My AB763 doesn't like NFB

[Blind Lemon]

First build in quite a while. Got to thinking about a single channel AB763 so I dialed up the Hoffman layout and used Pro Rev transformers.

Got it all finished up and the startup went really smooth, that great BF tone. (Thank you Mr Fender and Mr Hoffman)

Except ................ At about half volume it sounded gravelly and a little farty. Blocking distortion??????????
Don't know if that's right but that how I proceeded. (Changed out the tubes first to known good ones, didn't help.)
Did almost all the Aiken's suggestions. No help.
I have a 3 way NFB switch on it and had been running it on the normal 820 and for whatever reason decided to change it to different setting and the
less NFB I fed it the clearer (relative) the sound got. Removed the NFB and the gravelly, farty sound went away.
So my question is, "why doesn't my amp like NFB?"
BL

[sluckey]

Hey Blind Melon! Good to see you. Maybe you just need to swap the OT primary plate leads.

[Blind Lemon]

Thanks, been doin great, been retired a year last June and don’t miss it a bit.


New even occurred to me to swap the plate leads, usually it’s the squeal when you know to swap’m.


I’ll give it a try in the morning.


BL

[Blind Lemon]

WOW that was a loud squeal.
I don't think swapping the plate leads from the OPT is the thing to do.
Let me get a few thing s together and gather my thoughts.
BL

[sluckey]

Haha! Well now you know for certain. I always swap the OT plate leads on a build with NFB just to be certain. It's usually obvious with AB763 amps but some amps it's much more subtle.

Since your amp sounds better without NFB, I'd start by closely examining the PI cathode circuit for wiring error or incorrect valur component. A very popular mistake to make on this amp is to use a 47K or even 470K resistor rather than a 470Ω resistor for the cathode resistor.

Are you using Hoffman's layout?

[pdf64]

Where on the chassis did the NFB switch get put?
How about just disconnecting all wiring to the switch, and putting it stock?

To ensure NFB rather than PFB, just need a sig gen and scope.
The primary leg that’s in the same polarity as the secondary ’hot’ output/s connects to the ‘upper’ output valve on the schematic (make sure not to swap the phase splitter outputs around).
Other than the AB165, all regular valve guitar amps follow that same pattern.
https://el34world.com/charts/Schematics/files/Fender/Fender_pro_ab763_schem.pdf

[Blind Lemon]

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.

[Blind Lemon]

Oh, and I pulled the tubes for the trem and reverb so they weren’t contributing to the sound


BL

[pdf64]

Could a recording of the noise be made?

[Blind Lemon]

Give me a bit, getting a haircut.
BL

[Blind Lemon]

Not sure how to upload it, can't do a .mov.
BL

[pdf64]

Try YouTube then copy a link to the thread.

[Blind Lemon]

Not sure you'll get anything out of this.
Toward the end on the low E you can hear it.
Its different in person.
https://youtu.be/NNMqbsX7UlY

[Willabe]

Part of that sounds like vibration from something in the amp?

I can hear some fizz/fuzz also, but much more vibration sound.
 

[66Strat]

It sounds more mechanical than electrical to me. Maybe a baffle board rattle?

[mresistor]

Ditto

[Blind Lemon]

Chassis is isolated setting on the bench and happen in two different areas of the room.

I thought it might be a speaker at first but played through two different speaker cabs.
Same story.
BL

[Willabe]

What is the switch by the rectifier tube socket? Is that for switching between tube/SS rectifier? I can't really see it, but it looks like you have 4 x 1N4007's on the socket?

And you have 2 switches on the front of the chassis on the power amp end, power and standby?

Are you sure you have all that wired up correctly?

If you haven't seen this yet, here's a great way to double check your builds wiring from Doug;

https://el34world.com/Forum/index.php?topic=17701.0   

[Blind Lemon]

"What is the switch by the rectifier tube socket? Is that for switching between tube/SS rectifier? I can't really see it, but it looks like you have 4 x 1N4007's on the socket?"Bingo
"And you have 2 switches on the front of the chassis on the power amp end, power and standby?"Correct again
"Are you sure you have all that wired up correctly?"Have a look, I am filled with al sorts of doubts right now.
On "off" it tube on "On" its SS. Its my understanding with the switch to ON the diodes have a lower resistance path.

[Willabe]

The yellow wire going from the rectifier pin 2 to the switch, that should be moved to pin 8.

Pin 8 is the rectifier tubes cathode connection.

[Blind Lemon]

I'll move it, but electrically it the same??

BL

[pdf64]

I'll move it, but electrically it the same??

BL

What’s the rectifier type?

[thetragichero]

maybe a dumb question but: are your phase inverter ground and output jack ground on the same ground node?

[Blind Lemon]

64 - 5U4


Chassis ground different points.


BL

[Willabe]

I'll move it, but electrically it the same?

Only on some rectifier tubes.

Some rectifier tubes have an indirectly heated cathode, 5AR4/GZ34 and 5V4. They have a heater and a cathode. A 5Y3 and 5U4 have directly heated cathodes, the heater is the cathode.

With the 5 volt indirectly heated rectifier tubes we use, they use pin 8 as the cathode connection. Pins 2 and 8 are the heater connections. But pin 8 is shared by the cathode and 1 end of the heater. So you want to pull the B+ current from pin 8, not from the heater, pin 2.

If you try and pull the B+ current from pin 2 with an  - indirectly -  heated rectifier tube, then your trying to pull all the B+ current through the heater, instead of from the cathode. And that heater is not made for that current draw.

So if you want to be able use different rectifier tubes, then the B+ should always be taken from pin 8.   

[Willabe]

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.

So now it makes the same noise when your using either the tube rectifier or the SS rectifier? 

Does it make the noise with the tube rectifier with that gator jumper removed from across the metal heat sink R?   

[Willabe]

And now that your sure you have the phasing correct on the OT's primary wires, I'd shorten that brown OT primary wire a good inch or so, to get it away from the other power tube socket and the heater wires.

[Willabe]

I'd also try lifting the grid stopper R's on the power tubes, pins 5 and 6, up in the air above the tube sockets.

Then lift those 2 blue input grid wires up in the air to get them away from the screen grid wires and away from the yellow -FB wire going to those 3 -FB R's.

See if that helps any.

Edit; Those power tube grid wires are pretty long, grid wires are sensitive to picking up noise and causing problems. You did twist them up nice and that will give them some protection, as they are out of phase from each other. This is good.

But input grid wires should be kept away from output plate wires because the input is out of phase from the output of a tube. And screen grids are also out of phase with the input grid, screens are in phase with the plate. The screens have way less current than the plates, but I still would keep the grid wires away from the screen grid wires. And running them parallel to each other is the worst way to run them for lead dress. The least interaction with each other would be at 90 degrees.

You might end up having to use shielded cable for those 2 grid wire runs.   

[Willabe]

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 sound and if I remove NFB altogether it still made that noise.

So at 1st the noise went away when you disconnected the -FB, but now it it makes the noise with or without -FB hooked up? 

[Willabe]

Since your amp sounds better without NFB, I'd start by closely examining the PI cathode circuit for wiring error or incorrect valur component. A very popular mistake to make on this amp is to use a 47K or even 470K resistor rather than a 470Ω resistor for the cathode resistor.

Did you do this yet?

Are you using Hoffman's layout?

 

[Blind Lemon]

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.

So now it makes the same noise when your using either the tube rectifier or the SS rectifier? 

Does it make the noise with the tube rectifier with that gator jumper removed from across the metal heat sink R?

Makes the same noise it all Rect configs. Have not moved to pin 2 yet.

[Blind Lemon]

I'd also try lifting the grid stopper R's on the power tubes, pins 5 and 6, up in the air above the tube sockets.

Then lift those 2 blue input grid wires up in the air to get them away from the screen grid wires and away from the yellow -FB wire going to those 3 -FB R's.

See if that helps any.

Edit; Those power tube grid wires are pretty long, grid wires are sensitive to picking up noise and causing problems. You did twist them up nice and that will give them some protection, as they are out of phase from each other. This is good.

But input grid wires should be kept away from output plate wires because the input is out of phase from the output of a tube. And screen grids are also out of phase with the input grid, screens are in phase with the plate. The screens have way less current than the plates, but I still would keep the grid wires away from the screen grid wires. And running them parallel to each other is the worst way to run them for lead dress. The least interaction with each other would be at 90 degrees.

You might end up having to use shielded cable for those 2 grid wire runs.

Will do

[Blind Lemon]

Since your amp sounds better without NFB, I'd start by closely examining the PI cathode circuit for wiring error or incorrect valur component. A very popular mistake to make on this amp is to use a 47K or even 470K resistor rather than a 470Ω resistor for the cathode resistor.

Did you do this yet?


Yes PI values are correct, double checked

Are you using Hoffman's layout?

 

Yes, Hoffman layout except moved bias pot and stretched board in the middle to get componentsmore in line with tube and to keep wires running to tubes shorter.
BNL

[66Strat]

Not sure you'll get anything out of this.
Toward the end on the low E you can hear it.
Its different in person.
https://youtu.be/NNMqbsX7UlY

Does it just happen when playing on the low E string? Have you tried playing through a different amp? It sounds like a mechanical rattle to me. Maybe string or bridge saddle rattle? There wouldn't by chance be a drum kit in the room? I've heard similar sounds coming from a snare drum.

[Blind Lemon]

It was first noticed using another guitar and was reproduced through a couple different speaker cabs.


No drums.


BL

[66Strat]

I've seen chokes and transformers develop rattles. When I was messing around with cars, I would use a long socket extension to isolate noises such as stuck valves/lifters to the offending cylinder by holding one end next to my ear and the other against the engine rocker cover. Maybe you could try something similar with a non-conductive material such as a wooden dowel rod to rule out or isolate any extraneous mechanical noise.

[Blind Lemon]

Wish I had a scope to isolate it to a certain part of the circuit. I made the changes suggested above to no avail. Even rigged up a listening amp to try and find it and it didn’t show where it was.




God, I don’t want to have to pull the board to check the underside wiring.


BL




Hey and I haven’t said it yet but I really do appreciate all the input.

[Willabe]

We've seen several amps that had a bad silver mica treble cap in the tone stack, or had a bad ground connection that acted like yours.

I'd try changing out the 250pF TS treble cap. Clip a heat sink on the caps lead when soldering and get in and out quickly.

Do your chassis grounds have a good internal star washer between the chassis and the terminal strip?   

[Blind Lemon]

Swapped out 250pf and checked grounds. No love.
Going to peel the board back and see if this could have something to do with it.
BL

[sluckey]

WAIT!!! Don't pull the board yet.

See attachment. You used a 10Ω where a 1M should be. Looks like you did the same for the two 1Ms in the PI circuit too. Confirm and replace.

If we had hi-rez pics like your last one we would have been through days ago.   

[mresistor]

I don't know Sluckey that third band looks green to me.  But at the top of that 1M resistor is there a connection underneath the board?

[sluckey]

Yeah, probably is. 

[66Strat]

I see Brown first band, Black second band, Blue third band, and Gold fourth band. This would correlate to a 1 Meg, 5% resistor. IDK as to underboard connection.

[dude]

Looks green to me when l enlarge it, kind of blue when not.
It’s usually a simple error in these cases and sometimes can be very hard to find.

[sluckey]

I see Brown first band, Black second band, Blue third band, and Gold fourth band. This would correlate to a 1 Meg, 5% resistor. IDK as to underboard connection.

Brown/Black/Blue is 10Meg

[Willabe]

I see Brown/Black/Green.

[sluckey]

Will someone please put an ohm meter on this resistor?   

[66Strat]

I see Brown first band, Black second band, Blue third band, and Gold fourth band. This would correlate to a 1 Meg, 5% resistor. IDK as to underboard connection.

Brown/Black/Blue is 10Meg

So it is. I get confused with multipliers and the second digit.

[Blind Lemon]

I don't know Sluckey that third band looks green to me.  But at the top of that 1M resistor is there a connection underneath the board?

Brown/Black/Green 1 Meg, no under board. Under board is between the .1 and 47K.
BL

[Blind Lemon]

Will someone please put an ohm meter on this resistor?   

.992M
BL