem with reverb once in a Princeton Reverb build where I did this and it helped.

[PlateVoltage]

Hopefully this explains better, follow the thick black lines.
/Leevi

Yes that’s perfect! I’m going to try it tonight.

[PlateVoltage]

Hopefully this explains better, follow the thick black lines.
/Leevi

I soldered it up and it still hums.

[Leevi]

The grounding of the RCA connectors must be isolated from the chassis. Use only one common ground for them. Since they are not isolated models try to take them temporarily out of the chassis and test then.
/Leevi

[mresistor]

The grounding of the RCA connectors must be isolated from the chassis. Use only one common ground for them. Since they are not isolated models try to take them temporarily out of the chassis and test then.
/Leevi

Leevi  I've built several AB763 models and have owned many that uses those jacks and all have had no reverb hum. Currently I have a Super Rev head I built and it's dead silent.  I've built several Princeton Reverbs too with no hum on the reverb execept for the one I noted above.  None of them have had jacks "islolated" from the chassis. The center signal part is insulated.  So I really don't see that they need to be isolated on this Super Reverb build.

[mresistor]

I soldered it up and it still hums.

PV  do you see in your pic all those ground rings between the switchcraft jacks and the chassis. And they have all the wires connecting each one.  To me that is super redundant. Why? Because every one of these jacks is grounded to the chassis by physical contact to it.  I have a bag full of those silly rings because I never use most of them and I have observed that Fender didn't use them in Blackface amps either. Fender used those washers with teeth that bite into the metal on contact. I would unsolder all those gound wires from the ground rings and take all but one of them off the amp.  I would keep one only to ground the resistor.  Look at the snippet I posted above from and AB763. It shows ONE physical ground and the resistor is tied to it. I would also terminate the ground of the reverb transformer directly to the transformer mounting bolt, keeping that ground wire as short as possible. 

[Leevi]

I've built several AB763 models and have owned many that uses those jacks and all have had no reverb hum. Currently I have a Super Rev head I built and it's dead silent.  I've built several Princeton Reverbs too with no hum on the reverb execept for the one I noted above.  None of them have had jacks "islolated" from the chassis. The center signal part is insulated.  So I really don't see that they need to be isolated on this Super Reverb build.

I also have built several AB763s and other types of reverb amps too. Normally the non-isolated jacks work well and that is definitely the way you should build the amp.
The cases where I have had reverb hum issues have mostly been solved by changing the grounding around the recovery stage. The root cause for the hum is ground loop. It can be solved in different ways and an isolated jack is just an extreme one. There are some threads in this forum handling reverb hum and grounding.
https://el34world.com/Forum/index.php?topic=17645.0
/Leevi

[mresistor]

Looking at the pictures..   on the Reverb footswitch jack there is supposed to be a 220k resistor to ground..  I think there is one there but there is a capacitor too in parallel with it.  I'm wondering why the cap?

[PlateVoltage]

Looking at the pictures..   on the Reverb footswitch jack there is supposed to be a 220k resistor to ground..  I think there is one there but there is a capacitor too in parallel with it.  I'm wondering why the cap?

There is a resistor there for sure. I’m going to try your suggestion today with the ground from the reverb transformer.


Would that yelllow capacitor cause any hum?

[tubeswell]

If you disconnect the pan return cable from the chassis and turn the reverb level all the way up, is it still hummy?

[sluckey]

That cap on the RCA phono jacks was added much later to the CBS master volume amps. It was not on any AB763 amps. And the way all four of those phono jack grounds are wired together could possibly create a ground loop. Look at this pic for an example of a hum free reverb circuit...

     https://el34world.com/Hoffman/images/Img_8415.jpg

It's a Princeton Reverb but the reverb circuit is identical to the Super Reverb. I suggest you copy this pic and also eliminate that black ground wire that goes toward the board. No guarantees but does this improve the hum?

[mresistor]

There is a resistor there for sure. I’m going to try your suggestion today with the ground from the reverb transformer.
Would that yelllow capacitor cause any hum?

I don't know.  It's tied to the grid of the recovery triode and that is sensitive. 


I would also try what Tubeswell is suggesting  above   try to isolate where the hum is getting into the circuit.


Slukey   do you suppose they added that cap to quiet the switching of the reverb pedal switching?

[PlateVoltage]

Looking at the pictures..   on the Reverb footswitch jack there is supposed to be a 220k resistor to ground..  I think there is one there but there is a capacitor too in parallel with it.  I'm wondering why the cap?

I relocated the ground and it’s still humming.

[PlateVoltage]

If you disconnect the pan return cable from the chassis and turn the reverb level all the way up, is it still hummy?

If I disconnect the reverb input on the back of my amp it still hums when reverb is maxed. 

If I disconnect the reverb output on my amp it does not hum when reverb is maxed.

[mresistor]

So the hum is coming from the driver, cables, pan..    interesting..     and  may I ask   why? are you turning the reverb up to max?  Doing the Dick Dale thing?     the stock fender reverb is really crazy reverby and I don't think I've ever had occasion to use it past midway in stock form.  I think it's going to be a little noisy all the way up in most any fender clone amp.  Mine has a tad of hiss at that setting which I attribute to the recovery triode. I'll try my PR and see if it also has a little hiss at max.
 

[PlateVoltage]

That cap on the RCA phono jacks was added much later to the CBS master volume amps. It was not on any AB763 amps. And the way all four of those phono jack grounds are wired together could possibly create a ground loop. Look at this pic for an example of a hum free reverb circuit...

     https://el34world.com/Hoffman/images/Img_8415.jpg

It's a Princeton Reverb but the reverb circuit is identical to the Super Reverb. I suggest you copy this pic and also eliminate that black ground wire that goes toward the board. No guarantees but does this improve the hum?

No it still hums.

[PlateVoltage]

So the hum is coming from the driver, cables, pan..    interesting..     and  may I ask   why? are you turning the reverb up to max?  Doing the Dick Dale thing?     the stock fender reverb is really crazy reverby and I don't think I've ever had occasion to use it past midway in stock form.  I think it's going to be a little noisy all the way up in most any fender clone amp.  Mine has a tad of hiss at that setting which I attribute to the recovery triode. I'll try my PR and see if it also has a little hiss at max.

Oh yeah man I love playing surf. Yeah it’s more than a little hiss. It’s like a strat when its not properly grounded hum.

[tubeswell]

If you disconnect the pan return cable from the chassis and turn the reverb level all the way up, is it still hummy?

If I disconnect the reverb output on my amp it does not hum when reverb is maxed.

Yup ‘pan return cable’ is the same thing as ‘reverb out’. It’s the cable going from the pan’s output transducer back to the reverb recovery stage.


So the problem is the cable/pan shielding. Use your R meter to determine DC continuity in the cable shield. The shield should be connected to ground, but only at one end of the cable.

[PlateVoltage]

If you disconnect the pan return cable from the chassis and turn the reverb level all the way up, is it still hummy?

If I disconnect the reverb output on my amp it does not hum when reverb is maxed.

Yup ‘pan return cable’ is the same thing as ‘reverb out’. It’s the cable going from the pan’s output transducer back to the reverb recovery stage.


So the problem is the cable/pan shielding. Use your R meter to determine DC continuity in the cable shield. The shield should be connected to ground, but only at one end of the cable.

I’ve tried three different sets of rca cables and it hums not matter what. Does that narrow it down or add more confusion? 

[tubeswell]

I’ve tried three different sets of rca cables and it hums not matter what. Does that narrow it down or add more confusion? 

It may be to do with the ground connection for the cable shield; Or the ground connection to the pan. If the pan case isn't properly grounding, it could be the pan's output transducer is get unwanted EMF coupling from another source, like the PT. With one R-meter probe on the outside of the pan, and the other probe on the amp chassis, do you get low/zero resistance?


The reverb cable RCA jack sleeves on the pan and the amp are all supposed to be wired so that the pan, and both cable shields, are grounded, BUT the connection isn't supposed to make a ground loop between the pan and the chassis. You want the ground connection to go from the amp chassis into one cable shield, and then from that cable shield to the pan, while the other cable shield is grounded either at the pan or at the amp chassis, but not at both ends of that cable). I.e. 1 of the 4 RCA jack sleeve connections should be ground-isolated.


If it all is grounding as it should, it could be you just need better quality reverb cables or try ferrite beads/rings on the cables.


Another thing you can do (as I mentioned earlier) is solder a 68k grid stopper right to the grid pin of the reverb recovery triode (i.e. it would logically go between the 'reverb-out' (reverb return) RCA jack tip and the socket grid pin for the recovery stage. This will roll off unwanted HF and stop stop the high-end hash/noise. The reverb recovery triode is the most noise-sensitive triode in this circuit and its trying to amplify a very weak output transducer signal - along with any noise floor that's in the signal. When you dime the reverb level its adding all the noise that's in there into the main signal path.


Another possibility is a 'hummy triode' in the reverb recovery tube - in which case try swapping that tube for one of the other (channel input) preamp tubes, to see if one of those is quieter. BUT if this was the problem, you'd notice it with the reverb return ('reverb out') cable unplugged - so I doubt this is the problem from your verification to-date


(I'm not saying any of these is 'the answer' -  trying to eliminate options looking at it remotely)

[pdf64]

Is the reverb and trem footswitch connected to the RCAs at the back of the amp?
If so, how about if they’re unplugged?
The reverb will still work.
Point being that the footswitch or cable may be picking up nasties.

[PlateVoltage]

I’ve tried three different sets of rca cables and it hums not matter what. Does that narrow it down or add more confusion? 

It may be to do with the ground connection for the cable shield; Or the ground connection to the pan. If the pan case isn't properly grounding, it could be the pan's output transducer is get unwanted EMF coupling from another source, like the PT. With one R-meter probe on the outside of the pan, and the other probe on the amp chassis, do you get low/zero resistance?


The reverb cable RCA jack sleeves on the pan and the amp are all supposed to be wired so that the pan, and both cable shields, are grounded, BUT the connection isn't supposed to make a ground loop between the pan and the chassis. You want the ground connection to go from the amp chassis into one cable shield, and then from that cable shield to the pan, while the other cable shield is grounded either at the pan or at the amp chassis, but not at both ends of that cable). I.e. 1 of the 4 RCA jack sleeve connections should be ground-isolated.


If it all is grounding as it should, it could be you just need better quality reverb cables or try ferrite beads/rings on the cables.


Another thing you can do (as I mentioned earlier) is solder a 68k grid stopper right to the grid pin of the reverb recovery triode (i.e. it would logically go between the 'reverb-out' (reverb return) RCA jack tip and the socket grid pin for the recovery stage. This will roll off unwanted HF and stop stop the high-end hash/noise. The reverb recovery triode is the most noise-sensitive triode in this circuit and its trying to amplify a very weak output transducer signal - along with any noise floor that's in the signal. When you dime the reverb level its adding all the noise that's in there into the main signal path.


Another possibility is a 'hummy triode' in the reverb recovery tube - in which case try swapping that tube for one of the other (channel input) preamp tubes, to see if one of those is quieter. BUT if this was the problem, you'd notice it with the reverb return ('reverb out') cable unplugged - so I doubt this is the problem from your verification to-date


(I'm not saying any of these is 'the answer' -  trying to eliminate options looking at it remotely)

I have swapped new tubes in and out with no change.

I checked the cables and it’s grounded at both rca jacks on the chassis, grounded at the input on the reverb pan but not grounded on the pan return to amp. So I should be good there right?

I am going to try the 68k grid resistor. I of course know where the rca solder point is but can you circle or make a diagram to show where exactly I attach it to on the socket grid pin?

[PlateVoltage]

Is the reverb and trem footswitch connected to the RCAs at the back of the amp?
If so, how about if they’re unplugged?
The reverb will still work.
Point being that the footswitch or cable may be picking up nasties.

I still get the hum whether the footswitch is plugged in or not. 

[pdf64]

Soundclip demonstrating the issue?

[Williamblake]

If the hum goes and comes with the reverb being turned to ten or not i would start moving cables about in the reverb return path to see if the hum goes away maybe slightly. I would also shorten the transducer at the tank just to see what amount of hum you still get with the reverb pot being turned up or down. I also tried keeping the base out of the reverb return with a 1uF cathode resistor bybass for reverb return but that didn´t sound good.

You already got better advice on grounding but in my world i let input and output transducers do their thing and connect the tank itself to the chassis and keep the transducers disconnected from the tank.

[tubeswell]

I checked the cables and it’s grounded at both rca jacks on the chassis, grounded at the input on the reverb pan but not grounded on the pan return to amp. So I should be good there right?

Yep

I am going to try the 68k grid resistor. I of course know where the rca solder point is but can you circle or make a diagram to show where exactly I attach it to on the socket grid pin?

See attachment

[PlateVoltage]

Soundclip demonstrating the issue?

https://m.youtube.com/shorts/2wr_4OVFmjI?feature=share

[PlateVoltage]

I checked the cables and it’s grounded at both rca jacks on the chassis, grounded at the input on the reverb pan but not grounded on the pan return to amp. So I should be good there right?

Yep

I am going to try the 68k grid resistor. I of course know where the rca solder point is but can you circle or make a diagram to show where exactly I attach it to on the socket grid pin?

See attachment

Right on thank you. Does it need to be 1/4 watt 68k resistor?

[tubeswell]

I checked the cables and it’s grounded at both rca jacks on the chassis, grounded at the input on the reverb pan but not grounded on the pan return to amp. So I should be good there right?

Yep

I am going to try the 68k grid resistor. I of course know where the rca solder point is but can you circle or make a diagram to show where exactly I attach it to on the socket grid pin?

See attachment

Right on thank you. Does it need to be 1/4 watt 68k resistor?

Any wattage you want

[tubeswell]

FWIW heres one of my surf amps (rehoused 73 Quad Reverb) with heavy duty reverb cables.

[pdf64]

What are the type / model details noted on the tank?

[PlateVoltage]

What are the type / model details noted on the tank?

It’s a brand new “Surfybear” Accutronics pan.

[acheld]

Hi there platevoltage.

There should be a model number somewhere giving some of the design characteristics, and this will define the impedance requirements  -- see:  https://www.amplifiedparts.com/tech-articles/spring-reverb-tanks-explained-and-compared   a great explanation of the differences between the various tanks.

[pdf64]

Thanks for that!
I think the 5th character of the model code should be A or C, to ensure that the tank metalwork is connected to the output shield.

Or just check it with a meter.

[mresistor]

Is this it? 
aka https://signalchain.com.au/products/accutronics-surfypan


It is input insulated and output grounded supposedly like the original 4AB3C1B


 

[PlateVoltage]

Is this it? 
aka https://signalchain.com.au/products/accutronics-surfypan


It is input insulated and output grounded supposedly like the original 4AB3C1B


 

Yes that is exactly what it is.

[PlateVoltage]

I checked the cables and it’s grounded at both rca jacks on the chassis, grounded at the input on the reverb pan but not grounded on the pan return to amp. So I should be good there right?

Yep

I am going to try the 68k grid resistor. I of course know where the rca solder point is but can you circle or make a diagram to show where exactly I attach it to on the socket grid pin?

See attachment

I tried the 68k resistor and it didn’t change anything. I gotta say I’m pretty stumped.

[tubeswell]

I checked the cables and it’s grounded at both rca jacks on the chassis, grounded at the input on the reverb pan but not grounded on the pan return to amp. So I should be good there right?

Yep

I am going to try the 68k grid resistor. I of course know where the rca solder point is but can you circle or make a diagram to show where exactly I attach it to on the socket grid pin?

See attachment

I tried the 68k resistor and it didn’t change anything. I gotta say I’m pretty stumped.

Maybe try rolling a few more pre-amp tubes in the V4 slot (and/or the V1 and V2 slots)? - to find one that is 'low-hum'


Have you also tried better quality/heavy-duty reverb pan cables? (and or ferrite beads/rings?)

[mresistor]

can you upload a recording of the hum ?

[PlateVoltage]

can you upload a recording of the hum ?

https://m.youtube.com/shorts/2wr_4OVFmjI?feature=share

[PlateVoltage]

I checked the cables and it’s grounded at both rca jacks on the chassis, grounded at the input on the reverb pan but not grounded on the pan return to amp. So I should be good there right?

Yep

I am going to try the 68k grid resistor. I of course know where the rca solder point is but can you circle or make a diagram to show where exactly I attach it to on the socket grid pin?

See attachment

I tried the 68k resistor and it didn’t change anything. I gotta say I’m pretty stumped.

Maybe try rolling a few more pre-amp tubes in the V4 slot (and/or the V1 and V2 slots)? - to find one that is 'low-hum'


Have you also tried better quality/heavy-duty reverb pan cables? (and or ferrite beads/rings?)

I have moved tubes around but will try it again. I do have heavy duty rca cables on it now I got from British Audio.

What are ferrite beads/rings?