My AB763 doesn't like NFB

[sluckey]

   I really didn't think you messed that up.

[Blind Lemon]

Hell, I messed something up.       This thing is kicking my but.
I was able to make that change without lifting the board.
Here is the change.

I'm going to post some highres. of the guts.
BL

[Blind Lemon]

First set:

[Blind Lemon]

#2:

[Blind Lemon]

#3:

[Blind Lemon]

Ok the gold 25W dropping resister is bypassed because I wanted to take it back to stock voltages through the tube rectifier.
The amp makes the same distorted noise in any rectifier configuration.
The amp sounds clean with no distorted noise with guitar volume full up and the amp at about 10 o'clock.
Found the distortion with reverb and Trem operational, removed those tubes to rule those areas of the circuit out.
Same distortion whether tubes are in or out.
BL

[Blind Lemon]

For good measure here's the transformer side.

[PRR]

> that third band looks green to me. 

Go solar! Take it out in the sunlight! No indoor light is full spectrum (even the ones that claim they are). Incandescent is blue deficient ("is it green or is it blue?"). "Warm LED" is tailored to mimic incandescent, but has different flaws. Most fluorescents are really just a couple color spikes. A mix of incandescent and "daylight" fluorescent is not too bad, but both technologies are going out of style. I had large skylights in my last work place and it made a REAL difference in color reading.

[Blind Lemon]

Some Voltages:

[sluckey]

I don't see any red flags in your voltage chart.

[Blind Lemon]

I didn’t think so either, nothing that would cause concern.


I have another OPT I’m going to slave in tomorrow.


BL

[Blind Lemon]

Not the OPT.


BL


Starting to get frustrated.

[Soulfetish]

hey, took a short break to check in. It sounds to me like it could be ultrasonic oscillation. A scope would confirm. In lieu of that. Try connecting a 100pF capacitor across the phase inverter plates and see if that helps.
It would be best to actually compensate the output stage/feedback loop, but this usually helps as a generic value for audible instability

[Blind Lemon]

Gave it a shot, thx for the suggestion.
BL

[Soulfetish]

Wha' happened?

[premiumplus]

In the middle of the picture, the black wire coming in from the bottom right at the filter cap ground terminal looks like it might be a cold solder joint...maybe reflow that and double check all grounds for solid connection. I've had cold solder joints do very strange things. Also, the black wire wrapped around the white and yellow wires in the tone circuit looks like it is grounded at two places, or is that in the Bright circuit? Maybe you've got a ground loop issue?

[Blind Lemon]

Sorry guys, I took a little break from this. Had some other things to get done and thought I might have a fresh perspective when I got back.


Premiumplus: Thanks for taking a look at the pics, I’ll reflow that joint.


BL

[Blind Lemon]

Here we go, got a little time, have messed with it a little to no avail.
Still has a trailing distortion at about 1/3 volume and above.
Seems kind of drastic ....................... RIP THE GUTS OUT

[sluckey]

I feel your pain.   

[Blind Lemon]

Blank Slate. Not going to use any of the old components.
Going to use more spacing and the bias supply will be more traditional and off board.

BL

[Blind Lemon]

More

[sluckey]

I see from your pics that you are pressing the turrets the hard way. Put the pointy tool in the vise and mount the other piece in the drill chuck. Now you can load every turret into the board without them falling out. And you can go lickity split with the press. Watch Doug's short video...

&t=290s&ab_channel=EL34XYZ

[shooter]

Blank Slate.

fwiw;
did you do continuity tests on your material?
sneaky currents can cause complex problems

[Blind Lemon]

slucky: I've seen that before and need to change my setup.
Shooter: Man, I've built an awful lot of amps out of this stuff. I would really doubt it. But thx for the
             the suggestion. All suggestions are welcome.
BL

[Blind Lemon]

Boards ready to populate:

[Soulfetish]

I used under board wiring in my amp. Without easy access to the underside of the board, i will never do it again

[sluckey]

I used under board wiring in my amp. Without easy access to the underside of the board, i will never do it again

If you do it like Blind Lemon did you never need access to the underside again.

[Blind Lemon]

Populated:

[dude]

I’ve always used under the bd wiring, usually solid core, 20g. If you bend the wire over the turrets, you lock it in, like in BM’s new bd. If l have a low clearance bd like in an AO-43 chassis, l use a few dabs of silicone if l think a chance of snagging under the bd wires.
I feel for BL, probably something simple that can be very frustrating and difficult to find. I came close to doing what BM’s doing but so far found the culprit, usually something simple. I feel for ya..!

[Blind Lemon]

On the under board wiring I usually weep a little thin CA under the wire to hold it down (up).
BL   (not POOP  )

[dude]

On the under board wiring I usually weep a little thin CA under the wire to hold it down (up).
BL   (not POOP  )

You got me, "Bowel Movement", I was thinking Blind Mellon,  Just showing my age, sorry about that

[Blind Lemon]

WELL HELL 
Same issue. All new board and components. New wiring to the tubes except heater wiring. I use 18g heater wiring and I re-flowed all solder joints. Pulled all the pots to double check values and wiring, all controls work as they should. I am thinking of rewiring the rectifier tube socket and rerouting the wiring, although I have wire rectifiers like this before. Disregard the diodes at this point. Probable move the grounds around on the power transformer.
Don't know if I mentioned it previously but I have also slaved in a new OPT.
BL

[Blind Lemon]

Also, this is built in a Twin chassis, I have an old Pro Reverb cab and wanted to be able to use it. Pro and Twinn reverb have the same size chassis and I couldn't come up with a suitable Pro Reverb chassis so I got the Twin chassis.So if you look at the way the tubes are laid out it is V1,2,3 and then reverb transformer then V4 and V5. Could the placement of the reverb transformer be causing issues? I'm grasping here.
Would lifting the power to the reverb transformer be a way to test?
BL

[sluckey]

I'm using Hoffman's single channel AB763 layout as a reference.

A couple suggestions to help narrow down the problem...

1. Totally bypass the reverb circuit. Follow the red wire from V1-6 to the board. Disconnect this red wire from the board. Now disconnect the red wire from V3-6 socket. Connect the two dangling red wires together and tape the exposed connection. Now you have V1 connected directly to the PI. Remove V2 and V3. Play guitar. Is the problem still there?

2. Totally bypass the tremolo circuit. Remove V5. Disconnect the wires from the INT pot wiper and right lug. Connect the two dangling wires together and tape the exposed connection. This connects the bias supply directly to the two 220K grid resistors. Play guitar is the problem still there?

[Blind Lemon]

It's going to be a bit, got to go out for a couple hours.
BL

[Blind Lemon]

Yes, after removing the reverb and trem circuits the noise it still there.
I hadn't replaced the first 47uf cap in the power supply so I replaced it. That didn't help either.
Even change speaker cabs thinking that maybe in all this I hurt the speaker in the 1X12 so swapped to a 2X12.
I had replaced the tubes with new ones after I replace the board also.
BL

[Blind Lemon]

Slaved in a new choke, didn't help. 
Lifted the ground to the tone stack, didn't help. 
Heater wiring?????
Rewire rect tube socket????
Power transformer?????
BL

[shooter]

are we still here;

Removed the NFB and the gravelly, farty sound went away.So my question is, "why doesn't my amp like NFB?"

if so a scope should see it.

[Lectroid]

@All,

I have nothing to contribute but encouragement.  This is an epic battle for the soul of BL's amp.  Don't give up.  We're all rooting for you.  As my granny said, "it's always in the last place you look."  Yeah, hokey but--she was not wrong...   Keep at it, I know you'll figure it out.

[Blind Lemon]

Shooter: No and don't have a scope.
Lectroid: Thx for the support.
I have a new Bandmaster power transformer, it doesn't have tube rect taps but I can make it work.Going to slave it in.
BL

[thetragichero]

find yourself an old, slow, boat anchor scope. absolutely LIFE CHANGING when it comes to troubleshooting things like these, and probably would've cost less than you've already thrown at this build. i'd say a scope is third on the list to anyone looking to troubleshoot/build (which frankly is about 80% troubleshooting anyway) amps to a meter and lightbulb limiter. find your local ham radio club/electronics surplus store and you're likely to walk away with one for less than the cost of a tank of gas

[Blind Lemon]

 
No luck

[shooter]

what about OT, it's "inside" the NFB loop, you already eliminated everything else inside the loop.
*assuming the symptoms are still;  works fine with NFB disconnected*

[SILVERGUN]

How is the secondary of the OT grounded?
If it's just through the speaker jack to chassis, try a separate wire

[Blind Lemon]

From post #61

Not the OPT.


BL


Starting to get frustrated.

[Blind Lemon]

Shooter: I know this is a long thread, see below from post #3.

Here's a couple pics of what I got. AB763 with a SS/Tube rectifier switch and NFB multi tap switch. (Robinette)

Put in a dropping resistor because in SS mode the voltages were too high. It makes the same noise both ways so I jumped the dropping
resistor and leave it on the tube rect for the stock Pro voltages.  Hope the way I explained that made sense.

When I was chasing the blocking distortion issue (?) I swapped out some caps and grid resistors. (Aikens)

Everything is back to the way the amp started out life.

When I got it back together and disconnected the NFB switch and sub'ed in an 820 NFB resistor it still made the gravelly farty soundand if I remove NFB altogether it still made that noise.

Soooo, I think I went down the wrong rabbit hole
the noise is most noticeable at half volume playing an F note at the 13th fret.

[SILVERGUN]

Also, what's going on with the blue wire coming off of the wiper of the bias pot?

Nevermind, I see that goes to the intensity pot, as it should

[Blind Lemon]

SG:
What post and what pic are you referring to?? The bias supply has been completely redone and the tremolo and reverb have been bypassed.

There are no blue wires on the bias suppl, the the bias adjustment pot is connected to the pair of 220k resistors.

BL

[SILVERGUN]

I would still try a separate ground connection for the speaker jack, just to rule it out.

[Blind Lemon]

I may have a line on a couple of scopes I can borrow, he said he had a Ballentine 1010A and a Tektronix 455.
He not sure of the shape either is in. Going to pick up in a bit.
BL