My AB763 Fender Deluxe Reverb Clone has low volume in normal channel.

[72Blazer]

The normal is just not loud anymore.  You have to peg volume to reach any breakup.  Vibrato channel is loud/operating as expected.  I have checked bias, swapped in new V1 tube and also tried just swapping V1 and V2.  All tubes check out ok on my tube tester.  I have verified good resistors in channel one.  I did not check the caps but I really don’t suspect them.  Voltages are good on pins of V1.  Resoldered the ground and measured continuity.

The only mods to the circuit is Sluckey’s Trem-O-Nator and reverb and tremolo in both channels.

On the Ab763 circuit, I am used to the normal channel being louder than the normal  channel but that is not the case here.  Big difference.

Thoughts?
Vr
J.

[stratomaster]

Post voltages as outlined in my response to your last thread.

Post schematics as you've altered the circuit.

Post photos of the build if you have them.

[Latole]

Forum rules

Before posting amp problems on new builds, read this.

https://el34world.com/Forum/index.php?topic=19361.0

[72Blazer]

Thanks fellas.

Here are some pics and another symptom:  The treble pot on the normal channel does not affect tone until it is turned about 50%-then I get more treble but not full effect it should

[72Blazer]

More

[72Blazer]

Missed this one

[72Blazer]

Voltages

[Latole]

Post schematics as you've altered the circuit.

Schematic is a must fo us

You have to check all your work with the schematic or an original layout

[72Blazer]

See Layouts attached.  Again, this amp has Sluckey's TON mod-which works fantastic.

[stratomaster]

In circuit board layout there are concepts called clearance and creepage.  They exist to minimize crosstalk and potential for shorting.  From what I've seen in these photos I think it would be good to read to on these concepts and integrate those ideas into your component lead management, lead dress, and layout. 

I'd be willing to bet a lot of your mystery problems will vanish once you go through this exercise. 

[AlNewman]

Looks like your 2nd jack switch may be open?  I'm not sure what it does, because the layout posted doesn't match your amp.  How do your inputs work? 

[Latole]

Before modifying a circuit, it might be better to make sure that the original circuit works properly.

Sorry to say it, but looking at your photos I'm not surprised that this amp has problems.

[mresistor]

I have never seen anything like this  wouldn't the resistor be better installed down on the board?  The leads are so close to other leads.. 

[Latole]

I have never seen anything like this  wouldn't the resistor be better installed down on the board?  The leads are so close to other leads..

Very bad job , not suprise about the issues.

[stratomaster]

Does your Tremolo work with that particular optoisolator? I had a hell of a time getting that type to do anything I wanted out of the trem.  I got severe volume drops with higher Intensity settings.

[mresistor]

Did the normal channel volume drop after the reverb and tremolo was tied to the normal channel too?

[72Blazer]

Gents:  Thanks for time looking at the amp and your comments-really.  This is a long post, but I want to address all the comments as you took the time to critique/recommend.

I think I need to better articulate what I have stated in the opening post of this thread.  My exact words were:  “The normal is just not loud anymore.” The amp is healthy with the exception of my perceived lower volume in the normal channel and the newly discovered issue with the treble pot in the normal channel/operational effect.  All voltages seem good to me/based on original layout.

The tremelo channel is working like a charm.  Nice tone-loud-and all control s are operational.  The amp is dead quiet and tremelo/reverb is present in both channels (by choice).

Let me address all the comments:

Latole:  Thanks.  In my opening post, I could have better articulated that this volume issue is new to me/as I best recall.  This amp has been working for two years although it had noise and Sluckey recently recommended the following:  “…V4A and V4B cathodes share a single 820Ω/25µF. Separate the cathodes and give each one it's own 1.5K/25µF. Does this help?”  I did this and the noise issues were resolved-although it is a hasty fix that I will revisit/tidy up.  I am contemplating reversing that “band aid” fix to see if this is potentially affecting the normal channel volume.  See image6.jpeg to see what I am referring too.

All:  As far as the multiple comments about the lead dress, I moved a lot of the lead dress specifically and with the intent that you all could see the wiring more clearly.  Especially the control pots.  This has not affected the amp's operation/issue with the lower volume in channel 1 (normal channel).  Lead dress will be returned as it was and given more attention in general.  Per the comments received.  Again-thanks.

Stratomaster:  (1) I will look up “clearance and creepage” .  I admit I have never heard of those terms.  (2) Yes Sluckey’s TON is working and it is wonderful. 

AINewman:  I concur, both input jacks (low impedance side) are open.  I’m surprised I am not getting noise because of this.  I will repair/replace as needed.  Thanks.
mresistor:  (1) Ref: your screenshot:  The resistor is installed on the board.  The capacitor leads have been moved to get more clearance from the other components. (2) you ask a good / valid question in reply #15.  I will investigate that too.  Thanks
Lastly:
I am not a trained electronics technician and I have a full time job.  I've had no schooling other than the sometime hard knocks here on this website. 

I just love music, guitar, and these classic amps.  This is just a hobby for me. 

I currently have all 12 amps I have built.  3 Super Reverbs, 3 Bassman’s, 1 deluxe reverb, 1 Fender Pro 5E5-A, 1 Champ, 1 Marshall 18 watt, one JCM 800, and 1 Marshall 2550.  Some are tried and true working great/low noise.  My favorite amp thus far is my 6G6B Bassman which has some mods I learned from watching Psionic Audio’s YouTube videos.  Recently, as you have seen I have been revisiting the AB763s as I have struggled tremendously with this circuit). 

I built all these amps over the last 14 years.  My intent recently is to make them better and quieter. I have experiment with PPIMV master volumes and determined the hard way I’d rather use pedals. 

I will never build the AB763 circuit again!

This feedback has helped tremendously over the years.

Respectfully,
J.

[tubeswell]

I wouldn’t give up on account of the criticism here. Perfection is forged through striving, which is meant to be hard. Lessons learned are the main things to walk away with. I agree the lead dress looks terrible. Part of getting a good outcome is OCD attentiveness to being neat and tidy. Method is discipline.

[tdvt]

As far as the lead dress, the first thing that I am noticing is the large amounts of excess lead length, both the components & the wires.

I would think things would be far less busy & easier to trace/troubleshoot if much of that was edited out.

Elegantly simple is what I try to end up with. I am always reminding myself that about playing too...

[72Blazer]

Discovered something:  On the normal channel, if I disconnect the 6.8K resistor entirely from the bass pot-I get my full volume back.  But without the resistor in place the treble and bass pot do not work/affect the tone.  I tested the treble and bass pot and they have a good sweep.

IOW, the 6.8k resistor is really dulling the tone and has a drastic affect on the volume of the normal channel.

[AlNewman]

Your tone knobs won't have an effect on the tone without that resistor because that is the only path to ground.  Basically you just turned on a raw switch.

[tdvt]

Your tone knobs won't have an effect on the tone without that resistor because that is the only path to ground.  Basically you just turned on a raw switch.

Agreed, tone stack is out of circuit that way. Is the 6.8K still 6.8K?

Regardless, maybe temp in a 25K or 50K pot in that position, find your sweet spot & replace with a fixed resistor of that value.

[acheld]

I currently have all 12 amps I have built.

I admire this!   Well done.   

As far as thoughts about the AB763 design, my personal bias is that it is an incredibly versatile platform.  It ain't perfect at everything, but if you understand this circuit, you can take your variations to the stars and beyond. 

[pdf64]

... Regardless, maybe temp in a 25K or 50K pot in that position, find your sweet spot & replace with a fixed resistor of that value.

The amp's performance has been  reported to have changed.
It's much better to try to find and fix the actual root cause issue behind this, rather than to modify the original design to make up for the signal reduction.

[mresistor]

Agree with pdf64. In fact I would take all the mods out and see if the original circuit is working properly and if not then troubleshoot it. 

[tdvt]

I agree with you two, first things first.

Get it running as designed/drawn, then proceed with tweaks.

It wasn't clear to me whether the Normal channel lower volume was actual or perceived, when compared to the Vibrato channel.

[72Blazer]

Issue solved.

First off to clear some things up and statements I made:  My conclusion now is that the lower volume in the normal channel must have always been the case.  Remember I am revisiting this amp after it has set on a shelf for awhile. 

Also, when looking contemplating removing some mods, I realized that I did not have reverb/trem in both channels (no mod) like I thought.  When I was looking at/realizing that, I noticed a wire end coming up from the bottom of the main board at point "Y" on the layout where it connects with the .01 cap.  Indicative of an "under the board" jumper wire, I quickly determined it was and should not be there.  I then traced that wire to it's origination point which is where the 2 x 220k resistors meet (look at where point "X" and "Y" are on the layout.  I totally removed this wire and issue with the lower volume in the normal channel has been resolved to include with the 6.8k resistor (on normal channel bass pot) connected to ground.

I do not have the knowledge to understand the reasoning behind the cause and effect of the mistaken jumper wire connection but if anyone can explain or cares to explain, I'm all ears-although it will probable go in one ear and out the other.  lol

There were quite a few suggestions on how to troubleshoot this and I do appreciate them all.  I was torn between completely turning the circuit back into a stock AB763 OR removing one mod at a time.  I chose the latter.

I have good/loud volume in both channels and all tone controls function.  The TON mod is still working great.

Thanks all.

Vr
J.

[stratomaster]

I don't understand your description of the connection that should not be there.  There absolutely should be a wire from Y to Y.  That's the point of the Y. 

Can you draw this out, preferably on the schematic?

If you connected to the wrong end, then maybe you were bleeding signal to ground through the Intensity pot without the benefit of the 3rd gain stage. That will reduce gain, but not to an objectionable level.  That's how amps like the Showman are wired on the vibrato channel. There's no lack of volume there

[Dave]

I have a small, unoffensive comment about lead dress. Sure, yours could use some work. But... I have seen a lot worse on factory Fenders from the 70's. Just saying.


Dave

[72Blazer]

Here is a quick illustration of the "ERROR" jumper wire that was installed and then subsequently removed resulting in the "FIX".

Vr
J.

[stratomaster]

Interesting. Yeah, that looks like it'll put about a 30k load to AC ground at the end of the normal channel mix resistor, while bypassing the mixing resistors entirely for the vibrato channel. 

This bled normal channel signal to ground (about 85% of it) while also reducing a bit of attenuation on the vibrato channel.  Also it would've injected high voltage right into the PI grids if it weren't for the small input cap that I always considered overkill at 500v-1kV or whatever the schematic value is. 

This definitely explains it.

[tdvt]

Here is a quick illustration of the "ERROR" jumper wire that was installed and then subsequently removed resulting in the "FIX".

Glad you were able to track that down & resolve the issue. That mis-connected wire could have been really tough to find. Good job pushing through.

[72Blazer]

Thanks fellas.  A real knucklehead mistake I must say .  Wish I could travel back in time to 1981 and pick an electronics technician job instead of the radar "scope dope" job I picked when I enlisted in the Air Force.  Perhaps then I might understand the the detailed explanation stratomaster stated.

Y'all Rock on!

Vr
J.

[stratomaster]

Thanks fellas.  A real knucklehead mistake I must say .  Wish I could travel back in time to 1981 and pick an electronics technician job instead of the radar "scope dope" job I picked when I enlisted in the Air Force.  Perhaps then I might understand the the detailed explanation stratomaster stated.

Y'all Rock on!

Vr
J.

Your mistaken connection made it equivalent to adding a 250k volume pot to the end of the channel and turning to up to about 2.

[72Blazer]

Got this amp stable and quiet in both channels.  Also it has zero noise present with all the controls turned down.  Reverb and the TON vibrato are great.  I am pleased.

However, the normal channel has a little too much bass IMHO and not enough treble.  It does seem the treble pot really needs to be turned quite a bit (8) and the bass on (2) to get the brightness I want.

At any rate, the normal channel is stock per the schematic way above in this thread. 
.1uf cap, 0.33uf cap, 250pf silver mica, and the .o47uf cap.   

Any recommendations on substituting different value caps in this tone stack too brighten it up?

EDIT:  Actually, I was playing an Epiphone Les Paul Standard.  I just tried it with my G&L Asat and I can live with the tone with that guitar.  It would still be nice to know what mods could be done to the normal channel to make it a little brighter.  Thanks

Vr,
J.

[tdvt]

If it is the stock circuit, you can reduce the value of the bypass cap(s) on V1 which will attenuate some bass, depending on the value selected.

Stock value is 25uF, & you can go down to less than 1uf, with a range of 1uf-10uF being a typical mod. No cap at all would lower your gain for that stage.


Try out this calculator to choose a -3dB point.  LINK

[Esquirefreak]

Or try 330-470pF treble cap in the tone stack. My ears usually prefer 390pF for some reason. It lets a little more of the low treble/high mids come through.

/Max

[stratomaster]

Swap the 0.033 and 0.047 caps. Move the 0.047 to the mids position in the tonestack for a significant reduction in sub bass and low mids that tend to get muddy.

The 0.033 as a coupler reduces bass on paper as well, but in practice you probably won't hear much of an effect. It's just an economical change because both parts are already on hand and installed in an easy to service amp.

[mresistor]

Try a simple bright cap    install a 120pf cap from volume pot center terminal to the right side terminal.  See how it sounds to you. Like in the bigger amps.


Were you pugging the epi Les Paul into #2 input?  Try that as well.  I like to plug my twin humbucker guitars into #2 input. Single coils into the #1 input.


Pic lifted from SR AB763

[pdf64]

47nF is the correct value for the mid cap of a DR AB763 https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_ab763_schematic.pdf

Yes, a bright cap seems a good idea to brighten up the response.

However, unless there's a buffer between guitar and amp, plugging into the 2 input may tend to reduce the treble output from the guitar, due to the much lower input impedance.