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Vintage threads from the first ten years

Open the feedback?

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6/10/2000 10:19 PM
Screamer	Open the feedback? I've read about cutting the feedback wire from the power amp output to it's input. Is this a simple and worthwhile mod? I have a Carvin Xamp. Do I even have that? How would I find out?

6/10/2000 11:54 PM
Screamer	I have located the loop feedback on the schematic. It runs from the speaker output to the poweramp. It has a resistor (10k) and a capacitor (250pf). Should I cut it out?

6/11/2000 1:48 AM
Don Symes	Try putting an SPST switch across the resistor first. Experiment at LOW volumes first and keep RG's tube amp debugging FAQ handy - you may be unleashing oscillations at frequencies too high to hear. Good luck

6/11/2000 2:54 AM
Screamer	Replace the resistor with the switch? Keep the capacitor? Or eliminate the whole thing? I did read RG's debugging page....Maybe I shouldn't try this myself?

6/11/2000 10:25 PM
jason	Don, Wouldn't putting a switch across the resistor short out the resistor there by connecting the secondary of his OT to the PI (if it's connected like a typical fender NFB loop). I think he want's to get rid of the NFB, not increase it. Maybe I'm missing something. jason

6/11/2000 11:14 PM

It seems to me from what I've read, unless I misunderstood, which is quite possible, that the thing is to get rid fo the feedback loop all together. It is a mid-80's Carvin x-60b head. i am not familiar with the Fender circuit, I suppose I could get a schematic and compare the two Which one should I look at?. If there was a way, I would post a picture of the schematic.I think I might need to have someone other than myself work on it, so I don't end up frying myself or burning something out. Also it looks like if I replaced the resistor with a switch, it would take the resistor out of the circuit and leave only the capacitor because it looks like they are wired together.

6/11/2000 11:38 PM

quote:
"Also it looks like if I replaced the resistor with a switch, it would take the resistor out of the circuit and leave only the capacitor because it looks like they are wired together."

Exactly - my bad, hope you didn't try it.

As you've already figured out, you want to be able to cut the WIRE, not the RC circuit.

The big risk here is that the amp will go unstable and oscillate without the feedback - possibly at frequencies you can't hear. IIRC the AmpDebugFAQ has a section about how to detect oscillation.

Don't be afraid, just be careful.

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