Filter caps and Unnatural harmonics

[earthmud]

My SF72 vibrolux has a unnatural harmonic that I can't seem to get rid of. It rides the note and dies before the note does. Reminds me of a fuzz. It's been in the shop three times and here's whats been done. Filter caps replace last year, complete retube, new OT,and electrolytic caps have been replaced but it's still there. I have checked all cables speakers guitars. The amp sounds fine clean but if I plug in any overdrive pedal here comes the noise. The techs in this town are limited (and so is my cash already spent 400$) and I don't want to keep replacing original parts just to see if thats the problem. According to one of webers books these noises are almost always related to DC power supply filtering. Is it possible that the Illinois filter caps that were installed last year by a different tech could be bad. The noise was not there when I bought the amp 2 years ago. Here's the section from webers book and it does make the noise when hitting bflat 15thfret. What should I have my tech do now. He's as baffled as I am. The amp sounds great otherwise and I really want to keep it. Any help would be greatly appreciated.

http://books.google.com/books?id=lG8361HKf6kC&pg=PA164&lpg=PA164&dq=guitar+amp+buzz+at+high+volume&source=web&ots=2gPquTrXge&sig=97vDjamevv2zmDef1R4DCjZBIaY&hl=en&sa=X&oi=book_result&resnum=7&ct=result#PPA19,M1 (http://books.google.com/books?id=lG8361HKf6kC&pg=PA164&lpg=PA164&dq=guitar+amp+buzz+at+high+volume&source=web&ots=2gPquTrXge&sig=97vDjamevv2zmDef1R4DCjZBIaY&hl=en&sa=X&oi=book_result&resnum=7&ct=result#PPA19,M1)

[phsyconoodler]

It is doubtful you have a bad filter cap.If it's ghost notes that you are experiencing then the amp can be UNDR filtered,but that's not likely unless the values are wrong.
The .1uf driveline coupling caps could be the culprit.I change them in many amps and it not only stops lots of issues,it can dramatically increase the volume of the amp.

[Gatorman352]

If you read what Gerald says he says that at the higher registers the coupling caps could be the issue. Having the Electrolytic caps changed is great but coupling caps are not Electroytic caps they are non-polar caps. Current amps most use film type or Orange Drops. I would change the caps GW refers to. Although I'm not a expert, I have built several amps and wouldn't hestitate to try that. Do you know what the value of the Power tube grid coupling caps are?

Why don't you take some pictures of the guts of your amp and post it here? With all the advanced builder on this site they might be able to point you in the right direction.

BTW: I've read all of GW's books and he strongly is against Illinois Caps. He recommends Sprague, Mallory or German Caps ( probably F&T ). I have mixed feelings on this subject because the prupose is to filter ripple after the A/C is transferred to DC to smooth out the DC.

Be careful if you work on your amp because those filter Caps can hold a charge long after the power is disconnected.

[phsyconoodler]

I have use illinois caps in many amps with zero issues.Maye GW ran into a bad batch once and condemned them.
I had some bad Sprague Atoms once and they caused a lot of grief.
Point is you can have a bad batch in any type.
illinois caps are in many devices all over the world and they are just fine.

[Structo]

Just a thought and you are probably aware of this but I don't suppose it is Stratitis?

On strats particularily, when the pickups are too close to the strings ( mostly the neck pup) it can have those type of overtones.

Has this amp always done this?

It could also be the grid stoppers on the input since you say it only does this when the preamp is overdriven.
To me since you say it only does this with pedals in front, I would lean towards the problem being in the preamp.
Coupling caps, bypass caps, snubbers, grid stops.

[earthmud]

http://www.thegearpage.net/board/showthread.php?t=433858&highlight=vibrolux


Gut shots here.


Structo-it does it with my tele also and no the amp has not alawys done this. I can't pinpoint when it started but I know it was after the first time I took it to the shop. It's a very subtle noise.

Gator- My tech is the one who decided to change the electrolytics. I don't know the value of pwr tube coupling caps are.

Psy-my tech mentioned trying the coupling caps but for some reason changed the electrolytics and OT first. He is the best tech in town unfortunately.

So change the coupling caps first and if thats not it try the filter caps?

[doctorjp]

The first thing I would do would be to rotate a known good tube(s) through all of the preamp positions and the PI to see if it goes away. Forgive me if this has been done umpteen times but it may be worth a try and it is very satisfying when it works! A tube doesn't have to be microphonic to be noisy.

jp

[Jade]

Wow.. He changed the OT to fix ghost noting? That's kinda like changin the motor in your car because the tires are out of balance.

Anyways, I'd suspect that your problem is caused by an interstage oscillation caused by poor lead dress. If the amp went to the tech without making this noise, and came back with a problem, the issue was probably caused by the tech moving some wires around to access whatever it was he was fiddling with, and caused your problems. A good session with a chopstick would likely remedy your problem. Although in some cases, I've seen amps that just wouldnt settle down under high gain settings without adding a grid stop resistor on one, or more stages.

I'd recomend you find a new tech, even if it means mailing it off to someone. It should be easily fixed by a competent tech.

Of course I could be completely wrong.. but the OT replacement makes me think otherwise.

Best of luck getting it back to good!

OH, and +1 on the IC caps... nothing wrong with them at all. They're a high quality cap, and I've used em for years with no issues.

Jade

[earthmud]

The reason he tried the OT was because it was quick and therefore zero labor charge. He's been really cool and charged me for parts only after the electrolytic caps and OT were replaced. If it didn't work then he'll put the original OT back zero charge. I hear ya about the motor thing though. That was kind of my thought also. I'm going to talk to him about the grid stop resistors on monday and give him another shot. He has been working on amps for thirty years and his customer service is great. He always has me come in and play the amp with both of us there and listens to any opinions I have. It was a different tech that changed the filter caps and who may have caused the issue in the first place. Thanks for all the input and if he can't fix it maybe I'll send it to you:D. Here's a link to a recording of the noise( pleaser for give my low budget recording). The noise usually comes as soon as the note is hit a fades within a second or two. It's a light fizz uder the note. Pardon the hiss but if you turn the volume up you should be able to hear it. Thanks again. Jason
http://www.soundclick.com/bands/default.cfm?bandID=621096
Wow.. He changed the OT to fix ghost noting? That's kinda like changin the motor in your car because the tires are out of balance.

Anyways, I'd suspect that your problem is caused by an interstage oscillation caused by poor lead dress. If the amp went to the tech without making this noise, and came back with a problem, the issue was probably caused by the tech moving some wires around to access whatever it was he was fiddling with, and caused your problems. A good session with a chopstick would likely remedy your problem. Although in some cases, I've seen amps that just wouldnt settle down under high gain settings without adding a grid stop resistor on one, or more stages.

I'd recomend you find a new tech, even if it means mailing it off to someone. It should be easily fixed by a competent tech.

Of course I could be completely wrong.. but the OT replacement makes me think otherwise.

Best of luck getting it back to good!

OH, and +1 on the IC caps... nothing wrong with them at all. They're a high quality cap, and I've used em for years with no issues.

Jade

[Jade]

Don't send it to me... I couldnt hear the noise! LOL I honestly didnt hear any ghost noting, which is usually more of a out of tune octave pedal type sound, if that makes any sense.

Didnt mean to slag your tech too much. Since I'm not there to hear the amp in person, he probably knows what's up much better than I do. That OT swap just seemed odd to me. But what do I know, I cant even hear the problem LOL. I admit though that my laptop has some pretty crappy little speakers.

Good Luck getting it right!
Jade

[phsyconoodler]

I hear a noise when the higher notes are played.However,I hear a lot of fret buzzing on your guitar too.
I think it needs grid stopper resistors on the power tubes.The ones that go from pin 1 to pin 5.I had a bassman 5F6A that made a noise on the decay of the note and it had no grid stoppers.I put in 1.5k ones and the noise stopped completely.
It sounds like a distortion as the note is decaying.It has been confused with a bad voice coil in the speaker at times.

[RedRock]

Sometimes a small value cap (100pf to 1000pf) from pin 5 to ground on each of the
power tube sockets will stop oscillation.

Could it be speaker cone cry?

Try using shielded wire from the phase inverter coupling caps to the
inputs of the grid stopper resistors on the power tubes.

I have used IC caps many times, and they have been fine.

[earthmud]

Thanks for all the advice guys, The recording doesn't pickup the noise that well since it's a unnatural distortion under distortion. I'm going to take some of your idea's to my tech tomorrow and we'll see what happens. It's not cone cry. I have swaped out several speakers. Jade no offense taken at all. I completely understand what you mean. I really appreciate all the help a will update the thread after we try a few more things. Thanks, J

[Jade]

Thanks for all the advice guys, The recording doesn't pickup the noise that well since it's a unnatural distortion under distortion. I'm going to take some of your idea's to my tech tomorrow and we'll see what happens. It's not cone cry. I have swaped out several speakers. Jade no offense taken at all. I completely understand what you mean. I really appreciate all the help a will update the thread after we try a few more things. Thanks, J


Thanks!

Good Luck with it. Lot's of good suggestions here. As easy as it is to armchair quarterback an amp problem over the net, I think any tech thats been at it a while will tell you, sometimes there's noises that happen that can just be a real PITA to track down. Let us know what it turns out to be when it's fixed.:BEER

Jade

[banjoze]

earthmud, did you ever make any progress on this?

[RedMan]

Poorly balanced power tubes is the number one cause of ghost notes given the filter caps are good.