Guyatone GA-1100 Fender Twin clone "ghost note"

[vatch23]

I have an old Guyatone GA-1100, its an eighties clone of TR and everything is converted to AB-763 about a year ago.
Now suddenly there is a second note playing along. I have to pick pretty hard before it occurs and its more pronounced or maybe audible on F#and upwards on the second string.It doesnt die out slowly but ends pretty abruptly.
First I took the chassis out because I thought something was vibrating/resonating maybe even magnetically from those big magnets on the JBL E-120s.But NO, the sound was still there, on both channels and all inputs. Then I swapped all preamptubes to no avail. The I tried different speakers :sound still there.The amp was recapped and retubed about a half year ago and sure this amp can get really hot, but can caps really dry out in half a year with only about an hour a day of playing ? I also tried to rebias a little hotter but the annoying second tone is still there.....
Yesterday I recapped the doghouse again , didnt remove the trailing note.......Then I changed all Tubes pre and power, still no difference.
Waldo on GDForum suggested a powersupplyproblem and advised to take it to a qualified tech................
Help!

[vatch23]

OOOps I posted in the wrong part of the forum, can anyone tell me how to move the posting?
I tried to delete this one and make a new one in "Maintainance and repair" but cant even find a "delete" button....

[g1]

No need to repost, you are in the right "general area" .
The first thing that struck me was your mention of "second string", so first off have you tried another guitar or verified that the guitar is absolutely not the problem?
Secondly, caps can fail at any time. They can be dead on arrival, go bad during first uses, or during warranty period, or 50yrs. down the line. So you can not rule them out as a possible cause just because they are fairly new.

[vatch23]

Yes I have tried with different guitars, no difference, I recapped the doghouse yesterday, no difference, re-soldered all ground-connections, no difference, made a 20 foot speaker-cable and put the speaker(s)in a isolated room, no difference. What I noticed just now was that I earlier had put one 100 Ohms resistors between each filament and ground (atV1) and when I removed it a very loud hum appeared,.....maybe the PT is somehow, or wrong connected, or failing slowly?

[g1]

The bias filter caps are not usually in the doghouse, did you replace them? And preamp tube cathode bypass caps?

[R.G.]

Ghost notes are caused by intermodulation. It can come from speaker rub (yep, you tried different speakers) but also from having power supply ripple modulate the actual signal. Single ended circuits are intolerant of power supply ripple, as the signal on the plate is actually referenced to the B+, so ripple on the B+ feeds the next stage along with the plate voltage of the tube.

If this happened suddenly, I would do a lot of the checks you've already done, eliminating speakers, cabinet rattle, microphonics and failing tubes, then I'd go looking at power supply ripple. The method I'd use is to put an AC-coupled oscilloscope probe on the B+ feeds to each stage and see how big the power supply ripple is. Of course, this is as dangerous as doing dentistry on rattlesnakes if you're not already skilled with both high voltages and oscilloscopes.

The sudden onset of the symptom makes me suspect a sudden failure of the choke filter to the screen supplies, if it has this setup. A shorted choke would put dramatically more ripple on the screens of the output tubes as power goes up, but would not change the DC voltage on the screens much, perhaps not noticeably. Open caps would cause more ripple too, but that's also unusual.

You've covered most of the usual suspects. Time to look at less-usual suspects. And remember that it's not worth dying for.

[vatch23]

I am sorry but circumstances have made me go help an old friend out for a week. I will continue with this amp as fast as I get back. Thanks for all replies so far!

[vatch23]

How can I check if the filter choke is shorted? Without an oscilloscope that is.

[vatch23]

I have 5 months old filtercaps, biascap and preamptube cathode bypass caps. BUT I replaced them all to check and the problems still there. On the the preampfilaments (ac) there are two 100 ohm resistors to ground allthough tahat winding has a centertap and when I removed them A loud 50 Hertz hum is heard.........

[DrGonz78]

On the the preampfilaments (ac) there are two 100 ohm resistors to ground allthough tahat winding has a centertap and when I removed them A loud 50 Hertz hum is heard.........

Is the Center Tap connected to ground? It would seem that it is not connected since it is now humming after removing the virtual center tap(the two 100 ohm resistors).

[vatch23]

There is no centertap on the preampwinding on the PT. It is probably a replacementtrafo, not done by me.
And i think the hookup is a bit weird, I have tried to make a computerdrawing of it, its clumsy but understandable:

[vatch23]

This thread is continued at
Grateful Dead Music Forum ? View topic - Twin "extra tone"?