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Screeching preamp in DIY amp(s)

4/12/1999 12:28 PM
Aaron	Screeching preamp in DIY amp(s) I recently built a 2x6L6 amp using trannies from an old Peavey Classic VT. I built the power amp like a blackface Fender, except with a resonance control and a Marshall-style PI and presence control. I built the preamp fairly identical to one input of the 1959/1987 Plexi design with some minor differences. I used a .018u coupling cap between the first and second stages, and an 820 ohm cathode resistor on the first stage with a bypass cap (forget the value). I also put in a master volume (1M pot downstream of the tone stack). The rest is identical to the 1959/1987 preamp. Anyway, my problem is this: With a 1M gain pot, the amp sounds really, really good until I get the gain up to about 3. The preamp is mildly overdriven at this point. Above that, and it emits a nasty high-pitched squeal. I thought that maybe I was just overloading the 2nd stage grid, because in the Plexi, with the inputs paralleled, the unused channel provides a parallel path to ground, which I suppose knocks the incoming signal down some, right? So I paralleled a 470k resistor with the 2nd stage grid to ground (effectively adding the effect of the second input with the volume all the way to zero), and the thing still began to screech at gain = 4. I tried knocking the B+ down from approx. 315V to about 280V by adding resistance in the power supply. Got more distortion, same screech. What is going on here??? More background: I have a Marshall 2203 that I modded for more preamp distortion by increasing the first 2 plate resistors and dropping the second stage cathode resistor from 10k to 4.7k. I tried a 3.3k resistor there, but the amp screeched the same way my DIY does. Why is this screeching happening and what can I do to fix it?

4/12/1999 12:28 PM

Aaron

Screeching preamp in DIY amp(s)

I recently built a 2x6L6 amp using trannies from an old Peavey Classic VT. I built the power amp like a blackface Fender, except with a resonance control and a Marshall-style PI and presence control. I built the preamp fairly identical to one input of the 1959/1987 Plexi design with some minor differences. I used a .018u coupling cap between the first and second stages, and an 820 ohm cathode resistor on the first stage with a bypass cap (forget the value). I also put in a master volume (1M pot downstream of the tone stack). The rest is identical to the 1959/1987 preamp.

Anyway, my problem is this: With a 1M gain pot, the amp sounds really, really good until I get the gain up to about 3. The preamp is mildly overdriven at this point. Above that, and it emits a nasty high-pitched squeal. I thought that maybe I was just overloading the 2nd stage grid, because in the Plexi, with the inputs paralleled, the unused channel provides a parallel path to ground, which I suppose knocks the incoming signal down some, right? So I paralleled a 470k resistor with the 2nd stage grid to ground (effectively adding the effect of the second input with the volume all the way to zero), and the thing still began to screech at gain = 4. I tried knocking the B+ down from approx. 315V to about 280V by adding resistance in the power supply. Got more distortion, same screech.

What is going on here???

More background: I have a Marshall 2203 that I modded for more preamp distortion by increasing the first 2 plate resistors and dropping the second stage cathode resistor from 10k to 4.7k. I tried a 3.3k resistor there, but the amp screeched the same way my DIY does.

Why is this screeching happening and what can I do to fix it?

Replies:

Doc This amp uses negative feedback fro... -- 4/12/1999 12:44 PM