My AB763 doesn't like NFB

[SILVERGUN]

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

Sorry, BL.
I got hit with a couple phone calls and wound up editing my post while you were posting. I was referring back to Reply #6 for pics.
And I'm just here to throw darts that you haven't yet.
I can't believe this is still going on. Good luck.

[shooter]

the noise is most noticeable at half volume playing an F note at the 13th fret.

that sounds like a rabbit hole of it's own 


might figure out what frequency F on the 13th fret equals and use that as your input signal for scoping.
your description sounds like a harmonic distortion which will be hard to "see" on a scope. 


when volume get to near max does the amp become normal?

[shooter]

Scopin;
Put ch 1(or A) on the tip input jack, ground to chassis
Put CH 2 (B) on the Speaker +, ground to chassis
 
Set your input signal amplitude to 100mV rms (everyone has their favorite #)
Leave it alone
Set tone pots to 5.  Bring volume pot from 0 to 10 real slow. 
See any oddities? Get a screenshot, image and post
If nothing jumps out
Vol 5  tone pots 0 to 10
Oddities?
 
 
The 2 pics are from my last build.  The 1^st is what you should see, both in and out the same.
The 2^nd pic is an intermittent harmonic distortion when volume gets ~~ 7-8


It’s still there a year later, I just don’t “warranty” volume >7

[Blind Lemon]

Hey guys, got the scopes. Will mess with them in the morning after I get the slaved in choke and power transformer out and the original parts back in.I'm going to configure the circuit without the trem and reverb as slucky suggested and try to find the distortion from there.

Since this is kind of a new start with this issue and this tread is getting so long, and not everyone has read all the post I was thinking of starting anew thread restating the issue, in the first post, as I have become educated with what it is and when this distortion happens.

What do you think, new thread or keep this one going?

Either way nothing is going to happen till tomorrow morning.
BL

[dude]

Why wound you even consider a new post…? Work out the issue right here. You have the greatest minds helping you.  I know this is very frustrating for you but a component or wire mishap, or whatever is is the culprit, you just need to find it. You’ve rebuilt the amp, obviously you repeated the problem..? Maybe not, but…, keep the post here.
You just need to step aside for a day or so, go over every wire again, check components, there’s a mistake somewhere..?

[Blind Lemon]

Ok I'll keep it here.
Working putting the original choke and PT in the circuit.
Also the scopes work, but the 455 I can only get channel 2 to read anything. The Ballentine works on both channels.
Back to the amp.
BL

[sluckey]

Have you tried another guitar with this amp?

What other amps do you have?

[Blind Lemon]

Yea, I've tried 3 different guitars with 3 different cables, 3 different instrument cables and the same number of speaker cabs.
Amps: Starting left to right.
TW Rocket no cab

EL86 Plexi no cab

Mongral no cab

5881 Princeton /correct transformers to support the 5881s - on floor

Stack:

Revibe


6V6 Plexi

TW Express like amp with 6V6s

EL34 Plexi w/1 input

50w HP Tweed Twin w/BF Super OPT

Fender Reverb unit

And one that I didn't build in black tolex behind the tweed is a hot rodded Princeton.


Not showing off, but these are a few of the amps I've built. Mostly scratch built with new parts and transformers.
I enjoy building and and playing.

BL

[sluckey]

Nice stable! My question was a leading one. I'm hoping that you can rig up to be able to patch your AB763 preamp into another power amp, maybe the 6V6Plexi (or another amp with a LTP PI). And also patch the 6V6Plexi preamp into the AB763 power amp. Still trying to determine if your issue is in the preamp or power amp. Look at this schematic for an idea how to easily do the patching, or just sit the amps side by side and use gator clip leads.

     http://sluckeyamps.com/november/november.pdf

On another note... I know we've been over this before but I gotta ask again since you have all new components on the board. There are five 1M resistors on the board, but my eyes only see two. The other three look to be 10Ω to me. I've attached your board with the resistors circled. Resistors 1, 4, and 5 look like brown/black/black (10Ω) on my monitor and phone. Resistors 2 and 3 look like brown/black/green (1M) to me. I'm probably just seeing them wrong but will you please verify they are all 1M resistors? See attached pic.

[Blind Lemon]

sluckey:
Yes I could slave in another amp if need be. Was hoping the scopes would be of use before all that. 
Yea, those one meg resistors have a sort of forest green color to them.
Here is a pic of the 1meg color and probably what you are use to seeing.
Going to eat a quick bowl of venison stew and be right back. Hell was hoping to get out to do some bow hunting today. Not happening.

BL

[Blind Lemon]

So here is where I am right now.
The amp is back with the original components, but the tremolo and reverb are jumped and removed from the active circuit with those tubes pulled. As suggested slucky post #83.
What I am hearing now that I have some history and have done some critical listening. At lower volumes (~8:00) there is a fuzz/grrr on top of the note then as the note kind of dies and a feedback bloom starts it sound like a box of rocks shake. I had a 5751 in it and put a 12ax7 in and it does it more.
So gain seems to have a hand in it.
BL

[Blind Lemon]

In these photos of the scope display the bottom trace is the input and the top trace is the probe @ the output of the 220k riz going to the input grid of the power tube.I am no expert at this, but is that fuzz on the top trace distortion? BTW it happens at the output of both 220K riz, 180' out from each other. I wonder why..........
I'm generating the freq. with a plucked open low E.
Help me understand or confirm what I am seeing.
BL

[shooter]

your input signal (bottom trace?) looks like it's missing a ground reference


and ya, those RF bursts are not good.  measure again at the "left side" of the coupling caps that feed the PI


MAKE SURE the scope is set for AC coupling.  there will be biggish DCV there

[Blind Lemon]

Got a few screen shots.
I'm using a freq. generator of of my iPad.
82hz (low E) @~ -4db
amp has volume control at about 8:00, using a speaker for load, I also have a bigass (technical term) 8 ohm load I could use.

Bottom trace is always the input.
First photo @ output jack.
BL

[Blind Lemon]

After PI.

[Blind Lemon]

Before PI

[sluckey]

PI input good. PI output bad. Looks like the problem is in the PI. Do both PI outputs look like crap?

[Blind Lemon]

yes

[sluckey]

Disconnect NFB wire and post new pics of PI outputs. Instead of calling them PI 1 and PI 2, call them pin 1 and pin 6.

[Blind Lemon]

Coming your way

[Blind Lemon]

Here you go

[sluckey]

Much better without NFB. I seem to remember that you have modified the NFB to some fancy robrob circuit? If so, can you bypass that and just use the standard AB763 circuit, ie, 820Ω and 100Ω (or 47Ω) voltage divider network? Also, how is your speaker jack grounded. I like to run a dedicated ground wire from the jack over to the power ground point, even if I'm using a Switchcraft 12A jack that automatically grounds to the chassis.

[Blind Lemon]

Not a great pic.
Thats the NFB to circuit wire flying up.

[sluckey]

Disconnect that NFB switch from the speaker jack and connect a 820Ω resistor from the speaker jack directly to the 100Ω onboard resistor, just like Hoffman did. Also run a dedicated ground wire between the speaker jack and your power ground point.

[Blind Lemon]

Did it.
Jumper wire to PT ground and 820 to the 100 on the board.
Same, same.
When I lift the 820 it cleans up.
BL

[Blind Lemon]

I'm replacing jacks.

[sluckey]

With the scope on the output does the signal amplitude change when you connect the NFB versus no NFB?

I'm beginning to suspect power grounding scheme. Can you take some more closeups that show every filter cap and it's ground connection. Probably take 3 or 4 pics to show it all.

[Blind Lemon]

Jacks didn't help
Give me a couple and I'll get pics
BL

[SILVERGUN]

Take a look at the ground path through the bias pot body, if that is still the way you have it set up.
Not only does that ground the bias circuit, but it handles the output tube grid leaks as well.

[Blind Lemon]

Here they come.

[Blind Lemon]

Nodes B,C

[Blind Lemon]

Preamp end

[Blind Lemon]

SG: Checked it a couple days ago and made sure it had one of the big lock/grounding washers under it.
BL

[shooter]

If the jacks don't work;
try putting a 500pF to .001uF across both PI plate R's (one cap on each R)

[Blind Lemon]

With the scope on the output does the signal amplitude change when you connect the NFB versus no NFB?

I'm beginning to suspect power grounding scheme. Can you take some more closeups that show every filter cap and it's ground connection. Probably take 3 or 4 pics to show it all.

Sorry, I missed this.
Yes volume decreases quite a bit.
BL

[sluckey]

Where is the PT HT center tap???

I don't like the B and C ground point. It requires current for those caps to travel through the chassis to get back to the HT center tap ground point.

Ideally you want the PT center tap, first filter cap, B cap, C cap, bias board ground, bias pot ground, and that power ground buss on the board to all be connected to the same chassis ground lug. Try moving the B and C grounds IAW attached pic first.

[Willabe]

The 1st B+ cap's ground is going through the chassis over to the PT B+ CT ground.

The B+ CT should go directly to that 1st caps ground lead and then run a wire over to your power amp chassis ground. Don't move the cap, move the PT CT and run a wire back to the chassis power ground.

Do what Sluckey wrote 1st and see if that gets rid of your problem. If it doesn't or only gets part of it then try what I wrote.

[Blind Lemon]

No luck on both accounts.
BL

[dude]

I always grounded the CT, nodes, A,B,C all to the same point with the speaker ground, bias ground. The preamp filter ground opposite end of CT, run a buss (picking up other grounds) hanging free on one end all to bolt on chassis near the input, l even run a short lead from input ground to that bolt. Never had any noise or any issues.

[Blind Lemon]

This amp was never noisy at idle or low volume. But I consolidated my power grounds to one place.
BL

[Willabe]

Did you test it after moving those grounds with the speaker jack still jumpered over to the chassis power ground?   

Are you using bias vary trem? If so that needs to get grounded with the power amp grounds.

How did you ground the power tubes K's?

[Blind Lemon]

Changes sluckey suggested.

[Blind Lemon]

No luck until I added this.
BL

[Blind Lemon]

Sorry.......... 

[Willabe]

The B+ CT should go directly to that 1st caps ground lead and then run a wire over to your power amp chassis ground. Don't move the cap, move the PT CT and run a wire back to the chassis power ground.

Leave the black wire you ran from the 1st B+ cap's ground, but move the B+ CT to that 1st caps ground lead. Your in this far, It may or may not fix it, but, it makes a difference.

[Willabe]

Guess your done for the night.   

[Blind Lemon]

This took care of it.

[Willabe]

    YES!!!!!!


Across the LTPI's plates?

[Blind Lemon]

So my first question was >>>>>> Why doesn't my amp like NFB.
I'm going to stop for this evening and take a look at it again in the morning, but the amp sound a whole hell of a lot better now.
BL

[Blind Lemon]

Yes, across the PI plates.
BL