oscillation and ghost notes

[Geezer]

Funkalicousgroove
Junior tube assistant
Posts: 79
(4/23/04 3:40 pm)
 oscillation and ghost notes
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 My current project amp is having a problem I can't find: I have an AB763 (Well, for the most part...) that is a scratch build. The tone is spectacular but I get oscillation and ghost notes above the 14th fret, it sounds almost like a Ring modulator. Gerald weber suggests that power filtering causes this, but the amp is Brand new as are the filter caps. Does anyone have any ideas as to what may be causing this?
 
Tiny Daddy
I will work on all amps
Posts: 633
(4/23/04 4:06 pm)
 Re: oscillation and ghost notes
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 Have you tried it without feedback?
And I hope you are not using a 500pF cap to drive the phase inverter as on some schematics?
 
Funkalicousgroove
Junior tube assistant
Posts: 81
(4/23/04 4:28 pm)
 Re: oscillation and ghost notes
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 No, I tried a .022 this time because I used 5uf cathode bypass caps on the 1st 2 gain stages. I have a .047 after the tone recovery amp that I'm going to switch to a .022 because It puts too much bass into the reverb circut, and I'll pro'lly switch the .022 driving the PI to a .01 because it kind of "rolls" as it is now. Do you think the feedback could be the cuprit?
 
Tiny Daddy
I will work on all amps
Posts: 634
(4/23/04 5:20 pm)
 Re: oscillation and ghost notes
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 Don't know if it's the feedback until you disconnect it. And the wire from the speaker jack to the 820 ohms feedback resistor radiates signal and needs to be kept away from other wiring.
Could it be a microphonic tube?
I assume your layout places the preamp at one end of the chassis and the output tubes and power supply at

[Geezer]

I assume your layout places the preamp at one end of the chassis and the output tubes and power supply at the other end?
Or maybe it needs an additional power supply bypass cap (.22uF should work) that is physically closer to the preamp tube.
 
Funkalicousgroove
Junior tube assistant
Posts: 83
(4/23/04 6:26 pm)
 Re: oscillation and ghost notes
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 It's not a tube, the wire is away from the circutry, it's like a ring modulator on certain notes. My lead dress is good, as well as my layout, I can email you a pic of the chassis. I don't understand what you mean by power supply bypass cap---like a filter shunted to ground after the plate load resistor? will it affect tone like a silverface fender?
 
Tiny Daddy
I will work on all amps
Posts: 635
(4/23/04 6:50 pm)
 Re: oscillation and ghost notes
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 I mean add an extra capacitor in parallel with the last 20uF but place it near the first tube, from B+ to the negative end of the cathode bypass cap. The idea is to bypass higher frequencies locally so they do not have to travel through other (inductive) wiring.
I should have asked before, do both channels vibrato and normal have this problem? And if you pull the preamp tube for one channel, does the other still do it?
 
tubejunky
Posts: 18
(4/23/04 8:01 pm)
 Re: oscillation and ghost notes
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 Something I haven't given much though to, but could definitely be the source of a serious headache is the potential for the circuit board to become conductive. What type of board material are you using? Have you checked this out?
 
Funkalicousgroove
Junior tube assistant
Posts: 85
(4/24/04 3:20 pm)
 Re: oscillation and ghost notes
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 The amp

[Geezer]

The amp is single channel with no vibrato, I'm using g-10 garrolite for the board. I disconnected the Feedback with no change in Buzz, It almost sounds like a Cone rubbing-This speaker works with other amps. I am using a 4.7 k feedback resistor so there is less signal in the negative feedback anyway. I changed the coupling cap after the tone recovery amp back to stock fender .022. The amp has a PPI master and when I turn the input volume way up the Ring modulator sound subsides somewhat (I can still hear it a bit but not as pronounced)
 
Funkalicousgroove
Junior tube assistant
Posts: 86
(4/24/04 3:32 pm)
 Re: oscillation and ghost notes
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 I had another problem with this amp in it's earlier stages with noise and I thought it might be my 1st filtering stage because I get no reading with my cap meter. I am using 2 220uf 350v caps in series to get 110uf at the 1st stage, they are bypassed by 270k resistors to balance voltage. I'm going to change them to 80uf because the amp is too stiff, I'll let you know if this fixes my oscillation. in the mean time I'm open to suggestion-
 
Tiny Daddy
I will work on all amps
Posts: 636
(4/24/04 4:27 pm)
 Re: oscillation and ghost notes
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 Maybe start simplifying the circuit by removing the post-PI master volume, see if that helps. Or perhaps the output transformer is arcing?
Or jump the signal around a preamp tube so that tube is no longer in the circuit. Maybe feed a signal in from another amp or other source to determine where in the signal chain the problem occurs. Or tap the signal out at various points and feed to another amp. Use .1uF cap and shielded wire.
 
Andy Poupart
Senior tube assistant
Posts: 141
(4/24/04 11:08 pm)
 Re: oscillation and ghost notes
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 You'vd d

[Geezer]

You'vd done the obvious and simple things, right? I mean the input tube grid stoppers are mounted directly on the tue socket, and you've used shielded wire from the input jacks to the preamp tune grids? So many oscillation problems are simply solved by these two techniques.

 
Funkalicousgroove
Junior tube assistant
Posts: 87
(4/26/04 10:07 am)
 Re: oscillation and ghost notes
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 I'm pretty sure the issue is in the power filtering, I'm going to buy a scope and a dummy load and see if I can find it. The PPI master is part of the original design of this amp. This is not the first one of these I've built, but for some reason this one wants to fight back. I suspect one of the caps in the first stage of filtering may be the culprit. The OT is Fresh out of the box and of excellent quality, I doubt that it is the problem.