Deluxe Reverb Gremlin help

[TubeGeek]

I am converting a pcb deluxe reverb to hand-wired for a buddy. 


I am finished assembly and I am stumped with a problem. 


Everything is working except when I turn the volume up past 8 on the vibrato channel, I hear a nasty distortion. I can't explain this one either…If i am plugged into input 1 on the vibrato channel, vol at 8 and then turn up the vol pot on the normal channel, the noise gets worse.


The noise does not happen on the normal channel. I can play that channel at full level and the amp is not farting out.


I took a short video and posted it on my youtube so that you can hear for yourself. 

i=jzHU4UaW7dK51pM7


Any pointers? I am not having much luck troubleshooting this one.

[Latole]

Did you try a colder bias on power tubes ? Like 50 % power dissipation ?

[pdf64]

My guess is that it's oscillating at ultrasonics, but all the heavy overdrive is making it difficult to be sure.
Have you got a scope to check for this?
Do the V2 or V4 idle cathode voltages change a little, according to whether the volume is set above or below 8?
It may be helpful to repeat the video, but as you turn the amp volume up, try turn the guitar volume down, in order to avoid the sound becoming overdriven.
That may allow the artefacts caused by oscillation to become more apparent.

[TubeGeek]

I thought I had it sorted out tonight.  The amp played fine for about 45 minutes.  I posted a reply to what I found and then went back to play it and the gremlin is still there. 


I thought maybe the problem was that I had run the OT centre tap wire through the same grommet as the OT secondary wiring.  I thought it was causing the oscillation.  I do recall when measuring plate voltage on pin 3 of the 6V6 socket this past weekend, on one socket the amp would start oscillating when my DMM touched pin 3.


Back to troubleshooting...

[TubeGeek]

Did you try a colder bias on power tubes ? Like 50 % power dissipation ?

Yes, I tried this tonight and it makes no difference where the bias is set.

[TubeGeek]

My guess is that it's oscillating at ultrasonics, but all the heavy overdrive is making it difficult to be sure.
Have you got a scope to check for this?
Do the V2 or V4 idle cathode voltages change a little, according to whether the volume is set above or below 8?
It may be helpful to repeat the video, but as you turn the amp volume up, try turn the guitar volume down, in order to avoid the sound becoming overdriven.
That may allow the artefacts caused by oscillation to become more apparent.

I would agree that I am chasing an oscillation. 


I do have a scope to use.  This'll test me…I don't use a scope very often. I think I need a refresher on the scope.


Cathodes on V2 and V4 do not fluctuate.


I tried what you say with the guitar and it sounds normal.  Something I noticed a minute ago though, if the vol is at 10, there is no gremlin but if I back it off to 9, the gremlin appears.  The gremlin also becomes really apparent if I turn the un-used normal channel up to 10. It is unmistakable at that point.

[Latole]

Perhaps it's simply a question of arranging the wires better.
Ideally, they should run along the chassis and cross each other at 90 degrees, never in parallel for some. Otherwise, this will lead to oscillations and noise.

Also a colder sorder joint

Look here , this guy build a DR like yours and he work fine ,.......not the first time


https://www.guitarscanada.com/threads/watch-as-i-build-a-deluxe-reverb.307747/page-2

[TubeGeek]

Deluxe reverbs are hard to keep clean.  The past 20 deluxe reverbs I’ve built have turned out good.  I know the wiring ain’t perfect but it ain’t that bad is it?  I’ve been moving a lot of the wiring around thinking it was this.  Perhaps I rushed this build as I was fitting it in between repairs and now it’s become a bit of a headache. 

[Latole]

The wires may not be bad, but in some amp circuits they are very critical. It has to be perfectly positioned.
This may be the reason why the amp works well but not perfectly.

Don't neglect to redo some soldering either.

[stratomaster]

What is that pot along the back panel?  Looks like you've modified this circuit and deviated from a known good wiring scheme. 

Do you have a schematic of what you actually built?

Voltage survey would be handy too.

The pin 3 think could indicate inverted OT primaries. Disconnect NFB and retest.

There are quite a few threads indicating a positive feedback problem with reverb recovery and the 3rd stage of the vibrato channel, could be good to investigate.

There are also threads indicating an issue with the reverb driver causing unwanted artifacts in the clean signal, so also something to investigate. 

[Latole]

What is that pot along the back panel?  Looks like you've modified this circuit and deviated from a known good wiring scheme. 

One wire is connect to positive of output speaker jacks !!!

[SEL49]

What is that pot along the back panel?  Looks like you've modified this circuit and deviated from a known good wiring scheme. 

One wire is connect to positive of output speaker jacks !!!

It's just a variable NFB control.

[Latole]

What is that pot along the back panel?  Looks like you've modified this circuit and deviated from a known good wiring scheme. 

One wire is connect to positive of output speaker jacks !!!

It's just a variable NFB control.

You can't put just anything as a value, otherwise it will cause instability. You need to know more.
And as requested, a schematic as constr

[TubeGeek]

I spent some time with the amp tonight, trying to make progress.  I have narrowed it down to the reverb circuit.  Why?  When I turn up the reverb pot I begin hearing a severe high pitched oscillation.  When the reverb pot is maxed out, there is distortion.  When I strum the guitar and turn the reverb pot down to 2 or 3, the distortion goes away. 


Tried another known good tank. Same thing.


I did reverse the OT primaries which only squealed like crazy, so I have them correct.



Something else that I find strange is that if I fully remove v1 from the socket, the vibrato channel tone sounds way better.  I do still get the oscillations when I turn the reverb pot up, even when v1 is removed, so that is where I think I should focus on.


I removed the variable NFB pot for now. At least until it's working 100% proper.


I moved most wiring around, trying to make the oscillations worse or better.  When I move the reverb transformer blue wire, the squeal changes a bit.


Done for the night.


I am reading all the replies and taking your ideas into consideration. I just haven't connected the dots quite yet.


I appreciate the replies thus far.


Here is some info about this project:
As for what layout I have been following…I started with a copy of the fender ab763 schematic and layout as well as a copy of the mojotone layout in front of me. The mojo layout is a clearer example of wiring and I find it handy.  I haven't followed the layout exactly though. I made the eyelet board for this chassis.  The empty area in between the PT and main board was going to be used for a VVR install at some point.  My buddy also prefers to use SS rectification so I have an adapter plugged in to the rectifier position.  There's also some empty eyelets on the board where we were going to experiment with cathode biasing.  Basically I am just in the beginning stage of working closely on my friends custom amp.  We are going to be doing a lot of experiments on this thing.  I am contemplating a full re-wire at this point because of all the wiggling around and moving wires I have done.  I know I can get the wiring much much cleaner, especially if I use under the board jumpers and teflon coated wire.  I chose to use above the board wiring, which is a bit messier looking but I haven't had problems on many other builds similar to this.  Above the board wiring also makes it easier to try mods and make changes. I was trying to knock this project out quick because I pushed several repairs to the side to do this. Turns out, the opposite it seems for this project.

[TubeGeek]

What is that pot along the back panel?  Looks like you've modified this circuit and deviated from a known good wiring scheme. 

One wire is connect to positive of output speaker jacks !!!

It's just a variable NFB control.

You can't put just anything as a value, otherwise it will cause instability. You need to know more.
And as requested, a schematic as constr

Just a 5k pot for nfb control.


I removed it until troubleshooting is finished.


I used a combo of the ab763 fender Schematic/layout and mojotone's layout for reference.  Didn't follow either to a T, I made little adjustments here and there. Nothing I thought would cause an issue but it appears I've possibly done exactly that.


I am considering a full re-wire after i find the problem.

[TubeGeek]

What is that pot along the back panel?  Looks like you've modified this circuit and deviated from a known good wiring scheme. 

Do you have a schematic of what you actually built?

Voltage survey would be handy too.

The pin 3 think could indicate inverted OT primaries. Disconnect NFB and retest.

There are quite a few threads indicating a positive feedback problem with reverb recovery and the 3rd stage of the vibrato channel, could be good to investigate.

There are also threads indicating an issue with the reverb driver causing unwanted artifacts in the clean signal, so also something to investigate.

I do have a voltage chart but it's so messy that I need to do a new one to share.  Will do soon.


Reversed the OT primaries, got worse, so I do have them correct.


"There are quite a few threads indicating a positive feedback problem with reverb recovery and the 3rd stage of the vibrato channel, could be good to investigate."


This is probably where the problem lies after what I've learned tonight.

[pdf64]

Routing the full speaker signal near V4 circuits creates a scenario for unintended coupling, leading to positive feedback loops and instability.

A good rule is to build things straight, then add mods, one at a time.

[Latole]

I do have a voltage chart but it's so messy that I need to do a new one to share.  Will do soon.
 

Voltages are not the issue IMO.

[Latole]

Routing the full speaker signal near V4 circuits creates a scenario for unintended coupling, leading to positive feedback loops and instability.

A good rule is to build things straight, then add mods, one at a time.

100% right .

[emerson909]

I spent some time with the amp tonight, trying to make progress.  I have narrowed it down to the reverb circuit.  Why?  When I turn up the reverb pot I begin hearing a severe high pitched oscillation.  When the reverb pot is maxed out, there is distortion.  When I strum the guitar and turn the reverb pot down to 2 or 3, the distortion goes away.

You could try putting a bypass cap around the recovery 220k grid leak resistor similar to the A1172 circuit. This solved a similar problem I had in an AB763 build, the output volume would drop and distort as the reverb was increased above 5 which I assumed was due to ultrasonic oscillation. The circuit shows 2000pF but I only had 3300pF to hand which worked fine.

[TubeGeek]

Re-wire in progress.

[HotBluePlates]

I spent some time with the amp tonight, trying to make progress.  I have narrowed it down to the reverb circuit.  Why?  When I turn up the reverb pot I begin hearing a severe high pitched oscillation.  ...

Look at your wiring of V3 Pin 2 to Pin 7 (highlighted in pink).



Connect these pins with a shortest possible piece of wire, that goes straight across the socket.  Having push-back insulation on that wire is probably a good idea.


The looping path it takes now places it in-parallel to the Reverb Transformer input (which is the output at V3 Pin 1 and Pin 6), and likely couples the driver's output to its input in a way that's causing the reverb oscillation.


It is okay to have the long, looping Blue wire connecting V3 Pin 3 and Pin 8 due to the low impedance (and small, if any, AC Voltage) at this part of the circuit.

[Merlin]

Connect these pins with a shortest possible piece of wire, that goes straight across the socket. 

8:58 for instruction on how to wire the reverb socket:

=538

[Latole]

Look at your wiring of V3 Pin 2 to Pin 7 (highlighted in pink).



Connect these pins with a shortest possible piece of wire, that goes straight across the socket.  Having push-back insulation on that wire is probably a good idea.

You are right and that is Fender show in their amp layout.
My answer no 6 about wiring.

[mresistor]

Interestingly I have a 68 DR on the bench this morning and here is how they wired V3 in Corona in 68. It's not noisy nor having any issues.

[SEL49]

Same for the 64 DR. Grids and cathodes crisscross the socket and plates loop around the socket with wire touching the chassis.

[TubeGeek]

Complete re-wire finished.  I am happier with how it is now, in that regard.


I do still have oscillations in the reverb circuit.  This time I get the high pitched squeal as soon as I turn the reverb pot up. Before I had to turn up to 7 before I heard the oscillation.


 


I am frustrated and more determined to fix it now.  First thing I am going to do is move the blue reverb transformer wire to pin 6 and see if that is the gremlin.

[Willabe]

Looking at an old BF DR and a new DR the verb jacks are moved a bit. On the amp your working on the verb tank output RCA is right in front of the verb driver tube, so input and out put are right next to each other. And that can cause oscillation.   

Maybe that's why on the new DR's they put a piece of the vulcanized fiber board as a shield be tween the verb driver tube and the verb RCA jacks?

[TubeGeek]

I spent some time with the amp tonight, trying to make progress.  I have narrowed it down to the reverb circuit.  Why?  When I turn up the reverb pot I begin hearing a severe high pitched oscillation.  When the reverb pot is maxed out, there is distortion.  When I strum the guitar and turn the reverb pot down to 2 or 3, the distortion goes away.

You could try putting a bypass cap around the recovery 220k grid leak resistor similar to the A1172 circuit. This solved a similar problem I had in an AB763 build, the output volume would drop and distort as the reverb was increased above 5 which I assumed was due to ultrasonic oscillation. The circuit shows 2000pF but I only had 3300pF to hand which worked fine.

I've seen this on older amps.


I added the cap and it makes the oscillation worse, which may help me track it down.  But man it is hard to listen to that pitch for more than a minute or two.

[TubeGeek]

Looking at an old BF DR and a new DR the verb jacks are moved a bit. On the amp your working on the verb tank output RCA is right in front of the verb driver tube, so input and out put are right next to each other. And that can cause oscillation.   

Maybe that's why on the new DR's they put a piece of the vulcanized fiber board as a shield be tween the verb driver tube and the verb RCA jacks?

Well hmmm, that does seem possible eh?


The reason I have the rca jacks there is because I chose to not re-use the existing 1/4" foot switch hole in the chassis.  Laziness I guess…I didn't want to figure out how to wire up the 1/4" jack.  Drilling 4 new holes and installing rca jacks seemed like a better route .  Now I am not so sure.  Now I'm looking at how to use the original 1/4" jack…


Also, tried that piece of vulcanized board around the rca jacks, it didn't help any.  I still have that from the teardown.

[Willabe]

The reason I have the rca jacks there is because I chose to not re-use the existing 1/4" foot switch hole in the chassis.  Laziness I guess…I didn't want to figure out how to wire up the 1/4" jack.  Drilling 4 new holes and installing rca jacks seemed like a better route .  Now I am not so sure.  Now I'm looking at how to use the original 1/4" jack…

Ok, so that 1/4" jack hole with a stereo jack is for the on/off trem/verb foot switch?

So where are the jack holes for the verb tank's cables? I don't see them. 

[TubeGeek]

The original holes for the rca jacks are located under the main board on this chassis.


My options are to drill new holes to the right of the 1/4" jack.  That could mean I need to remove the back panel due to thickness but we'll see.  Maybe not.




It's either drill holes through the back panel or remove the panel and drill.  Currently in deep thought about it 



Gonna try something else before I drill anything.

[TubeGeek]

Removed the reverb foot switch rca jacks and re-used the 1/4" jack. That's working.  I have a plan to cover up these holes so it won't be too obvious or an eyesore.


Moved the tank in/out connections over to the first two rca holes. 


Plugged in and still getting the original distortion and fizziness I was chasing before the re-wire.  The amp seems a bit under powered as well.


Only thing different is that the reverb is no longer squealing at me.  That seems to have been solved or I can't hear that frequency anymore.


There is still an issue somewhere. I'm really struggling with this one for goodness sake.


Here is a new video of how it's sounding:

[Willabe]

Yeah, that's not pleasant.

But 1 down 1 to go. 

[TubeGeek]

Next steps:


All I can think of at this point is trying another reverb transformer and replacing the 10pF cap, 4M7 resistor, and maybe the 500pF silver mica in that path too.  I've already tried replacing everything else.


I'll do them one by one later tonight. I was hoping to get this figured out before xmas so I can relax the rest of the week. The owner has a big gig this friday and I'd like to be done with it asap.I need a break to clear my head.

[stratomaster]

Next steps:


All I can think of at this point is trying another reverb transformer and replacing the 10pF cap, 4M7 resistor, and maybe the 500pF silver mica in that path too.  I've already tried replacing everything else.


I'll do them one by one later tonight. I was hoping to get this figured out before xmas so I can relax the rest of the week. The owner has a big gig this friday and I'd like to be done with it asap.I need a break to clear my head.

I think you're getting overwhelmed. Take a step back and do some troubleshooting.

If the normal channel works properly but the vibrato channel doesn't then the problem is somewhere in the channel.  You can get creative and move the wire from the normal channel 2nd stage coupling cap to the mix resistor to the equivalent spot on the vibrato channel.  If the signal is clean and strong then you can suspect the reverb. 

You can also disconnect the entire reverb circuit.  Just lift the 500pf cap and the wire from the reverb pot wiper.  If removing the reverb circuit fixes the problem, then you can suspect the reverb circuit.

You can remove the tremolo circuit too in a similar way. 

Strip the channel down and add circuits back until it breaks. 

The beauty of a handwired amp like you've converted to is that you can accomplish all of these scenarios with a few alligator clips and a few minutes of your time.  Take advantage of it for troubleshooting purposes.

Isolate and conquer.

[TubeGeek]

"You can also disconnect the entire reverb circuit.  Just lift the 500pf cap and the wire from the reverb pot wiper.  If removing the reverb circuit fixes the problem, then you can suspect the reverb circuit."


Tried this. The problem goes away without verb.


Pretty safe to say the issue is in the verb circuit.  With the verb disconnected, the tremolo sounds fine.

[TubeGeek]

Found the gremlin!

The reverb transformer was the problem all along.

I replaced all reverb circuit components one by one on the board with no success. 

The last thing I tried was replacing the reverb transformer and that did it.  The amp is sounding and behaving like it should now.  I spent the last 20 minutes playing it and no issues.

That was hard on my brain.


I have never had this much trouble in a reverb circuit before.  Something that may be a dumb question but is it possible to simply reverse the reverb transformer primaries?  That's something I did not do with the original transformer.  Is it like an OT when swapping the leads?  Maybe that's what I have been hearing all along but never clued in. I actually did have the thought many steps ago but second guessed myself and thought, nah, that can't be it. I've never seen a bad reverb transformer before.

If I don't need to use my spare reverb transformer, I would like to keep it for my personal project down the road. I just happen to have one sitting on the shelf and could easily wire it up. 


For now, the replacement transformer is wired up but hanging off the chassis and on the workbench, not permanently installed.  I want to hear back on the primary wiring idea before I commit to using my spare.

[DummyLoad]

Found the gremlin!

The reverb transformer was the problem all along.

I replaced all reverb circuit components one by one on the board with no success. 

The last thing I tried was replacing the reverb transformer and that did it.  The amp is sounding and behaving like it should now.  I spent the last 20 minutes playing it and no issues.

That was hard on my brain.


I have never had this much trouble in a reverb circuit before.  Something that may be a dumb question but is it possible to simply reverse the reverb transformer primaries?  That's something I did not do with the original transformer.  Is it like an OT when swapping the leads?  Maybe that's what I have been hearing all along but never clued in. I actually did have the thought many steps ago but second guessed myself and thought, nah, that can't be it. I've never seen a bad reverb transformer before.


If I don't need to use my spare reverb transformer, I would like to keep it for my personal project down the road. I just happen to have one sitting on the shelf and could easily wire it up. 


For now, the replacement transformer is wired up but hanging off the chassis and on the workbench, not permanently installed.  I want to hear back on the primary wiring idea before I commit to using my spare.

Yes, to swapping primary on reverb opt.trans.

--Pete

[TubeGeek]

It's embarrassing to admit this, but the noise I have been chasing was a bad 12aT7 tube. The tube was brand new and I had swapped tubes around early on.  Clearly it threw me for a loop.


Along the way, things have been corrected and the reverb oscillation was real and solved by moving the rca connections. So that was a win.


The wiring was improved, that's a win.


I learned a few things as well. That's a win.


The reverb transformer is probably good. Doesn't matter now as I replaced it with a classic tone I have had on the shelf since they announced they were halting production.  I decided to upgrade the OT to a classic tone as well.


The amp is sounding just as it should.


Would it be wrong to delete this whole thread now?


Thank you all, for taking the time to help a guy out.

[stratomaster]

Awesome. Glad you got it sorted. Leave the thread up to help the next guy.