Adding a reverb in a fender champ circuit

[sebbes]

Hi guys,

I m struggling since few weeks now in adding a reverb circuit in my champ type build.. I think I have tried alll the schematics possible found in the web without good result.. Someone has already achived this mod? I removed the vibrato part and added a tube for the reverb.

This is my amp layout before the vibrato removed and all what I have tested

Thx lot for your help

[tubeswell]

Try this

[sebbes]

Try this

Thx for the help what does it mean "use a separate RK CK" ? Dont understood the therm :)

[tubeswell]

Try this

Thx for the help what does it mean "use a separate RK CK" ? Dont understood the therm :)

R=resistor
C=capacitor
k=cathode
Rk=cathode resistor
Ck=cathode by-pass cap
||='in parallel'


Using separate Rk||Ck is better for eliminating interstage feedback where you have a pair of cascaded gain stages (dunno if that's a problem with that particular schematic, but its what I'd do. YMMV


The main thing is to fix the wiring in that schematic (hence the solid blue lines and red crosses).

[tubenit]

You need to give us more information, please ………. IF you want us to help you.  You said the circuits didn't work?
What was not working in those circuits????   You haven't stated any specifics about what the problem is?

So what exactly is the problem with the reverb circuits you've tried?


Physconoodler reported a working Champ reverb using the circuit attached here. 

Here are some websites with detailed photos on adding reverb to a Champ circuit:

http://www.offsetguitars.com/forums/viewtopic.php?t=35226

https://sites.google.com/site/ntasher/vibro-champ-reverb

With respect, Tubenit

[tubenit]

As an FYI to everyone ……………………….    The guys on the forum love to help people troubleshoot amp problems or design a new circuit.   When the forum has been most helpful to someone, there are typically 4 things the poster has done.

1) provided the "actual" schematic he is using in the amp's current state

2) a high resolution photo of the chassis interior

3) voltages on the plates and cathodes of all the tubes

4) a detailed description of what is currently NOT working and a detailed description of what they have already tried
    to do in order to resolve the problem.

With respect, Tubenit

[sebbes]

You need to give us more information, please ………. IF you want us to help you.  You said the circuits didn't work?
What was not working in those circuits????   You haven't stated any specifics about what the problem is?

So what exactly is the problem with the reverb circuits you've tried?


Physconoodler reported a working Champ reverb using the circuit attached here. 

Here are some websites with detailed photos on adding reverb to a Champ circuit:

http://www.offsetguitars.com/forums/viewtopic.php?t=35226

https://sites.google.com/site/ntasher/vibro-champ-reverb

With respect, Tubenit

On the first schematic reverb was quite good but no volume  i mean the amp lost a lot of power

On the second no reverb at all

Latest schematic its original from the first link you sent to me.. the amps was sounding ok volume wise and reverb was there but way to deep  long decay and deep like a church and if you where putting more reverb was starting to feedback

[sebbes]

As an FYI to everyone ……………………….    The guys on the forum love to help people troubleshoot amp problems or design a new circuit.   When the forum has been most helpful to someone, there are typically 4 things the poster has done.

1) provided the "actual" schematic he is using in the amp's current state

2) a high resolution photo of the chassis interior

3) voltages on the plates and cathodes of all the tubes

4) a detailed description of what is currently NOT working and a detailed description of what they have already tried
    to do in order to resolve the problem.

With respect, Tubenit

Now I m on the second schematic.. but as nothing works maybe lot me know which schematic would be better to use and I will do and after that I will post you each updates in order.

Should I follow the new schematic from tubeswell?

[tubenit]

OK,  here is a DRAFT idea based on Tubeswell's post.   I have NOT done this mod myself. 

However, in an effort to help you out ………… I have drawn this up for you.  There is an editable SCH version attached also.

ExpressSCH is a FREE download that will open SCH files.  J-scheme on Mac's work to open SCH also.

I hope this helps out. 

Guys, IF you see any revisions needed to help the poster out,  PLEASE comment.

With respect, Tubenit

[tubenit]

On the first schematic reverb was quite good but no volume  i mean the amp lost a lot of power

That's because you had a 3.3M resistor in the signal path.  You need resistance between reverb send and reverb return. Since Fender uses a 100k reverb pot, they use this big 3.3M resistor to separate send from return.

Note the difference between Fender reverb's and the "one tube reverb" values.

Fender uses a 100k reverb pot and a 3.3M resistor    VS.  the one tube reverb uses a 1M or 2.2M pot and 150-220k resistor.   So, the "one tube reverb" 150k-220k resistor doesn't attenuate the volume as much as the Fender 3.3M does.

My guess is Physconoodler's "one tube reverb" insertion after the volume pot and prior to V1-7 would work fine.  It still allows V1b to add volume back after the reverb insertion point. 

In summary,  I'd use Tubeswell idea IF you want the Fenderish style reverb.  I'd use Physconoodler's IF you want one tube reverb.  Wired properly,  I think either circuit should work.

With respect, Tubenit

[sebbes]

OK,  here is a DRAFT idea based on Tubeswell's post.   I have NOT done this mod myself. 

However, in an effort to help you out ………… I have drawn this up for you.  There is an editable SCH version attached also.

ExpressSCH is a FREE download that will open SCH files.  J-scheme on Mac's work to open SCH also.

I hope this helps out. 

Guys, IF you see any revisions needed to help the poster out,  PLEASE comment.

With respect, Tubenit

Man you are awesome!!! Many many thx!!!

[sebbes]

Try this

Thx for the help what does it mean "use a separate RK CK" ? Dont understood the therm :)

R=resistor
C=capacitor
k=cathode
Rk=cathode resistor
Ck=cathode by-pass cap
||='in parallel'


Using separate Rk||Ck is better for eliminating interstage feedback where you have a pair of cascaded gain stages (dunno if that's a problem with that particular schematic, but its what I'd do. YMMV


The main thing is to fix the wiring in that schematic (hence the solid blue lines and red crosses).

Many many thx for your help! Will try one or the other schematics posted at his point on the thread! Hope that will work.. I m tired ahah

[tubeswell]

OK,  here is a DRAFT idea based on Tubeswell's post.   

One minor adjustment suggestion

[2deaf]

Try this

There are three descending lines at the high end.  The left descending line goes from the plate of V1B to the reverb driver via a 500pF cap, which is fine.  However, it also goes to the 3.3M/10pF combo with no coupling capacitor.  This will put a positive voltage on the grid of the mixer stage which will foul-up the bias.

The middle descending line goes from the grid of the mixer stage to nowhere, which is fine because that grid shouldn't be hooked to anything except what was already shown.

The right descending line is fine with me.

 

[tubeswell]

Try this

There are three descending lines at the high end.  The left descending line goes from the plate of V1B to the reverb driver via a 500pF cap, which is fine.  However, it also goes to the 3.3M/10pF combo with no coupling capacitor.  This will put a positive voltage on the grid of the mixer stage which will foul-up the bias.

The middle descending line goes from the grid of the mixer stage to nowhere, which is fine because that grid shouldn't be hooked to anything except what was already shown.

The right descending line is fine with me.

Right you are, it was late in the evening for me. Tubenit did a better job of my tired bumbling.

[sebbes]

Try this

There are three descending lines at the high end.  The left descending line goes from the plate of V1B to the reverb driver via a 500pF cap, which is fine.  However, it also goes to the 3.3M/10pF combo with no coupling capacitor.  This will put a positive voltage on the grid of the mixer stage which will foul-up the bias.

The middle descending line goes from the grid of the mixer stage to nowhere, which is fine because that grid shouldn't be hooked to anything except what was already shown.

The right descending line is fine with me.

Right you are, it was late in the evening for me. Tubenit did a better job of my tired bumbling.

So if I choose this schematic I have to follow 2deaf’s schematic correction?

[tubeswell]

Try this

There are three descending lines at the high end.  The left descending line goes from the plate of V1B to the reverb driver via a 500pF cap, which is fine.  However, it also goes to the 3.3M/10pF combo with no coupling capacitor.  This will put a positive voltage on the grid of the mixer stage which will foul-up the bias.

The middle descending line goes from the grid of the mixer stage to nowhere, which is fine because that grid shouldn't be hooked to anything except what was already shown.

The right descending line is fine with me.

Right you are, it was late in the evening for me. Tubenit did a better job of my tired bumbling.

So if I choose this schematic I have to follow 2deaf’s schematic correction?

Tubenit had already done the correction in his version

[tubenit]

Tubeswell, 2deaf & Tubenit are saying use this one. 

Be sure to post the results.  There are a lot of inquiries on the different amp forums about adding reverb to a Champ, so it would be nice to have another "proven" schematic for others to follow.

[kagliostro]

The arrangement remember a lot the VOX Cambridge

https://el34world.com/charts/Schematics/files/Vox/Vox_cambridge_reverb.pdf



Franco

[sebbes]

Tubeswell, 2deaf & Tubenit are saying use this one. 

Be sure to post the results.  There are a lot of inquiries on the different amp forums about adding reverb to a Champ, so it would be nice to have another "proven" schematic for others to follow.

Yes I will! Thx guys!

[2deaf]

I moved the insertion point for the negative feedback because I consider it poor form to include signal altering stuff within the negative feedback loop.  Stuff like volume controls, tone controls, reverb, and other effects. 

Should you happen to agree with me, you will need to reverse the stock wiring at either the OT primary or the secondary because the mixer stage inverts the stock signal.

[sebbes]

Hi guys,
I m on it right now. So unfortunately that doesn’t work. I can’t turn the reverb pot. It start to feedback really soon. But I can hear that some reverb is coming in.. The only thing I didn’t change is on the attached picture. Let me know if that can change something? I also had to invert the OT wires because it was impossible to run the amp in the other way.
Thx a lot

[ac427v]

The negative feedback insertion point that 2deaf cites would cause a problem. His solution should work. If you don't want to try it because of complexity or lack of parts you should test the amp without any negative feedback. Just disconnect the negative feedback wire and tape it off. Does the amp play? Reverb work? If not, it would help to have a precise description of what problems you are hearing, seeing, or (hopefully not) smelling. My version of this amp worked fine with tubenit's schematic with the negative feedback wired per 2deaf's correction.

[sebbes]

The negative feedback insertion point that 2deaf cites would cause a problem. His solution should work. If you don't want to try it because of complexity or lack of parts you should test the amp without any negative feedback. Just disconnect the negative feedback wire and tape it off. Does the amp play? Reverb work? If not, it would help to have a precise description of what problems you are hearing, seeing, or (hopefully not) smelling. My version of this amp worked fine with tubenit's schematic with the negative feedback wired per 2deaf's correction.

So yes we have done the negative feedback as described. Now it is removed and in this way, yes the amp works and the reverb works but too deep and far away and more you add the reverb the more you got hum

[tubeswell]

The NFB loop should be going to the cathode of V3a. Swap the primaries if you get high-pitched screeching.
The fact that hum increases as you increase the reverb level is because the reverb recovery stage is highly noise sensitive. (i.e. it has a high S:N ratio). So work on getting that down

Some ideas:
1) a bad tube in V3 
2) having the pan's (EMF-sensitive) output transducer close to the PT (turn the pan around)
3) insufficient/bad shielding on signal wire going to V3a grid
4) insufficient RF suppression on V3a (try a small ferrite bead on the pan cables, or a 33k grid stopper right at the grid pin of V3a)
5) bad solder joint somewhere on V3a (reflow joints)
6) bad plate resistor on V3a
7) be related to how you have the ground returns arranged. (redo your grounding - read Merlin Blencowe's grounding article)

[Willabe]

7) be related to how you have the ground returns arranged. (redo your grounding - read Merlin Blencowe's grounding article)

Link for Merlin's grounding page;

http://www.valvewizard.co.uk/Grounding.html

[sebbes]

Thx for the help tubeswell,
And any ideas about why I need to disconnect the negative feedback to make it work and why the sound of the reverb is not as it should be.I use a 8ab3c1b tank.
The effect is too deep and far away. I tried a super reverb tank which is 4ab3c1b the result is the same in term of effect but in term of volume it is not loud enough.
Thx

[tubeswell]

I didn't say to disconnect the NFB. I suggested moving it to the cathode of V3a


I don't know what you really mean by ' too deep and too far away'. Can you post a sound sample?


As for the overall volume, you could change the dry/wet voltage divider by reducing the 3M3 to say 2M2 or 1M, or 470k. The smaller this resistor is, the more dry signal and overall volume you will get. (but also the less 'reverb' you will get)

[sebbes]

I didn't say to disconnect the NFB. I suggested moving it to the cathode of V3a


I don't know what you really mean by ' too deep and too far away'. Can you post a sound sample?


As for the overall volume, you could change the dry/wet voltage divider by reducing the 3M3 to say 2M2 or 1M, or 470k. The smaller this resistor is, the more dry signal and overall volume you will get. (but also the less 'reverb' you will get)

I disconnected it because ac427v advice me to doing that because with it connected the reverb doesn’t work.. so I tested to disconnect it and yes the reverb is there. When I say far away it is like if the reverb is really behind the dry signal.. not really blended..  I will try to move it where you say me. I will try to do an audio to show you as well

[sebbes]

I didn't say to disconnect the NFB. I suggested moving it to the cathode of V3a

Need I to take the 47 ohm on the v1b to put with the NFB on v3a?

[tubeswell]

Need I to take the 47 ohm on the v1b to put with the NFB on v3a?

Yep. The 47R is part of the NFB loop


(you need to ground the Rk||Ck of V1b - when you take the NFB off that stage)

[tubeswell]

When I say far away it is like if the reverb is really behind the dry signal.. not really blended..

So there is not enough wet signal?

[tubenit]

IF you are saying the reverb is too "cavernous" and too intense.  Try paralleling a 1M resistor across the 1M dwell resistor effectively giving you a 500k value.  Is that more to your liking?

You can also try a 500p across the outside lugs of the reverb pot to see if that smooths out the reverb signal to your liking?

With respect, Tubenit

[tubeswell]

I think he's saying there isn't enough reverb (but I'm not sure)

[sebbes]

When I say far away it is like if the reverb is really behind the dry signal.. not really blended..

So there is not enough wet signal?

Not sure.. the sensation is quiet strange.. definitely not a fender kind of reverb.. Maybe tubenit is right when he says cavernous! Is that possible to send audio in the thread or videos?
Thx guys!

[2deaf]

I moved the insertion point for the negative feedback because I consider it poor form to include signal altering stuff within the negative feedback loop.  Stuff like volume controls, tone controls, reverb, and other effects. 

Should you happen to agree with me, you will need to reverse the stock wiring at either the OT primary or the secondary because the mixer stage inverts the stock signal.

That ain't right.  You would only reverse the OT wiring if the NFB insertion remains at V1B.  If the NFB insertion is at V3B as I drew it, the OT wiring would remain stock.

[Williamblake]

If it is going in the right diretion but reverb is too cavernous decrease base in the reverb and it might decrease hum in the reverb, too.

[2deaf]

So unfortunately that doesn’t work. I can’t turn the reverb pot. It start to feedback really soon. But I can hear that some reverb is coming in.. The only thing I didn’t change is on the attached picture. Let me know if that can change something? I also had to invert the OT wires because it was impossible to run the amp in the other way.

There is some confusion about what is the "A" triode and what is the "B" triode.  Traditionally triode unit 1 is pins 6,7, and 8.  Triode unit 2 is pins 1, 2, and 3.  Which triode is "A" and which is "B" seems to be completely dependent on the draftsman.  It doesn't even have to be consistent within the same drawing, apparently.

tubeswell must be calling V3a pins 6, 7, and 8 which would be the mixer stage.  If you connected the NFB to the reverb recovery stage (pins 1, 2, and 3) instead, you could get the symptoms described above.   

[tubeswell]

tubeswell must be calling V3a pins 6, 7, and 8 which would be the mixer stage.  If you connected the NFB to the reverb recovery stage (pins 1, 2, and 3) instead, you could get the symptoms described above.

Yes - I was going by tubenit's schematic (in which V3a is the mixer stage)

[sebbes]

If it is going in the right diretion but reverb is too cavernous decrease base in the reverb and it might decrease hum in the reverb, too.

How I can decrease that? As tubenit says?

[sebbes]

That ain't right.  You would only reverse the OT wiring if the NFB insertion remains at V1B.  If the NFB insertion is at V3B as I drew it, the OT wiring would remain stock.

Thx but I can’t find your drawing with the NFB at V3b.

[2deaf]

Thx but I can’t find your drawing with the NFB at V3b.

It's in Reply #13.  I'm calling the triode with pins 6, 7, and 8 valve 3B.  It is the last stage before the 6V6.

[Williamblake]

You can reduce base changing cathode bypass capacitors or by putting a high pass in front of the recovery stage or just mess the 500p "dwell"-capacitor. As hum is mostly generated in the recovery i'd start there.

[2deaf]

I moved the NFB in tubenit's drawing so that it is on a tube that is labeled V3a and so that it lines up with what tubeswell was saying.  Hopefully this will clear things up.

 

[tubeswell]

And (besides the earlier comments about the NFB) this is what I was saying about changing the 'dry vs wet' signal levels (prior to going through the 'mixing stage' - V3a)

[sebbes]

Thx 2deaf, williamblake and tubeswell for the update. I will keeping you in touch.

[sebbes]

And (besides the earlier comments about the NFB) this is what I was saying about changing the 'dry vs wet' signal levels (prior to going through the 'mixing stage' - V3a)

Yes understood, BUT, the nfb on V1 doesn’t work I don’t know why. No reverb.. the reverb works only when I disconnect it..

[tubeswell]

And (besides the earlier comments about the NFB) this is what I was saying about changing the 'dry vs wet' signal levels (prior to going through the 'mixing stage' - V3a)

Yes understood, BUT, the nfb on V1 doesn’t work I don’t know why. No reverb.. the reverb works only when I disconnect it..

Why are you putting the NFB on V1?

[sebbes]

And (besides the earlier comments about the NFB) this is what I was saying about changing the 'dry vs wet' signal levels (prior to going through the 'mixing stage' - V3a)

Yes understood, BUT, the nfb on V1 doesn’t work I don’t know why. No reverb.. the reverb works only when I disconnect it..

Why are you putting the NFB on V1?

Wanna say v1b as the first schematic right?

[sebbes]

Hi guys,
So I have tried to change the nfb on the v3a, that still doesn’t work.. is it really important to have it? because the better result I got at this point is when it is disconnected.