Modded AB763 BM Clone Ghost Note After Cap Replacement

[Thisismyname]

Modded BM clone I built for someone. During a gig, broken connection w/ preamp filter cap. Fixed it, improved it. Gave the amp back. A few weeks later, same symptoms. I found the same problem but different location.
This time it was the + lead of the lower B+ filter cap. It cracked right at the turret. Broken connection only when playing loud, otherwise a happy amp. I replaced it and fixed a structural problem I made with the filter cap board that caused this latest problem.

Now, when I play the amp with an attenuator and headphones, I'm hearing a ghost note at higher gain settings. This tone is most prominent on higher frets with the thinner strings. Sounds like a quiet distortion that has different intervals from the note I pluck relative to that note. Some notes, major 3rd, others, diminished 5th etc. Probably improper filtering.

Could there have been damage to another filter cap during this intermittent connection break?
I also forgot to form the new cap until after I turned the amp on and played for a few minutes with the vol past noon.

My main question, should I replace anything, and if so, what?

Thanks for any and all help :)

[jim]

You already know that low value current leaking electrolytic filters will create ghost notes at high volume. In addition, low quality radial filter caps often have thin friable leads that could break with vibration. A break could stress the other caps.  I would use 220/350V caps in the totem pole filter and then replace all the other filter caps to be sure.  Use axial F&T or Sprague.    Jim

[Thisismyname]

You already know that low value current leaking electrolytic filters will create ghost notes at high volume. In addition, low quality radial filter caps often have thin friable leads that could break with vibration. A break could stress the other caps.  I would use 220/350V caps in the totem pole filter and then replace all the other filter caps to be sure.  Use axial F&T or Sprague.    Jim

They are already F&Ts.

The breaking likely occured from gluing the caps down, but in a way where most of their weight was still on the leads. Ideally, I should have glued first, let it cure, then solder in so their weight was being supported by the glue instead.
The first break was an isolated cap. Looking back, I glued wrong.
This second break occured because I glued 4 filter caps together on a dedicated board, but not to the board itself because the balancing resistors were underneath. I overlooked the fact that gluing the caps like this pretty much combines the weight of the 4 caps together and that combined weight was supported only by their leads. My guess is the weakest lead or the one supporting most of the weight was the one that broke. I fixed that problem.

Could I replace one cap at a time and test until the ghost note goes away or just play it safe and get all of them?

[Latole]

I don't trust gluing filter caps.

Post pictures.

[jim]

Yes Post pics. Don't know what really happened but if leads are too long the caps will bounce unless you keep leads short and strap them down with cable ties or something.  Heat weakens glue as well.  All I'm saying is you need more filtering after the diodes and before the choke to kill the ghosting. Try 220uf/350v in series(keep the 220K balancing resistors) will yield about 100 uf at 700V.  It will give you a slightly tighter bass response .  Jim

[Latole]

Yes Post pics. Don't know what really happened but if leads are too long the caps will bounce unless you keep leads short and strap them down with cable ties or something.  Heat weakens glue as well.  All I'm saying is you need more filtering after the diodes and before the choke to kill the ghosting. Try 220uf/350v in series(keep the 220K balancing resistors) will yield about 100 uf at 700V.  It will give you a slightly tighter bass response .  Jim

I agree, 100mfds in series = 50 mfds = too small

[HotBluePlates]

Modded BM clone ... when I play the amp with an attenuator and headphones, I'm hearing a ghost note at higher gain settings. ...

The amp is probably fine, the "Ghosting" is simply a normal result of of distorting the power section hard with that attenuator.

On another forum, we had a 10-page thread where someone had a hard time believing this was normal for a vintage-style amp.  I can't recall if he had a Soldano or Dumble-style clone, but noted they didn't have the ghosting problem.

But in those cases, the power section distortion was minimized because of preamp gain & master volume, and the lower power output due to master-volume use caused less current-suck from the filter caps.  Add to that the much higher filter cap values in those amps, and suddenly the ghosting is much reduced.

What happens to cause ghosting is the burst of high output power causes the power tubes to suck big current from the reservoir and screen filter caps.  When current draw increases the hum level at those caps goes up, related to the pulsing DC the rectifier is supplying to try to re-fill the caps.  This increased hum is then modulating the desired guitar signal, and the distortion causes sum+difference tones to be created.  The "sum tones" get lost in all the other harmonics created by distortion, but the difference-tones become out-of-tune stuff you can hear.


What now?

You kill ghosting at high amp-power-levels by cranking up the filter capacitance for the power tubes.  Seeing 220µF per filter stage (4x 220µF caps in series-part parallel) is pretty normal for some high-gain amps that don't want the power section distorting or sagging.

But when you crank up the capacitance, the amp-feel gets stiffer, because power supply sag is greatly reduced.  So the amp may seem "stiff" or "brittle" at lower volume.

It's a tradeoff between vintage-feel for clean or lower-volume distortion, vs hearing ghosting when the amp is cranked.  FWIW, there's all kinds of ghosting present on classic guitar solos that is only audible when the guitar is isolated from the track.  But guys playing with attenuators and outside the context of a band/song-track start noticing these things that always happened, and didn't matter until they put stuff under a microscope.

[shooter]

the difference-tones become out-of-tune stuff you can hear.

...and the light bulb is now lit!  been curious for awhile, but not enough to go looking, so thanks.