Intermittent poping sound on tube amp when inputing a loud signal

[ELS122]

so I am building this guitar tube amp, it's not super high gain, having only 2 gain stages and an LTP PI.
it works ok but after like 20 seconds of it on, it starts making like short poping squealing sounds when I play loud.
I've narrowed it down to 1 output tube, I tried changing the tube between 3 other tubes, to no avail, tried changing the coupling capacitor from the PI, still the same, changed the bias adjustment range resistors, resoldered all the pins on the tube socket, still the same issue, only parts left that I haven't changed are the grid leak, grid blocker, and bias pot resistors, but I doubt they could cause this problem.

I don't have an oscilloscope, but I used a multimeter to measure the DC voltage on the grid of that tube, when I have to tube out, everything seems normal, when I have it in tho when the poppy-squealing noise happens, the DC voltage jumps up to sometimes even (positive) 60Volts, which would let me to believe that the tube is arcing from G2 to G1, (it's fixed bias amp so the cathode is grounded),
 
and if I remove that particular tube and use the amp then, then everything is ok, grid DC voltage is normal, so is the sound.
output tubes are EL34's, B+ is 680V, the screen voltage is 480V, I have biased them at 75% max dissipation. which puts the grid at around -35volts DC at idle

it seems it could be the grid leak resistor going open, but idk, would it cause the G1 voltage to jump up to 60Volts?!?, I measured it when the amp was cold and off and of then it measures ok, tried stabing it with a stick when the amp was on and no pops or anything

[Refrigerator]

I wouldn't rule out the resistors yet, might be worth measuring them to confirm that their resistance hasn't drifted if they're old.

[ELS122]

I already did, and all of them are well in 5% tolerance. I have also checked the bias board PCB for any signs of carbon tracking, cleaned all the flux off. still have that problem. I might change the grid leak resistor, just to rule out that

edit: I changed the grid leak and still has the pops, so it wasn't that.
only thing that I can think of that could be doing this now is the coupling capacitor, but I already tried changing that, and why would it not be arcing inside or whatever when the tube is out? 

I'm thinking it maybe is carbon tracking on the ceramic tube socket because that particular socket arced between the plate and heater wires before when I wasn't thinking straight and left the tube pins uninsulated and just floping around there 
but I doubt a slight mist of smoke on the tube socket could cause arcing between pins, but I'll clean it really well and see if that fixes it.

[Audiorepair]

You might want to borrow a scope to rule out (or in) ultrasonic oscillations.

[ELS122]

it's not that, it's intermittent shorting or opening somewhere, and it is heat dependant, it only happens when the amp has been on for a bit

[CaptDon]

If the output is class B push pull you have the bias way to high.
Even in single ended class A the bias is too high. If it were arcing
in the tube you would surely see it!! You may be causing the amp
to go into self oscillation. I have seen that happen at frequencies
as high as 100khz and it will surely destroy the output tubes by
melting the G1 grid wires from excess grid current. What is the
circuit layout? Transformer interstage coupling? Capacitor Coupling?
What is the output stage, single ended class A or push/pull AB2 or B?
You may need to change the gain vs. frequency negative feedback
loop as they often caused problems at certain load impedances.
Did you design it yourself or copy from somewhere else??

[ELS122]

it's a class AB1 amp, 4 output tubes (2 in parallel on each side), bias is fine,
when the pop happens, cathode current skyrockets just in the short moment when the "pop" happens, it only happens on that one tube, it's capacitor coupled,
it's basically a copy of the Marshall JMP lead amp, apart from some minor tweaks in the 1st stage and NFBL.
I can't see any flashes/arcs in the tubes.
well, the output impedance should be 15ohms but I have it connected to a 12ohm speaker for testing, doubt that the mismatch would cause pops like that...
I tried putting a completely dead tube in that place that barely conducts at all, after rebiasing ofc. it still had the pops but they were much more subtle,
but what was really weird, is that now the G1 voltage never went positive with that dead tube, it just went up 10volts, and cathode current also didn't skyrocket as much but that's expected since it's a dead tube.
I went over every solder join, used flux, and resoldered everything, cleaned the flux ofc. looked for bad traces, bad wires. everything was ok, and it still is having this popping noise.
all I can imagine now is that the coupling capacitor is shorting for that tube when G1 starts to draw current at loud volumes because it gets saturated...
weird that 2 coupling capacitors could have the exact same problem... but I'll try changing it anyway...
I even went to 6P3S tubes and still, that exact same tube was causing this problem.
I saw no arcing anywhere apart from the blueish glow that increased on those pops which would be expected...

edit: forgot to mention, the bias circuit is different, it uses 4 bias pots for each tube, and thus 4 separate coupling capacitors for each tube

and the amp is ok when I turn the volume down so it doesn't make those pops, bias is stable, volume too, no weird distortions, no red plating tubes, voltages are all normal... nothing out of the ordinary

[ELS122]

ok so I found the issue and the cause - changed the coupling capacitor and found that it was a 250V rated capacitor... 
also before that I measured 0 AC volts on G1, which I was pretty sure there was AC there before...
so yeah apparently when you overvolt a film capacitor, it goes open and can arc across afterwards

[Refrigerator]

ok so I found the issue and the cause - changed the coupling capacitor and found that it was a 250V rated capacitor... 
also before that I measured 0 AC volts on G1, which I was pretty sure there was AC there before...
so yeah apparently when you overvolt a film capacitor, it goes open and can arc across afterwards

Film capacitors have this self healing feature where they burn the metal layer off the film when they arc unitl the spark can't jump the gap anymore.
I was about to mention arcing caps but you said you checked capacitors so i decided not to because i thought it would be redundant. 

[ELS122]

I said I changed that cap, but the previous one was 250V rated too.
thank's for the info anyway! 

[ELS122]

ok so the problem came back after a few minutes of playing, and I continued to search for the problem, I drew a schematic here:
https://oshwlab.com/evalc2636/problematic-amp

(the 6N2P and 6N23P tubes, pin 9 is the shield between the 2 triodes, not the heater tap like shown in the schematic, so it's fine!)

I've changed:
the 2 main rectifying diodes for the main B+ supply from the old germaniums to some CO15... something (I forgot what was the actual name for them but I remember them being rated for 1.5kV and like 2amps), that didn't fix it, if anything it made it slightly worse perhaps.
changed the grid leak resistor and the ac ground capacitor on the PI NFBL side.
changed the CF resistor, the plate resistor for the stage driving it as well.
changed the coupling capacitor into the PI.
changed the NFBL presence pot.
changed all grid leaks on the output tubes.
changed all the coupling capacitors on the output tubes.

I made sure the output impedance is correct.
I grounded the place where the 470, 2 1Meg's, and 10k connect on the PI.
I grounded the NFBL circuit.
I measured all plate voltages, grid voltages, cathode voltages, everything seems normal (except the NFBL side of the PI which I'll explain down below)
I swapped tubes, tried a 6N23P instead of a 6N2P in the PI, it kinda made it a bit louder but still the same.
I moved all the shielded wires around to see if maybe there's some feedback somewhere somehow in some wire.
I moved the screen supply from B+, to B+1.
I tried biasing the output tubes super cold, and really hot,

still, after doing all that, nothing changed, still the same poping thing.

when it pops the volume gets lower, and then after around 1-2 seconds it starts fading back in, I then started measuring voltages to see which voltage related to what I was hearing, and that was the NFBL side of the PI, which the cathode voltage jumped really high, bias voltage was doing something super weird and my multimeter just showed overload on DC volts or AC volts (I measured the bias from the 10k to grid, not from the ground to grid because the 1Meg input impedance of my meter would probably mess with the readings a lot), the bias seemed super low, low being 0volts, and it was the same on both sides, but the NFBL loop side jumped all around, it never went positive really, it was always negative bias.
the plate voltage jumped high when it popped, and then slowly dropped exactly in relation to the volume fading back in,
the voltage on the 10k, 470, and 2 1Meg's, junction had exactly the opposite behavior of the cathode voltage, when it poped it dropped, and when it came back it increased.
this led me to believe it was the 470ohm cathode resistor, which changed that. still the same...

now I'm starting to think it's some sort of feedback loop within the NFBL side of the PI, but how could this be, even if I ground the NFBL network out?
the NFBL line goes right next to the plate wires for the output, the signal could have coupled over... but I doubt the signal would be nearly great enough to cause any oscillations if the other end of that wire (where the pots connect) is grounded.

I don't have an oscilloscope and I don't have any place I can borrow one from.

sometimes the amp pop's and then stays in some sort of state where it hums at 50hz, and it draws a ton of current from the wall, I would think that would be a clear sign of ultrasonic oscillation happening like one of you suggested was the problem already, the question now is tho, where is it coming from? 
and that has only happened like 2 or 3 times, which could be just a coincidence and I just had the instrument cable close to the OPT or something...

although now I'm thinking... maybe it is in my inputs because the junction of the grid and 22k get grounded not the junction of the 22k and 1Meg like shown in the schematic (couldn't find a 6.3mm jack symbol that was the same as the ones I have in the amp)... it could be causing some ground loop, and maybe it picks up something from the output since the 22k isn't there anymore and it won't cancel out ultrasonic frequencies.
the input wires going to the input jacks are both shielded, and both shielded wires are in another shield, all shields there are grounded at the input JACKS.

so I'll try wiring the inputs like normal tomorrow. meanwhile, feel free to suggest checking anything else.
for some reason, every amp I build is cursed with problems like this 


EDIT: so I tried just plugging in a random cable in the other input to break the contacts that ground the grid, that didn't fix it!


it's weird that these problems occur only if I have all 4 tubes in, if I remove any 2 it works like normal, I doubt it's oscillations in the output tubes because that PI side related to the behaviour exactly, meanwhile the output doesn't seem to do anything in relation to what I'm hearing, the voltage on the grid could be jumping up because of a spike in voltage on the PI plate, the screen and plate voltages relate to what the bias does, not what I hear from the output, so I really doubt it would be that.

I'm was thinking it was maybe within the power supply, but none of those caps are open, so it shouldn't be able to oscillate at a high frequency, the amp itself has nearly no audible hum from itself.
perhaps it's some feedback from the CF to the 1st stages? but again it would need to have open filtering capacitors to oscillate ultrasonically within the power supply I think...

maybe it's a grounding issue, but I've tested nearly every ground, and everything seems ok.

[Audiorepair]

Wiring layout/lead dress is very important in valve amps.

I once had an old head that would oscillate, turned out it was because some of the wiring routing had changed, and was no longer running close enough to the chassis.


You need a scope, as ultrasonic oscillations can start with small but very visible artifacts, then can suddenly go Pop at full power, causing the heavy 50Hz/60Hz ripple you can hear.

You can poke at the wiring, change the layout while observing the effect on the onset of catastrophic oscillation.

[ELS122]

every wire that has signal on it, is a shielded wire and is put as far from anything that can interfere with it as possible. the only exceptions would be the wires going to the output from the PI, but they're pushed right up to the chassis (I've checked and the insulation isn't damaged on those)
and the wire from the 10k on the PI, going to the 3.9k, and the NFBL circuitry which isn't shielded and is right next to the plate wires on the output (but as I already explained, grounding this doesn't help)

all the heater wires and B+ wires are string wrapped nicely 

I'm gonna take some pics and show you the layout!

[Audiorepair]

You need a scope.

[ELS122]

oh so me not having a scope is the issue of the amp? a scope will only show me what I know already, the oscillations are happening in the NFBL side of the PI, all the previous stages don't have any signs of the signal going through them while the oscillating is happening. and the NFBL side of the PI is the earliest stage where it happens, when I ground the NFBL the oscillations keep happening, which leads me to believe that it is not happening in the NFBL.
and I'm now pretty certain that the only way this could be happening is carbon tracking between tube pins and/or in the tube socket itself, between the plate and grid, most likely.

as a wise man once said:

there's no problem that can't be fixed with time and thought

oh and btw to this:
"You can poke at the wiring, change the layout while observing the effect on the onset of catastrophic oscillation."

I already told that in my post, I tried moving all the signal wires around, it didn't change


oh and btw #2, changing the NFBL amount doesn't change the oscillation at all, which would agree with what I am sure of, that it's happening in the NFBL side of the PI since that doesn't receive the NFBL signal (now it seems kinda ironic that I called it the "NFBL side"  )

[Audiorepair]

oh and btw to this:
"You can poke at the wiring, change the layout while observing the effect on the onset of catastrophic oscillation."

I already told that in my post, I tried moving all the signal wires around, it didn't change

You mean you tried moving the signal wire around and you didn't SEE any change.

You need a scope.

[ELS122]

oh and btw to this:
"You can poke at the wiring, change the layout while observing the effect on the onset of catastrophic oscillation."

I already told that in my post, I tried moving all the signal wires around, it didn't change

You mean you tried moving the signal wire around and you didn't SEE any change.

You need a scope.

YES! I didn't see ANY change, the only change I saw was when I changed the rectifiers to silicon which made it louder (as expected because the B+ increased), and changing the PI tube, which when I changed to a 6N23P from a 6N2P it made the amp louder, AND a lot more prone to this oscillation.

oh and by "see" I mean hear, measure (current draw of the amp and/or voltages on stages)

[Audiorepair]

Look,

you can have hf oscillations in a valve amp without ever knowing they are there, unless you can see them with a scope.

They can appear at the top and bottom of the waveform for just a short part of the cycle.

Eventually, given certain conditions, they can evolve into full blown full power hf feedback, which you then will notice.


You can observe these beginings of castastrophic oscillations with a scope, and see what makes it better or worse.
You won't hear or measure any of this with a multimeter.


You need a scope.

[ELS122]

Look,

you can have hf oscillations in a valve amp without ever knowing they are there, unless you can see them with a scope.

They can appear at the top and bottom of the waveform for just a short part of the cycle.

Eventually, given certain conditions, they can evolve into full blown full power hf feedback, which you then will notice.


You can observe these beginings of castastrophic oscillations with a scope, and see what makes it better or worse.
You won't hear or measure any of this with a multimeter.


You need a scope.

I already told you, I can hear and measure when it goes into this oscillation, it draws a ton of current and it hums. I can measure it too because the output tubes draw a ton of current, and I can measure that the oscillation starts in that one particular stage, which you don't care about, you only care about if I have a scope or not, instead of actually helping me troubleshoot this.

I see that I'm on my own here, thank's for nothing anyway!