"popping" Fender Twin Amp '57

[Sansteeth]

Hey everyone,
I stumped with this one repair on a Fender Twin Amp '57 Reissue... Schematics is there ->https://schematicheaven.net/fenderamps/57_twin_amp.pdf
It makes a "pop" every now and then, not often but often enough to drive its owner (and myself) crazy.
I'm only working with my own test tubes right now, so unfortunately the usual suspects are not guilty here...
Here's the thing:
- "pop" is nice and loud with only V3, V4 and the power tubes in, so VOL control has no incidence on it.
- the treble control changes the tone of the "pop" (brighter or darker)
- get this: with only V4 and the power tubes, you can STILL hear a faint(er) "pop"  AND its tone changes with the treble control (which is upstream of the signal chain...)

I'm thinking of a defective component in the power supply, I changed the first 10k dropping resistor (r44 on the schematics), still the same issue.
I NEVER trust silvermicas (c7 and C8 are on the tone stack) so I changed them for HV ceramics, still the same issue.
Next thing in my crosshair is the filter caps but they're F+Ts, not that they never fail, but they are the lowest for me on the list of things that can go wrong.
The thing that baffles me really is with V3 out, the treble control still influences the tone of the "pop"
What do you folks think?

Thank you so much

[tubeswell]

Could be a bad tube, possibly a bad output tube.

[stratomaster]

Check R25 for a bad solder joint, broken internal connection, or just replace it outright.

Inspect the V3 socket for signs of arcing.

Check C15 for a bad lead or solder joint.

Make sure the treble pot is tight and clean.

It's getting into the treble pot somehow. If those suggestions don't pan out, isolate that section entirely by strategically disconnecting then reconnecting the outer lugs of the treble pot. If that doesn't work, lift the 10M feedback resistor and repeat.

[Sansteeth]

Hey Tubeswell, thank you for taking the time to respond, as I was saying the amps right now is fitted with none of the tubes it came with to my shop, the symptoms absolutely sound like a bad tube, but with a third full tube swap (the customer tried to shotgun it by buying an entire new set of tubes earlier, to no avail), it would be an insane coincidence if it was all 3 sets of tubes.

Stratomaster,
Thanks for chipping in!
somehow I was also thinking of R25, so I'm going to replace it and see, however, with V3 out, I cannot see how it would still be the culprit, but at this point I'll believe in voodoo

[pdf64]

V4,5,6 are all in the nfb loop.
How about if the loop is opened?

To eliminate the power supply caps, just connect a good cap (smaller uF, at least as good V rating) across each in turn.

The treble control puts a 10nF cap bypassing V3 pin2 grid, so it seems reasonable that it would still affect things.

[Sansteeth]

Hey pdf64,

V4,5,6 are all in the nfb loop.
How about if the loop is opened?

thanks for bringing it up, every message I post I fail to mention that the NFB loop is indeed disconnected

To eliminate the power supply caps, just connect a good cap (smaller uF, at least as good V rating) across each in turn.

I'll do that and report back!

The treble control puts a 10nF cap bypassing V3 pin2 grid, so it seems reasonable that it would still affect things.

I'm not sure I understand what you mean?

[pdf64]

...

The treble control puts a 10nF cap bypassing V3 pin2 grid, so it seems reasonable that it would still affect things.

I'm not sure I understand what you mean?

If the nfb loop was intact, and eg V4 anode was popping somehow, the loop would feed the pop back to V3 pin3 cathode, which from the perspective of the feedback signal acts as a common grid stage. My understanding is that an AC bypass at the grid of a common grid stage will tend to raise the stage gain, as well as decoupling any resistor noise at the grid caused by residual grid current.
Or something ... 

[tubeswell]

Chopstick test for a loose/ cold solder joint

[Sansteeth]

Thanks for the explanation, I'll be honest I'm not super familiar with the set up of V3 or Twin Amps nor their tonestacks.
Since the "pop" is pretty random, it might take a while before I can report back and get to a proper conclusion (which makes the whole process extremely frustrating)
Thanks a lot for all the pointers folks and thanks for bearing with me!

Chopstick test for a loose/ cold solder joint

Brother, I've chopstick'd this amp so much it turned into a stir-fry 
Joke aside, I keep thinking of a cold solder joint because it's exactly those symptoms but so far I haven't come across a single dodgy joint

[pdf64]

A possible iffy connection might be at one of the valve sockets, ie if one of the valve pins wasn't being gripped sufficiently tightly.

[shooter]

since the "pop" is pretty random

any chance it correlates to using the standby switch??  (if the amp has one)

[pdf64]

That reminds me -
"THe failure of a standy switch is quityr common and one of the most puzzling problems that techs can come across because they rarely just burn out. The most common symptom is a ticking sound every few seconds."
From https://www.thegearpage.net/board/index.php?threads/will-putting-an-amp-in-standby-before-shutting-down-prolong-the-cap-life.1226042/post-15220533

I recall xtian, who frequents forms other than this, mentions this symptom / cause too.

The standby implementation in this amp is particularly egregious, as it hot switches the reservoir cap to the 5U4.

[Sansteeth]

A possible iffy connection might be at one of the valve sockets, ie if one of the valve pins wasn't being gripped sufficiently tightly.

That was also on my mind but I wiggled the hell out of those tubes and not a crackle, solid connections.

any chance it correlates to using the standby switch??  (if the amp has one)

I havent been using the standby switch (I kept it on all the time) but "not using it" doesnt mean it cant shows the symptoms you and pdf64 are mentionning. Dang, I never heard of that and the standby switch would really be last on my list of things to check for a random ticking sound but that's why I love this community because it allows us to pool some of oyr experiences and absolute oddball stuff we've come across.

In the meantime I strapped a 500V 10uF on the filter caps and the ticking sound was still there.
Then I got frustrated and just reflowed a bunch of solder joint and it didnt tick for like 15mn.
I'm off to the seaside for a couple of days but I will report back when Im at it again at the shop
Thanks y'all for the precious input

[tubeswell]

Chopstick test for a loose/ cold solder joint

Brother, I've chopstick'd this amp so much it turned into a stir-fry 
Joke aside, I keep thinking of a cold solder joint because it's exactly those symptoms but so far I haven't come across a single dodgy joint

Sometimes what looks like a good joint at first or second or third glance can still be a loose joint. Use your needle-nose pliers to grab each component lead and give it a decent wriggle

[passaloutre]

Not to pile on, but I had a random/intermittent popping/crackling, exactly the sound of the last few seconds of cooking popcorn...it was a loose solder joing coming off the plate of my preamp tube.

[RafaelBlock]

It makes a "pop" every now and then, not often but often enough to drive its owner (and myself) crazy.

- "pop" is nice and loud with only V3, V4 and the power tubes in, so VOL control has no incidence on it.
- the treble control changes the tone of the "pop" (brighter or darker)
- get this: with only V4 and the power tubes, you can STILL hear a faint(er) "pop"  AND its tone changes with the treble control (which is upstream of the signal chain...)


Thank you so much

At the start of the signal path the treble control fades in and out the effect of C2 on V3s grid, changing the tone of the noise. It could be C2 is leaky, try temporarily disconnecting it. Even with V3 removed, the treble control is still connected to the negative feedback loop. The louder pop with V3 installed sounds to me like a global noise that is louder because it has one more gain stage and either comes from C2 or the power supply. IMO once I've opened an amp up I just go ahead and replace the power supply caps unless they're obviously of recent vintage. If there's gunk on the board, use a qtip and alcohol and scrub it clean. DC can leak through gunge stuck on ancient flux residue. It also never hurts to just go up and down the board and re-flow every solder joint. These things were assembled by people of all stripes and abilities to minimal standards.

[Sansteeth]

Hey everyone,
I just got back from some needed holidays, thank you so much for your help and all your responses, as many have guessed before, it was a dodgy solder joint, I reflowed a whole bunch of solder joints and the "pop" never came back...
I though the chopstick method would easily show issues of bad connections/cold solder joints, and it usually does, but I realize it can be too subtle and go under the radar.
Again, Thank you so much every one

greg