200S Problem

[george]

I have a 200s which has developed a problem of generating unnatural harmonics (ghost notes). The problem only shows up  when playing the G and B strings. The harmonics ride on top of the note giving it a "ringmodulated" sound.

I re-capped the supply caps about 5 years ago and assume that it's now the coupling caps on the output stage tubes that are the issue. I was thinking about using a lower value on the replacement part. Anybody have any experience with this issue?

THX

[rick.heil]

Could be the coupling caps...that would be my first suspicion after the supply caps.

Might also be a "playing position" type of thing - some types of guitars, especially semi-hollows, can really get some odd "ghost note" problems as a result of some resonant frequency thing, i guess (i dunno, I'm not too familiar with the problem, thank goodness!).  Try changing your position in relation to the amp and see if that helps, or maybe a different guitar.  If it still does the same thing, I'd redo the coupling caps.

As for lower value caps, if you feel confident the rest of the amp (notably speakers) is up for the increased low-freq response, I wouldn't see why not.  I bet the originals had a higher value to account for leakage anyways...just have to make sure you don't go too low.

[JoeArthur]

I would check the guitar first, especially if I had just set the intonation.  If you have to move the bridge back (away from the neck) the part of the string that was sitting on the bridge may now be a kink in front of the bridge.

For the amp, I agree with Rick - power supply caps.  Then there would be the output tubes.  If it is a ring modulation effect, then there has to be some way that the 120hz ripple or 60hz heater "signal" is getting through.

Then I would check the bias voltage.  If the coupling caps are leaky then a positive voltage imposed on the output tube grid would have the effect of reducing the negative bias voltage.  A change in bias would help confirm a leaky coupling cap.

Usually reducing the bias voltage won't give the ghost note symptom.  A higher bias voltage can produce the symptom because of the increase in crossover distortion. 

You might also check the electrolytics on the bias supply.  Sometimes these are neglected during cap replacement.  Bad ones are a good way to impose a 60hz signal on the grids of the output tubes.

Good luck!!