Baffling issue. Can anyone solve this one?

[fiveightandten]

So here's a brain buster, if any of you are able to offer some ideas.

Summary:
I bought an amp used off ebay in "like new" condition, and it arrived as such. Though, shortly after plugging it in, the amp started having problems. I don't want to mention the builder, but this is a high quality hand wired, turret board style amp. It is constructed with solid core wire and NOS mustard caps, just FYI. The amp was built in 2014. I believe the seller that the amp was fine when he shipped it, and it's likely something got loosened up during shipping. He's paying for the shipping and repairs, so he has no reason to lie, as far as i'm concerned.

Condition:
It has a mechanically induced pop/crackle, accompanied with volume drops. Poking around on components didn't yield anything promising. Poking around and jiggling every accessible wire in the amp also yielded no notable observations. I used the o-scope and tracked it down to the 3rd gain stage, right before the PI circuit. I was thinking a broken solder or broken wire must be hiding under the turret board in this area. So I decided the pre-amp board probably has to come up and shipped it to the builder for repair.

Clarification:
I tried 2 different sets of power tubes, 3 different sets of pre-amp tubes, 2 different cabinets, 2 different speaker cables, and 2 different power cables. It does the same thing at my practice space, and home. I'd say it's definitely the amp.

Current situation:
I shipped the amp in a flight case to the builder. He had it for a week and found NOTHING wrong with it. He even emailed me a video of him playing the amp at high volume with no issue, and banging the thing on his bench without a peep coming out of it. Same amp, same tubes, no issue with it at all.

He then put the amp back in the flight case, and sent it back to me.

I pull the amp out of the flight case, plug it in, and it's making the SAME NOISE, as if nothing ever changed. It has the SAME PROBLEM. I sent him a video of it happening and we're both completely baffled.


Any ideas on how this amp can have a problem with 100% consistency in front of me, then be 100% fine for him, then have the problem when it's with me again? The plan is to send it back to him, but this is extremely frustrating, as you can imagine. I'm looking for any suggestions or ideas. I've never seen this happen before.

[Randall]

You haven't said if you have ruled out your instrument, cable, pedals, etc.

[Enzo]

Without knowing what it is limits what we can tell you. Imagine a car that is stuck in one gear. If we don't know whether it is an automatic or manual shift, then we don't know which of two very different tracks to look down.

But...

It makes this noise with NOTHING plugged into it other than the speaker? As in no guitar plugged in. Does it have an effects loop or other patch jacks? If so have you patched across them?

Ball up your fist and whack the top hard, any reaction? There should NOT be a reaction, any noise is evidence of something loose. Whack the end of it. If it has a reverb, turn it off, pick up one end of the amp a couple inches and drop it. All of these send different mechanical shocks through it.

What exactly do you mean by mechanically induced? You have to hit it? Or are you surmising this from high sound level vibrations? Does connecting the amp to a speaker say 30 feet away make a difference from a speaker right next to it? Knowing WHAT triggers this is a clue to where it is.

Take your amp someplace else, a friend's house, a gig, somewhere. Does it behave the same? We need to eliminate your house/environment. Oh... you did, OK.

Your scope may find it in a certain stage, but that can be misleading. A failed solder joint in a filter cap across the chassis can affect one stage. The stage is where it shows up, but the problem is elsewhere. Just like an intermittent outlet in your bedroom could be caused my a loose connection in the fuse box across the house.

You have a turret board. Remove a mounting screw or two so you can apply a little flex to it, any reaction?

You tried different tubes, but have you wiggled each tube while it runs?

Does ANY control affect the noise in ANY way? Can you turn the volume of noise up and down? Change the tone of the noise? Switch channels to find if the noise is channel specific. Can you bypass the suspect stage with a jumper and a couple part lifts? That will reduce system gain, but any remaining noise should still be audible if reduced.

[J M Fahey]

Condition:
It has a mechanically induced pop/crackle, accompanied with volume drops.

Howdoyouknowit´s mechanically induced?

Poking around on components didn't yield anything promising. Poking around and jiggling every accessible wire in the amp also yielded no notable observations.

That does not exactly confirm the mechanical hypothesis.

I used the o-scope and tracked it down to the 3rd gain stage, right before the PI circuit.

Tracked it down starting from where? Input jack or output jack?
Where in the 3rd stage?:grid/plate/cathode/2 or more?
By the way, what the break is "the third gain stage"?
Schematic please.

What do yu see in the scope which indicates thisproblem?
care to share a scope screen capture showing it?

In any case, what happens if you pullthe suspect tube?
No at tyhe speaker, but right there, at the socket legs ... does the interference/noise still appear?
Sorry for the ton of questions but it´s to avoid:
- "did you test xx?"
-"yes, it´s not that"

- "did you test yy?"
-"yes, it´s not that"

- "did you test zz?"
-"yes, it´s not that"

and so on and on and on.
Do you still find it

[Tom Phillips]

...I bought an amp used off ebay in "like new" condition... I don't want to mention the builder, but this is a high quality hand wired, turret board style amp. It is constructed with solid core wire and NOS mustard caps, just FYI. The amp was built in 2014...

All this mystery is counterproductive to allowing people to help without even having the amp on their bench. Stuff happens to even the best built amps. Full disclosure of the facts will help us help you so why not just go for it?

[fiveightandten]

Thank you for the replies. Apologies for any ambiguity, I'll try and clear some things up.

-The amp makes noise without a guitar or cable plugged into the input jack.
-The amp does not have reverb, or an effects loop. It's pretty bare bones Vol, B, M, T, Presence. It does have a footswitchable gain boost, fed by a cathode follower stage. The noise happens with or without the foot switch plugged in, and with or without that boost engaged. It seems to be after that part of the circuit.
-If I tap my hand on the head sleeve, the amp makes a loud pop and crackle.
-Poking around with the chopstick method seems to suggest the pop/crackle originates from the preamp turret board. If I tap anywhere on the board, it makes noise. However, no single component or wire seems to induce any popping or crackling. The preamp turret board is very sensitive to the tapping. The rest of the amp is not. However, the problem is bad enough that it will pop and crackle just from normal vibrations that happen when sitting on top of a cabinet.
-If the amp is sitting on top of the cabinet, it's unplayable. If it is isolated from the cabinet and not hit or disturbed, it's fine. This is what I mean by "mechanically induced".
-The noise is independent of the gain control, and independent of the EQ knobs. The scope doesn't show anything being present until very late in the pre-amp circuit. Then it's visible in the stage right before the PI.
-The amp has 4 12AX7s. If I pull #1 and 2, it has no affect on the noise. If I pull #3, the noise is gone. It seems to be in the 2nd half of that 3rd 12AX7, right before the long tail PI circuit.
-If you feel the builder is relevant, I can mention that. But they don't publish schematics, so it probably won't really be relevant.

Thank you again for the replies. I hope that clears some things up.

[Pedro Vecino]

As an external possibility to the amp itself: are you using the same power cable or a different one than the vendor? If it is disconnectable, sometimes they do not make good contact and are susceptible to generate noises...

[fiveightandten]

As an external possibility to the amp itself: are you using the same power cable or a different one than the vendor? If it is disconnectable, sometimes they do not make good contact and are susceptible to generate noises...

Thanks. I think I mentioned it in the original post, but I used 2 different power cables with the amp. I've tried multiple speaker cables, power cables, wall outlets, buildings, and cabinets. I've also tried multiple sets of pre-amp tubes and power amps tubes. Everything outside of the amp itself has been swapped out. I'm confident that the problem lies with the amp.

All the sockets and jacks are extremely tight and making nice connection. The pre-tube sockets were so tight that they scratched the gold plating off the pins of some pre-amp tubes I put in it. The amp has about 10 hours play time on it, according to the seller. That means it sat for most of the time. Due to this, I was suspect of the NOS mustard caps.

[Jazz P Bass]

I see no mention at all that you changed out the guitar cord OR the guitar itself.

[J M Fahey]

-If I tap my hand on the head sleeve, the amp makes a loud pop and crackle.
-Poking around with the chopstick method seems to suggest the pop/crackle originates from the preamp turret board. If I tap anywhere on the board, it makes noise. However, no single component or wire seems to induce any popping or crackling. The preamp turret board is very sensitive to the tapping. The rest of the amp is not. However, the problem is bad enough that it will pop and crackle just from normal vibrations that happen when sitting on top of a cabinet.
-If the amp is sitting on top of the cabinet, it's unplayable. If it is isolated from the cabinet and not hit or disturbed, it's fine. This is what I mean by "mechanically induced".

Ok, now we´re starting to talk

-The noise is independent of the gain control, and independent of the EQ knobs. The scope doesn't show anything being present until very late in the pre-amp circuit.

OK

Then it's visible in the stage right before the PI.

Ok, *where* on that stage?
Plate/grid/cathode?

-The amp has 4 12AX7s. If I pull #1 and 2, it has no affect on the noise.

You already said or implied that.

If I pull #3, the noise is gone. It seems to be in the 2nd half of that 3rd 12AX7, right before the long tail PI circuit.

Draw both halves of V3 and circuit surrounding them.
Tube amps are **simple** (except MB that is ) , so basically 3 resistors and 3 caps per triode, easy peasy.
We want the full path in each, from ground through cathode resistor through tube through plate load to supply node +V , plus coupling caps.
If any grid is directly connected to, say, a volume pot or tone control, we also want that.

IF you can reproduce the noise at will, consider the problem solved ... the time wasters are those which "works fine .... but ....... "
IF amp maker is "widely known" probably some tubehead Forum has already analyzed and modded it, publishing some hand drawn schematic in the process, so please tell us the maker.
This is NOT a "tear reputation to shreds" type Forum, quite the contrary; if anything, it´s better for said maker to have this amp repaired and you praising its sound rather than you (or any angry owner) venting frustration in a couple widely read pages.
Now THAT hurts sales and reputation.

Maybe a turretboard closeup showing the suspect area, also a suspect tube socket closeup may help.

[Tom Phillips]

...IF amp maker is "widely known" probably some tubehead Forum has already analyzed and modded it, publishing some hand drawn schematic in the process, so please tell us the maker. This is NOT a "tear reputation to shreds" type Forum, quite the contrary; if anything, it´s better for said maker to have this amp repaired and you praising its sound rather than you (or any angry owner) venting frustration in a couple widely read pages.
Now THAT hurts sales and reputation...

I strongly agree. Also post photos of the problem area and the model designation of the amp or model that it claims to emulate etc.

[fiveightandten]

Ok, now we´re starting to talk

OK
Ok, *where* on that stage?
Plate/grid/cathode?

I can see it on the scope on the plate, grid, and cathode (6, 7, 8). Only that one triode though. It's not on 1, 2, 3.

Draw both halves of V3 and circuit surrounding them.

Tube amps are **simple** (except MB that is ) , so basically 3 resistors and 3 caps per triode, easy peasy.
We want the full path in each, from ground through cathode resistor through tube through plate load to supply node +V , plus coupling caps.
If any grid is directly connected to, say, a volume pot or tone control, we also want that.

I don't have the amp with me, but I can draw that out.

IF you can reproduce the noise at will, consider the problem solved ... the time wasters are those which "works fine .... but ....... "

The problem isn't intermittent. It's totally consistent. The odd part, for me, is that he couldn't reproduce it.

IF amp maker is "widely known" probably some tubehead Forum has already analyzed and modded it, publishing some hand drawn schematic in the process, so please tell us the maker. This is NOT a "tear reputation to shreds" type Forum, quite the contrary; if anything, it´s better for said maker to have this amp repaired and you praising its sound rather than you (or any angry owner) venting frustration in a couple widely read pages.
Now THAT hurts sales and reputation.

I understand. I guess i'm just used to the witch hunts on The Gear Page, where a simple problem solving thread like this can destroy a builder's reputation for no reason.

Maybe a turretboard closeup showing the suspect area, also a suspect tube socket closeup may help.

Here's the suspected area (some of you will likely know who made this based on this photo). I have jiggled every lead flying to every pot, tube socket, switch and jack in the amp and none of them made a peep. There are obviously wire runs that I can't get at under the turret board. I suspected there was a broken wire, bad solder, etc. on the underside (a turret to turret connection). But that brings me back to why it would be fine on his bench and not fine with me.

I jiggled every single part in the amp. I couldn't find a single broken lead, broken wire, or anything else accessible without pulling the board up.

[fiveightandten]

I see no mention at all that you changed out the guitar cord OR the guitar itself.

See Post 6...

-The amp makes noise without a guitar or cable plugged into the input jack.

[tedmich]

"I shipped the amp in a flight case to the builder. He had it for a week and found NOTHING wrong with it. He even emailed me a video of him playing the amp at high volume with no issue, and banging the thing on his bench without a peep coming out of it. Same amp, same tubes, no issue with it at all. "

If the maker of the amp can't even confirm the "problem" much less fix it with the unit on his bench for 1 week, what chance does anyone here have of doing so remotely?

Zero is my bet.

[Tom Phillips]

...The problem isn't intermittent...

Well, in electronics speak it is intermittent because because it is not a continuous bad signal or loss of signal. This kind of stuff is one of the nightmares of troubleshooting. We all have our own ways of dealing with such situations. In my experience a hairline crack in a solder joint is the most common cause of this type of problem. They are harder to isolate when the joints are covered with the red coating, as in your amp, and when you are not free to clean off the joints for inspection or re-soldering. When touching several locations exacerbates the problem then you need to make (or wait for) the problem to get worse so that you can tap or wiggle with less force to narrow down the area where the problem is. It could also be inside a component where the lead wire is bonded to the guts. Heating and / or cooling the whole amp or individual areas can sometimes make the problem mote sensitive to wiggling a wire or component.

The photo you posted is pretty good but it's always hard to see clearly when zooming in unless the photo is super high resolution. Check out the attached commented photo.


Cheers,
Tom