troubleshooting fuzz, oscillation AB763 clone

[MrPink]

Started up the amp that I played very loud just an hour before and now it's just fuzz and oszillation, specially on the low end. whole thing sounds like a cheap fuzz pedal
swapped tubes around, even power tubes, cleaned tube sockets, nothing changed. disconnected nfb loop

tapped around with a pen looking for some changes moving wires etc. did this quite a bit. specially on sockets
Screen and Grid resistors are fine. voltages on v2 v4 v6 are normal. no tubes in other spots. ss rectifier
power tube voltages are fine. bias is fine. even speaker and speaker cable are fine. wiggled around tubes while strumming looking for bad connection in a socket, didnt find none.
nfb resistor measures 82 ohm in circuit instead off 820. this is weird, as i put it there myself and it's 820 definetly. guess it got to do with the circuit? but anyhow disconnecting the nfb circuit didnt make a change.
more hiss and background noise than before as well. used to be dead quiet, now there is some.

don't know what happened here as it was all good just an hour ago, glad for any hints on where to start trouble shooting

[acheld]

Try switching the blue and the brown wires to the power tubes.

[MrPink]

not long enough to be switched.. also it used to work like it is
just finished resoldering power tube sockets, still only fuzz

[Dave]

Pull all the preamp tubes and see if it still does it. That will eliminate the possibility that you developed a preamp issue and thought it was a PA issue.


Dave

[MrPink]

It only does it with a signal, otherwise it just hisses more than before and the volume pot became a bit scratchy.
playing a guitar through it sounds like either fuzz pedal or broken speaker, very metallic with endless sustain/oscillation

[separateness]

Maybe an open in the NFB path? Barring that the way to do it is to go stage by stage , divide and conquer to zero in on where the issue is. A scope would be helpful. Make sure every grid has has its path to ground (input resistor, tonestack/volume pot etc) intact.

[tubenit]

It will be an easy troubleshooting thing to do.


Check to see if the reverb tank cables are hooked up properly. If they are backwards, it can give a fuzzy sound.


With respect, Tubenit

[MrPink]

I don't know what exactly was the issue, but resoldering all the nfb loop, all ground connections and replacing the 100ohm nfb resistor turned out to solve it, now all is good again. hiss also disappeared. I have a feeling it was a bad ground for the nfb loop and not the resistor
also i added the robrobinette ice pick mod cap (220pf as i had this laying here) to the 820ohm nfb resistor

now i'm curious about oscillations in general
I have 510ohm screen resistors installed, read somewhere that 1kohm might improve things oscillationwise? a lot to learn here...
anyhow, all is sounding good now, thx

[pdf64]

...
also i added the robrobinette ice pick mod cap (220pf as i had this laying here) to the 820ohm nfb resistor

now i'm curious about oscillations in general
I have 510ohm screen resistors installed, read somewhere that 1kohm might improve things oscillationwise? a lot to learn here...

If you're going to get serious about this, especially if tinkering with nfb loops, you need basic bench kit, eg sig gen, dummy load, scope. Ideally a reactive load, that behaves similarly to a real cab, too.

[mresistor]

Mr Pink  you look lovely   nice long hair   pretty   woman I have no idea how you got here..