Farfisa Tube Amp High Pitch Oscillation RVRB Problem

[sarakisof]

As i mentioned i don't get signal at transformer input too.

Tried to probe at trafo's input (so to find if it is the curlpit) for about some secs (no signal again by the way)

.
So the problem could be earlier (tube pins 3,6?).
ECL86 no output...

As for the hot resistor, i said it gets hot (maybe even hotter than 200C) only when i put my diy probe at trafo's input. In any other case, it runs cool normal.
That's why i was asking if that's normal. Maybe my diy probe tracer (cap resistor inside cable)?
When no probing there, either probing elsewhere or no probing at all just playing guitar, it is normal cool.

[sarakisof]

Repeated many times today probing at transformer input/ecl86 pin3,6 i get no signal. Only crackling sounds. So no ecl86 output. By chopstick tapping at ecl86 while probing there i can clearly hear "tap tap" from cheap probe-amp's speakers. Microphonic ecl86?

[Rob Strand]

As for the hot resistor, i said it gets hot (maybe even hotter than 200C) only when i put my diy probe at trafo's input. In any other case, it runs cool normal.
That's why i was asking if that's normal. Maybe my diy probe tracer (cap resistor inside cable)?
When no probing there, either probing elsewhere or no probing at all just playing guitar, it is normal cool.

If the problem *only* occurs when you connect the probe to the transformer input it sounds like your probe is loading down the circuit.  Perhaps to the extent where it is pulling  current from the HV supply then through the probe to ground.   That would cause high current on the HV rail and shunt out the audio signal on the plate of the tube.

You could confirm that by permanently connecting a multimeter to the HV supply or plate and watch the
voltage drop when you connect the probe.

Things that would cause that issue with the probe are low impedance input or even a low voltage cap
breaking down and shorting.

Repeated many times today probing at transformer input/ecl86 pin3,6 i get no signal. Only crackling sounds. So no ecl86 output. By chopstick tapping at ecl86 while probing there i can clearly hear "tap tap" from cheap probe-amp's speakers. Microphonic ecl86?

You can get microphonics from taps in many ways.  It's likely to be that.   

Do the DC voltage check I mentioned above and review your probe circuit.

The possibilities don't stop there.    Another possibility is the capacitance of the probe is loading down the plate and causing the circuit to oscillate.   One way around that is to put a 10k resistor in series with the probe.   You might lose a bit of level on the probe but the idea is to trade that against capacitive loading on the tube plate which could trigger oscillation.

[sarakisof]

Do the DC voltage check I mentioned above and review your probe circuit.

Just did it. By just touching the gnd /sleeve probe only to any of transformer's inputs it immediately shorts (0.4 volts) with scratching at speakers (short sound) and power resistor gets immediately ultra hot. This doesn't happen with the hot probe.

My signal tracer is like this one, with some slight variations at values if i remember well. Haven't opened it yet as i had put double heat shrink back then, but it's a series resistor, cap with hot and resistor parallel hot to gnd probe.

[sarakisof]

And makes sense as it shorts pin3 or pin6 of the tube 
By probing hot at pin6 and gnd to gnd -thsts the correct way i guess :p - i get only deep buzz.
If ecl86 isn't dead after having shorted its grid and anode pentode i guess it's the HighLander 

Don't know what i was thinking, i was confused by secondaries which are actually signal-gnd. I must have shorted out the tube now right? And that could explain that i completely lost reverb signal --> no splash (in my first efforts troubleshooting arcing the other days).

[Rob Strand]

And makes sense as it shorts pin3 or pin6 of the tube 
By probing hot at pin6 and gnd to gnd -thsts the correct way i guess :p - i get only deep buzz.
If ecl86 isn't dead after having shorted its grid and anode pentode i guess it's the HighLander 

Yes it all makes sense.  The black clip of the probe is connected to mains ground via the guitar amp ground connection.
Since the Farfisa also ground connects to mains ground when you connect the black clip to anything that isn't ground
it is the same as shorting it to ground.   For the tube that's like shorting the plate or power supply to ground.   The
power supply would be pulling a lot of current through the 1.5k resistor and it starts to cook.

I doubt there's any damage to the tube since the all the faults divert current from the tube.

The buzz from connecting one lead is hard to remove.   The best you could do perhaps would be to connect the
black clip to the Farfisa ground.   Maybe through a 1uF cap or 10 ohm resistor.

[sarakisof]

The buzz from connecting one lead is hard to remove.   The best you could do perhaps would be to connect the
black clip to the Farfisa ground.   Maybe through a 1uF cap or 10 ohm resistor.

I connect black probe to Farfisa's gnd and red to pin6 (ang get buzz).
To make it simple, if i would tell you "prob at trafo's inputs", this is the only way to go?

[sarakisof]

Update: OK i have signal at transformer's input!
So we are here again:

So upto now
- reverb driver works, signal makes it to output of transformer
- reverb recovery works, signal passes through recovery amp

So the unknowns remaining are the connections to the reverb tank, both tank input and tank output.

Can you tap the reverb and hear a spash?   I thought a few posts back you said that wasn't happening (?).
That would point to a connection issue between the reverb tank output and the input of the recovery amp (AC151 etc).

I don't hear splash. I will check tank's output

[Rob Strand]

To make it simple, if i would tell you "prob at trafo's inputs", this is the only way to go?

The last few posts were a result of the probe causing shorts giving a fault impression of lost signal.

You mentioned this a few post back.

When no probing there, either probing elsewhere or no probing at all just playing guitar, it is normal cool.

If we discard the problems caused by the probe it seems to work?
So now I'm confused about what isn't working.

[sarakisof]

Yes "cooked resistor" and "no signal by probing at trafo issues solved.
Just read at my last "Update" post.I get signal at trafo's input, trafo's out, tank's input and gets lost at tank's out.
But i get signal when isolating the tank's out from recovery amp and putting signal to rca recovery amp.
It's like signal gets lost through springs somewhere between tank's input and output transducers. Tank also works.
Also, by probing at tank's output i can hear splash through probe amp's speaker.

[Rob Strand]

Just read at my last "Update" post.I get signal at trafo's input, trafo's out, tank's input and gets lost at tank's out.
But i get signal when isolating the tank's out from recovery amp and putting signal to rca recovery amp.
It's like signal gets lost through springs somewhere between tank's input and output transducers. Tank also works.
Also, by probing at tank's output i can hear splash through probe amp's speaker.

Something isn't making sense.

I guess the confusing thing is the test signal seems to go through the recovery amp but the splash of the reverb tank isn't going through the recovery amp.

What could be happening is the test signal is very strong and is making it's way through a damaged recovery amp.
The real signal from reverb tank is very small and if the recovery amp isn't *really* working then it might not amplify
the splash enough to be heard.

Measuring the DC voltages around the recovery amp would be useful (including the supply voltage on the low voltage electrolytic cap it's 100uF/25V or 200uF/25V).

[sarakisof]

Reverb problem solved.

That would point to a connection issue between the reverb tank output and the input of the recovery amp (AC151 etc).

You were absolutely right Rob. The tiny hot signal lead of the rca cable from tank's output to recovery amp had cut off right at the solder joint (in the amp area) with the years of use.
The thing that was fooling me was the fact that it was still making intermediate contact in some cases and i realised that, by moving around the cable before opening the amp (it is placed in another external chassis on the cabinet). Signal was coming in and out.
So when i tested by probing at the rca's it happened that timeme to make contact, fooling me that recovery amp was fine.

So reverb runs fine now. And the weird -but good- thing is that i have it running about half an hour playing some tones ("Splish Splash i was takin a bath" 🤣) and the ecl86 hasn't started arcing yet.
Who knows how much will this last. 🤣

[Rob Strand]

Reverb problem solved.

Good news.

The thing that was fooling me was the fact that it was still making intermediate contact in some cases and i realised that, by moving around the cable before opening the amp (it is placed in another external chassis on the cabinet). Signal was coming in and out.
So when i tested by probing at the rca's it happened that timeme to make contact, fooling me that recovery amp was fine.

Intermittent contacts are never easy.     It's not uncommon to move or push something and it starts working but the fault turns out to somewhere else.

So reverb runs fine now. And the weird -but good- thing is that i have it running about half an hour playing some tones ("Splish Splash i was takin a bath" 🤣) and the ecl86 hasn't started arcing yet.
Who knows how much will this last. 🤣

Yes, well good luck just same.   The reason for the arcing still isn't clear to me.

[sarakisof]

Yeap definitely the arcing still isn't clear. It came back just a while ago when i was making my final voltage measurements. By the way, once again, all voltages are perfect.
At the third power up (when it comes for HV i use to attach the alligators of multimeter before power up then power off, attach alligators to the next measurement point and so on, just for safety reasons. I mostly work in live circuit for heaters and other low volts points) and during tubes warming it made the common immediate "pop" and small arcing.
No arcing after that, but i decided to keep amp working without playing smthng. Just looking and try to hear strange noises.
I got two kind of noises, one "drippy splash pop" when rvrb was on.
And another "cracking/poping" like noise, with rvrb off and even volume turned off.

This is the drippy noise: https://we.tl/t-Vnk3xD4t91

And this is the crackly poping: https://we.tl/t-RYi4rLAbIX

[Rob Strand]

I got two kind of noises, one "drippy splash pop" when rvrb was on.
And another "cracking/poping" like noise, with rvrb off and even volume turned off.

This is the drippy noise: https://we.tl/t-Vnk3xD4t91

And this is the crackly poping: https://we.tl/t-RYi4rLAbIX

They both sound like a bad connection somewhere.    Could be a the tube socket or the connector between the two sections.   Could even be a failing component bad cap,  cracked resistor.

They two cases might have a similar origin if it i coming from the connector or the preamp section.

In the first case the pop could be occurring before the reverb and the reverb is adding reverb to the pop sounds.  Perhaps the second sound is the same as the first but without reverb.    I'm sure you could determine if that's
the case better than I can.    You need to adjust the reverb level while the pops are happening and see if the reverb
to can change one sound into the other.

At the third power up (when it comes for HV i use to attach the alligators of multimeter before power up then power off, attach alligators to the next measurement point and so on, just for safety reasons. I mostly work in live circuit for heaters and other low volts points) and during tubes warming it made the common immediate "pop" and small arcing.

If the arcing is more reliable at power-on it might give a hint.   The heaters are cold so the tubes current
will be low.   The plate voltages will be high due to low current.  Also the caps are charging up and perhaps
some take a while.    It would be interesting to  measure the ECL86 tube DC voltages on power up to see if any
'glitch' occurs when you get the arc.

[anotherjim]

Maybe a standby switch will help? That way (if you remember to do it) the heaters can get hot before you apply the HT.
Did you replace the ECL86 socket? If it's a phenolic socket, they can develop hidden insulation faults in the material. I guess ceramic ones can do it too if the surface glaze has been damaged.
If it is socket insulation, then as Rob says, the HT is offload and higher than normal until the heaters are hot which would be when insulation breakdown is most likely to start.

[sarakisof]

I had to be out for the weekend, but before leaving, i played with it about 2-3 hours testing its behaviour and waiting for those pops/splash pops to appear. For all those hours i got no splash pops at all, only the dry pops (like in the second video uploaded) but way far quieter.
If i didn't know, i would have not even realized it.
It's the crackling kind of fire burning dry cobs, but very quietly.

They two cases might have a similar origin if it i coming from the connector or the preamp section.

That's my thought too. But this last time of 3 hour use i got no drippy pop at all. 
So i wanted to do that

You need to adjust the reverb level while the pops are happening and see if the reverb
to can change one sound into the other

but i couldn't.
After 3 hours i powered off and on (property) again about three times but still no splash or even normal loudness pops to test. No arcing too.

Maybe a standby switch will help? That way (if you remember to do it) the heaters can get hot before you apply the HT.

Yes maybe. I realised that by normal use, appropriate and not rapid turn on and offs i get no arcing at all. I start thinking about it was from that, alongside with the  lack of stand by.  I just let the tubes cool about 3-5' and then i turn on again.

Did you replace the ECL86 socket?

Not yet.

So for now, just a slight crackling pop that you can only hear when not playing (and if you get close to speakers).
If loud pops and splashes are back i will try to determine if they come from the same source. I guess if they do and the drippy sound is the first with reverb (very likely), my troubleshooting efforts will be concentrated on preamp, PI area, but not in ECL86's circuit and socket.

[Rob Strand]

That's a good test.   Weird the reverb drips have gone clearly that shows the two faults are different.

So for now, just a slight crackling pop that you can only hear when not playing (and if you get close to speakers).
If loud pops and splashes are back i will try to determine if they come from the same source. I guess if they do and the drippy sound is the first with reverb (very likely), my troubleshooting efforts will be concentrated on preamp, PI area, but not in ECL86's circuit and socket.

The only way I can see a way of narrowing it down is to start at the output monitor various points in the circuit with an audio probe.  As you work back from the output towards the input you might find a point where the pops stop.

You could even find it's occurring on every stage.  That would point to a power supply issue.

Without narrowing it down you would have to swap out far too many parts.   You could end-up replacing half the unit only to find it's a cracked resistor or dirty connection.