what does it mean when your amp produces ghost notes?

[ode2no1]

i have a silverface bassman 50 that produces ghost notes an octave lower than what i'm playing when i'm doing lead stuff higher on the neck. i know it isn't the speaker cab cause i have a drri with the same speaker in it except that it's a 1x12 whereas the cab for the bassman is 2x12. i've also played the drri thru the 2x12 without ghost notes. i think i remember reading a few years ago that it's a cap issue. i suppose it's probably about time to change pretty much all of the caps anyway since the amp is at least 30 years old by now, huh? but is that it or could it be a number of things? i'd prefer to do as much work on the amp myself as possible.

[Boogdish]

It's definitely time to change those caps.  I wouldn't be surprised if that fixed your problem, but it's not the only possible cause.  Lead dress issues would be my second guess.

If you can't fix it, just tell the bass player to turn up so no one will notice. 

[R.G.]

Ghost notes are modulation of what you're playing by ... something.

This can happen with excessive ripple on the power supply, which is what you heard the "uh, dude, it could be, like, caps, dude" about. It can be other things like a misbias of the output tubes too cold causing crossover distortion, especially if one of them is too cold. It can also be a speaker that rubs or is otherwise ill. The first thing to do, which you have done, is to elminate the speakers. Next, if you are up for it, re-capping certainly won't hurt. But don't get discouraged if that's not perfection. Could be a combination of things.

[sixstringphil]

I had the same issue with a 67 bassman. I'm no amp tech, so I took it to one of the best guys around. He found that someone had done some kind of feedback mod that was intensifying the effect. With the mod removed, it was still noticeable, but drastically reduced. We tried many things, including a new transformer. Nothing seemed to completely eliminate the overtones, but one day it just seemed to go away.

[ode2no1]

it really isn't terrible or anything. the ghost notes aren't blaring, but i do hear them. it's kind of like the subtle octave effect you can get with the harmonic percolator. i was just worried that something kind of bad might happen soon. it very well could be the wiring of the building. i use a one spot and daisy chain about ten pedals and sometimes i get HORRIBLE noise from my distortion boxes when i'm not playing...but other days it's as quiet as if i were using a battery. i'll have to check the bias soon. i put in a matched pair of JJ's about 2 1/2 years ago and biased it up on the warmer side but the amp just sat there unused for almost 2 years. maybe it drifted.

as far as changing the caps...i believe the amp was recapped about 5 years ago, but when you take an amp in to be recapped it's usually just the giant power supply ones, right?

[amptramp]

it really isn't terrible or anything. the ghost notes aren't blaring, but i do hear them. it's kind of like the subtle octave effect you can get with the harmonic percolator. i was just worried that something kind of bad might happen soon. it very well could be the wiring of the building. i use a one spot and daisy chain about ten pedals and sometimes i get HORRIBLE noise from my distortion boxes when i'm not playing...but other days it's as quiet as if i were using a battery. i'll have to check the bias soon. i put in a matched pair of JJ's about 2 1/2 years ago and biased it up on the warmer side but the amp just sat there unused for almost 2 years. maybe it drifted.

as far as changing the caps...i believe the amp was recapped about 5 years ago, but when you take an amp in to be recapped it's usually just the giant power supply ones, right?

The octave effect can occur when the electrolytic filter capacitors lose capacitance (which they will do over time).  On one cycle, the capacitors will be drained, then on the next cycle, the reduced voltage will cause a lower current drain, then the next cycle will have the same output as the first, causing a component at half frequency.  This is especially important with feedback.  If the unit was recapped five years ago, it should not be a problem unless the technician skimped on the capacitors.

When it gets bad enough, there will be excessive hum and in extreme cases, a tendency to "motorboating", a type of oscillation caused by coupling between stages via the power supply.

Trace out the schematic and make sure it follows:

http://schematicheaven.com/fenderamps/fender_bassman50.pdf

Some technicians like to make their own mods and some mods will not help.

Recap usually means the coupling capacitors as well.  The Bassman 50 also has 2000 pF capacitors from grid to ground on the output tubes.  These don't seem to have any reason to be there except possibly to provide a dominant rolloff frequency to stabilize the feedback.  The small signal stages all have capacitors across the cathode resistors as well and these should be replaced.  In general, any electrolytic or paper capacitor should be replaced.  Small silver-mica capacitors are usually OK.  Ceramic capacitors do not belong in the signal or feedback path at all, so if there are any in there, they should be changed to film capacitors.

The distortion pedal problem could be feedback through the power supply.  Each pedal needs a good electrolytic at the input and electrolytics do not age gracefully.