AB763 Reverb Hums louder as you turn the Reverb up

[Platefire]

This is a Peavey Pacer cab and chassis I built a one channel AB763 Deluxe Reverb in with a 6G16 bias Tremolo. Got it out today and been jamming on it. It sounds great and runs nice and quite with the reverb turned all the way off. The minute you start turning the reverb pot up the hum get louder and louder.
I was playing around with the reverb send and return cables with the amp on. When I disconnect the send cable from the 12AU7 Driver there in no change in the noise. When I disconnect the return from tank to recovery stage it makes a loud squeal and the hum gets louder with it unplugged. Is this telling me the problem is in the recovery stage or even the recovery tube? or does this mean nothing?

So my first thought of action was to try a new 12AX7 in recovery and see if that helps. I was really just looking for a good excuse to talk to you guys and this seemed good as any and----I really would like to get the hum out of my reverb cause I might want to do some Dick Dale stuff:>) Platefire
BTW--Attached is a schematic and a couple of gut shots of the reverb hookup

[tubeswell]

So you tube-swapped the reverb recovery tube already?

[Searing]

Reverb transformer issue?  Wiring?  Red wire from the transformer goes to the board, and then underneath the board.  Are all of those joints securely soldered?  Is that wire securely soldered?  No idea?  But, those fallen leaves on the ground are making me anxious.  Summer is flying by. 

[Platefire]

Haven't tried another recovery tube yet. That will be my first move. I don't really think any of my wiring on the board is off but that would be something to check should everything else fail. The red wire from transformer to board can be clearly seen in the posted pictures to tie into the turret where it goes under board to power node "B". Off course you can't see the under-board wire but it is there.

My second thought after trying a new tube in recovery was "lead Dress"---two wires too close to each other causing interference? So good to hear from you guys and I'll post what I find with the tube replacement. Hoping not to pull the chassis but if I haft to, will dive in. Platefire

[Platefire]

Well, put in a new 12AX7 in V3. No change
 

[shooter]

was "lead Dress"---

It does look like your fil wires are spending a good distance parallel with the reverb tranny wires 

nbd, your schematic appears to short the output of the reverb tranny to ground.
there is a "box" icon your can use in .sch

[tubeswell]

Could be a noisy plate resistor on the recovery stage.


Or a bad/poor ground somewhere on that stage.


(Is the pan's (EMF-sensitive) output transducer situated away from the PT-end of the amp?)

[Platefire]

 Going to pull the chassis and set it up with a chassis stand hooked to a speaker so I can experiment with the lead dress to hopefully to get some better results there. Will be checking things you've already recommended but will start with lead dress and go from there. Thanks, Platefire

[Platefire]

Ok, I pulled the chassis and got it set up on a chassis stand next to the cab so I can hook the speaker and reverb connections up. First before firing it up I did continuity checks on all the grounds and they all seemed good. I moved a few wires around to provide more separation. Checked the wires running under heater wires and they are well separated the closest one at least 3/8" except where they join up on socket connections and they have to get close. Those heater wires are at least an inch or more away from those reverb transformer wires that appear to be running in parallel. From what I see I really don't see any red flags on lead dress. I can take some more pictures at your request to give you a closer look!
So after that I began to compare my schematic to the original AB763 Reverb and discovered a few things:
1-My driver tube was a 12AU7 and the AB763 schematic is a 12AT7. So I found a Groove Tubes 12AT7 and installed it in V2, no difference, same old hum.

2-I noticed on V2 12AT7 the two grids pin #2 and #7 are jumpered together. I check my amp wiring and they are jumpered also on my V2. Only problem my reverb input grid is connected to pin # 7 instead of #2 like on the drawing. So-----I don't see where that makes any difference since they are jumpered together. What do ya'll say on that???? So there is a problem on my schematic because it don't show the two grids jumpered together and should show my reverb input grid going to pin #7 like on my amp wiring.

3-On recovery tube V3b 12AX7, the cathode resistor on the original AB763 reverb is a 820 Ohm and on
my amp is a 1.5K (this could be a problem!)

I use a small amount of reverb along with a short delay pedal that stays on all the time. So with the amount of reverb that I use for myself, that reverb is perfectly quiet at my setting. It's just that when you turn it up to the splashy reverb the hum enters in and gets louder the more reverb you dial in. Even though I'll probably never use that much reverb, it bugs me that I think the hum shouldn't be there. wouldn't you feel the same way? Platefire

[shooter]

small amount of reverb along with a short delay pedal

is there an FX that I'm missing in schematic, or you using it in front of amp?

disconnect the return from tank to recovery stage it makes a loud squeal and the hum gets louder

Is the squeal continuous or just after it's unplugged?
If continuous, go away when you dial reverb pot to 0?

[sluckey]

2-I noticed on V2 12AT7 the two grids pin #2 and #7 are jumpered together.

Are pins 1 and 6 also jumpered? Should be. Are pins 3 and 8 also jumpered? Should be.

3-On recovery tube V3b 12AX7, the cathode resistor on the original AB763 reverb is a 820 Ohm and on
my amp is a 1.5K (this could be a problem!)

No

This topic comes up regularly. Use the search button. Your answer may just be a click away.

[shooter]

the schematic shows jumped, +1 for searching
fwiw here's a minor tweak'd version

[Platefire]

To answer Shooters question first:
1-No FX loop, it's in front of the amp but when I'm testing a amp like now, I go straight in. 2-The squeal of feedback sound is just when your disconnecting the rca to tank from the socket. The reverb only squeals with the reverb on, not when it's tuned all the way down(0).



Answers to sluckey's questions:
2-On V2 grids and cathode pins are both jumpered together.3-Ok, 1.5K can stay. I did fine the note in my folder regarding this that states "V3b cathode resistor should be 1.5K. The original DR AB763 uses an 820 Ohm, but the resistor is shared with another tube(V3a cathode in your circuit). So in this circuit, we are not sharing a cathode resistor!sluckey---I have been researching this on the web and it's a very common problem with Fender reverbs.I have Pro Reverb head that does the same thing. Everybody's you tube or post has a different solution that worked on their amp, but so far nothing has worked on my amp. I'm about ready to take a look at the tank. Qusestion----have you ever had a vintage fender amp or clone circuit where the reverb was completely quite all the way up to spaced out splash city reverb??? Platefire

[Platefire]

Shooter---somebody must have doctored that schematic for me and added that jumper. It wasn't  on my drawing on my PC, but I just added it in. After 11 years finally getting things corrected:>) Thanks, Platefire

[tubeswell]

1-My driver tube was a 12AU7 and the AB763 schematic is a 12AT7. So I found a Groove Tubes 12AT7 and installed it in V2, no difference, same old hum. 

2-I noticed on V2 12AT7 the two grids pin #2 and #7 are jumpered together. I check my amp wiring and they are jumpered also on my V2. Only problem my reverb input grid is connected to pin # 7 instead of #2 like on the drawing. So-----I don't see where that makes any difference since they are jumpered together. What do ya'll say on that???? So there is a problem on my schematic because it don't show the two grids jumpered together and should show my reverb input grid going to pin #7 like on my amp wiring.

Swapping the driver tube (and anything on the reverb driver side) won't affect hum if the hum is coming from the recovery stage.

... It's just that when you turn it up to the splashy reverb the hum enters in and gets louder the more reverb you dial in...

The hum is definitely located in the recovery stage.


Methinks it could be (in descending order of likelihood):

• bad return cable or
• EMF coupling into the pan's sensitive output transducer from a stronger EM flux field somewhere nearby (like the PT)
• unshielded signal wire going to the recovery stage input grid
• poor ground return grouping/routing or
• insufficient filtering/decoupling in the HT supply rail or
• bad plate resistor or grid leak resistor or
• bad solder joint or loose socket pin clamp or cracked hookup wire or
• a combination or two or more of the above

Because of the very weak signal from the pan's output transducer, the reverb recovery triode is the most sensitive gain stage in the amp in terms of S:N ratio, therefore its more susceptible to any noise like bad grounding or EMF coupling or insufficient power supply filtering.

Apart from checking the integrity of all the connections, and the orientation of the pan w.r.t. the PT location, it really helps to follow a 'galactic' ground return grouping (as suggested in Merlin Blencowe's grounding article) with attention paid to where each stage gets its power supply, in order to reduce the possibility of ground loop hum. Don't group the recovery stage grounds with the driver stage grounds (especially if the driver stage is hooked up to the screen supply node, and the recovery stage is hooked up to one of the preamp supply nodes).

[sluckey]

I have been researching this on the web

Use the search button ON THIS FORUM!

I had a PR back in the '60s and  a TR clone in the '70s. I currently have a TRRI and my clone TDR. Reverb was hum free on all. I had to roll several tubes thru the recovery socket before I got rid of the hum. Ended up with a lower gain 5751 tube.

[Platefire]

Yeah I have read some thread on the this subject already in which sluckey you were involved in. This seems to be a wide ranging subject which could pick up noise from a lot of sources. I have tried several new/NOS tubes in recovery even this morning--a 12AY7 and a 5751---no change.

For now I've put the amp away. Got a Hurricane breathing down my back right now. Lost power twice yesterday. Not sure how much of it we will catch in my Northwest location but I got my generator set up on my porch should I need it. Platefire

[sluckey]

Couple things to try...

1. Ground V3 pin 2. Do this directly on the socket. Still hum?

2. Disconnect that .003 cap from the top side of the reverb pot. Still hum?

One guy with exact hum issues swears he replaced the reverb driver and the hum disappeared. Put the old tube back in and the hum reappeared. Sounded like a fantasy to me but I just turned the page. Just sayin'...

You built this amp some time ago. Do you recall having hum issues when you first built it?

[Platefire]

Hay sluckey
Grounding V3 grid at pin #2 was what I thought the reverb on and off switch does at the rca jack where the reverb enters back into the chassis to the recovery stage. What difference moving it a few more inches to tube socket pin #2 to chassis ground do????? Wouldn't it turn off the reverb just like the foot switch does???

Also the disconnecting the lead of .003 coupling cap to pot would cut the reverb off completely returning to the grid point R2.
So yeah, it would get rid of the hum and reverb all together. I would perfer a little bit of non-hum reverb to no reverb at all 

Platefire

[wsscott]

Platefire--Did you ever resolve this problem?  I'm looking for answer on a similar issue.  Thanks.