My AB763 doesn't like NFB

[sluckey]

Looks like I missed a lot while out in the shop. I've set up my bench with my Phoenix (AB763 with an additional Ampeg preamp). Anyhow, I can now do some checks for comparisons. Here's what my PI looks like. 500Hz signal adjusted for 1V_RMS at PI input. Got about 30V_pp at each PI plate. I'm done for tonight but I can do this again tomorrow (just not during Alabama/LSU game).

I think your PI signal still looks crappy but I have no idea what signal level you have at the PI input. That cap across the PI plates is a band aid fix. It's just masking the problem.

[Willabe]

The grid wires from the PI to the power tubes are pretty long. Even though you have a nicely twisted pair of wires there and it is a stronger signal there, they could be picking up stay signals.   

And since the problem seems to be at/after the PI, at this point I'd try using shielded cable for those. See if that helps. Worth a shot.

[shooter]

I've started my herbal meds so;

they could be picking up stay signals.

why ONLY when NFB is applied?
agree the Cap is just a band-aid, maybe.
If it stops the tube from breaking into oscillation and doesn't have a noticeable tonal effect
I'd just leave it, play the amp till you have a good sense there ain't more issues unseen/heard.
there are plenty of "professional" amp schematics that did just that, "boss we're shipping it"  tell R&D to look into it next Revision

[Blind Lemon]

At the risk of biting the hand…….


Geaux Tigers


Sooners got a bye this week..


Boomer


BL

[sluckey]

Geaux Tigers

Always a fun game to watch. Can go either way. IMO the game to watch this year is SEC championship, Alabama and Georgia, providing Alabama can hang in there.   

[66Strat]

It seems like you've tested all of the likely possibilities. Any chance that the PI tube or tube socket could be bad?

[Blind Lemon]

Not saying that a socket couldn't be bad, new Beltons, but I would think unlikely. I've had 2 new JJs and a good old RCA in that spot.

Anyone have any revelations overnight on this issue? I'm pretty much still game.

sluckey, what model Siglent are you using and what are your thoughts on it.
BL

[shooter]

Play it with the cap in til you are convinced there are no other "issues".


If it plays good and sound right then get another Fat Tire and enjoy

[sluckey]

I have the Siglent Technologies SDS1202X-E. Bought it from Amazon in April of 2020. Chinese prices were on a roller coaster at that time. I bought when the price was at the bottom of the track.

It's a very nice scope. I haven't turned on my Tek 465B since I bought the Siglent. Takes a bit of time to get used to the features and controls. If you learned how to use all the features of those wacky Fluke 190c scopemeters that FAA bought, then the 1202 will be a breeze. But if you only have experience with analog scopes be prepared to spend a lot of time pushing buttons and twisting knobs with the 1202 (or any of the new digital scopes for that matter).   

Here's the setup I used to get those scope pics... HP204D Oscillator set for 500Hz sine wave at 400mV_pp amplitude connected to amp input jack. Scope triggering to external connected to sync output from HP Oscillator. CH1 connected to PI input cap. Amp volume adjusted to provide 1V_RMS on CH1. CH 2 probe connected to PI plates.

Here's the schematic of the my amp. Basically the same as yours except the added Ampeg preamp and improved bias supply. More info on my website if needed...

     http://sluckeyamps.com/phoenix/phoenix.pdf

And here's my bench. I'll leave it like this while you're sorting your amp...

[sluckey]

Basic question... Are you using a 12AT7 for the PI IAW Fender specs?

[Blind Lemon]

Yes I am, re: 12AT7

Cleaning the shop right now, going to get some pre PI and post PI readings in a bit.
Good game last night.
BL

[sluckey]

This time tell us what signal voltage levels you have at the PI input and PI plates.

[Blind Lemon]

Good afternoon.
Got some pics coming. I not sure I understand all I know about what I'm seeing.
BL

[Blind Lemon]

No signal input, input grounded through jack.
Both pics bottom trace PI input between .1 and .001 caps.
First pic top trace PI output, V5P1.
Second pic top trace V5P6.
10X probes
BL

[Blind Lemon]

Heres a couple more pics.
Bottom trace CH1, top CH2
Also, this is into an 8 ohm dummy load

[sluckey]

That could be anything from hi freq oscillation with some 60Hz hum inside the amp, to bad probes, bad scope, poor ground connection between the amp and scope, etc.

Get some confidence in your scope and probes to be sure they are presenting the picture they should be. Be sure to use the probe ground clip. Look at the 1V cal output on your scope. Is it a clean square wave? Then look at the 6.3VAC filament voltage. Is it a clean sine wave?

[Blind Lemon]

Probes showed clean 1v square wave and a clean 6.3 heater.
Probes are the same also, traces stack nicely on top of each other.
BL

[Blind Lemon]

Steve, all these test today have been into a dummy load and 500hz instead of 82hz. 300pf across the output of the 2 200k resistors.

Also the reverb and Trem have been put back in the circuit also.
BL

[Blind Lemon]

Swapped out that PI tube and can't reproduce the fuzz.
Oscillating tube?
BL

[Blind Lemon]

Well I've had a couple buddies come by and play the amp and although the PI seems to have some roughness on the scopeit sounds pretty dam6 good to the ear. 
I really appreciate all the help and suggestions when it came to trouble shooting this problem.

 First time in a looong time I've had to use a scope, for guitar or even audio amps I've never felt the need. Probably would be good to keep one and
and some sort of signal generator.

Been kind of torn between finding an old analog scope or just get one of the cheap digitals and some sort of signal generator.I've got my iPad but I would prefer to have a stand alone. Suggestions are well come. Any suggestions would be welcomed.

And this is my 700th post.
BL

[shooter]

I'd rather be without coffee than my scope    (and I LOVE coffee!)
If you want to spend time tweakin, or repairing, especially signal path stuff they are worth the time n money and the young'ns think you're really cool 


at end of day, the "sound-check" with multiple guitarists IS the final say

[SILVERGUN]

Been kind of torn between finding an old analog scope or just get one of the cheap digitals and some sort of signal generator.I've got my iPad but I would prefer to have a stand alone. Suggestions are well come. Any suggestions would be welcomed.

I recently tried to "upgrade" my bench with a PC based scope and I just hate it and I will be downgrading to analog in the near future.
I just don't even want to try to get used to it. I don't need to record a trace or need stupid high bandwidth or blah, blah, blah...
There's just something that feels right about the immediate response of the analog scope that makes me want to throw the PC version against a wall. There's also lots of discussion about this on the web.
I'll hang onto the PC unit for a backpack, service call unit, or a backup.


The delay with the USB throughput ruins it for me.

[PRR]

> And this is my 700th post.

Congratulations!

> I'd rather be without coffee than my scope

You need to figure what is *really* important. Drugs before toys!

> I've got my iPad but I would prefer to have a stand alone.

Tracfone will sell you Android-flavor phones $29 and less. Get a refurb LG, they are decent, I've owned 4 this decade. You may have to buy "minutes" but there is a $15 starter. Do not let it register itself on the cell-net, it will stop nagging in a week. --- Ah, today's best deal may be "Samsung Galaxy A01 (S111DL) - RECONDITIONED" plus a 30-day plan, $25+$15+$5 is 45 bux. (I can't buy an iPhone cable for that.)
https://www.tracfone.com/phone/samsung-galaxy-a01-reconditioned

[pdf64]

Been kind of torn between finding an old analog scope or just get one of the cheap digitals and some sort of signal generator.I've got my iPad but I would prefer to have a stand alone. Suggestions are well come. Any suggestions would be welcomed.

I recently tried to "upgrade" my bench with a PC based scope and I just hate it and I will be downgrading to analog in the near future…

I got a Picoscpoe usb thingy a few years back, as my old old scope was getting a bit dodgy. Thankfully it carried on, as it took me maybe a couple of years to get conversant with the Picoscope user interface, but I’m good with it now, and can get it to do what I want pretty quickly, including stuff beyond a regular old scope’s capabilities. Some aspects of it are frustrating still though, and I’m thinking of getting a Velleman.

[Soulfetish]

We carried the Velleman line when  I worked at You Do It Electronics. They’re kind of a Strange cat in that, some of the products they distributed were total garbage. But there were other occasions where they put out some pretty decent products that were much higher quality. My buddy let me borrow a small battery powered multi waveform generator, with a bandwidth of 1kHz to somewhere in the hundreds of kHz. Plastic enclosure, with a bnc jack, and a simple interface. But the output was clean! And way more accurate than I was expecting.