What's going on in this peavey schematic?

[leadfootdriver]

It's a Valveking. What are the diodes doing?

http://home.comcast.net/~leadfootdri...king%20212.pdf

[daz]

Appears it's a clipping circuit. Lots of marshalls have used them over the years including the JCM 800, 900, jubilee series and more. Just adds more distortion, and while most people hate SS distortion i found it sounded quite good in the marshalls i had.

[GainFreak]

You can find the same circuit in Peavey's XXX and JSX heads schematics as well. It's supposed to be a noise gate.

[olddawg]

It's an as symmetrical clipping circuit. Any signal across those diodes over .6 -.7v gets clipped.

[Enzo]

No no no...

Those diodes do not clip at all, they are a noise reduction circuit, as Gain Freak said.

To clip a voltage over .6v, they would have to be from the signal path to ground, not in series with it.

What they do is snip out a small part of the signal as it crosses zero volts. If you analyze the circuit you will also find they are shorted across by those switches when in clean mode. The small amount of crossover distortion they add is not heard in the lead chanel, but would be noticed in clean.

Noise is typically a very low level signal in the background of your music. What these diodes do is get in the way of that. The signal has to go over the half a volt the diode needs before it can pass. And since the signal goes both ways, we have two diodes cross wired. If you have say 30v of signal, yuou won;t miss that half a volt. But since the noise signal is typically lower than that half volt level, it cannot make it past the diodes. SO the noise is trimmed out of the signal.

[leadfootdriver]

Thanks for the reply's.

So in essence, when I have a lot of gain, but the guitar volume turned off, you don't hear the hiss in the preamp? I checked a JSX schematic, and they have a pot where the 1M resistor is.

I can get one of these for dirt cheap. I was considering one to beat around for like band practice purposes.

What do you guys think of them?

The clips I've heard on YT are nice on the cleans, but the people have them very death metal scooped out on the gain channel I go for a more full range tone.

[Enzo]

It is not really a gate, just a reduction circuit. If you want to hear their effect, tack a wire across one of the diodes, shorting it across. The control in place of the resistor allows you to dial it out. Adjusting the control to the zero resistance end is the same thing as my piece of wire shorting across the resistor.


SO it is dirt cheap? And can you sell it for the same dirt cheap if you hate it? Then buy it. Over at the Peavey forum there are some threads about modifying these.

[kleuck]

You have the same kind of circuit in every metal or high gain distortion pedal (usually with Ge diodes).

[leadfootdriver]

THis amp has a lot of ceramic caps in it, especially in all spots of the tone stacks, and a lot of coupling caps. I have to think that the amp would sound better with some film caps.

[olddawg]

No no no...

Those diodes do not clip at all, they are a noise reduction circuit, as Gain Freak said.

To clip a voltage over .6v, they would have to be from the signal path to ground, not in series with it.

What they do is snip out a small part of the signal as it crosses zero volts. If you analyze the circuit you will also find they are shorted across by those switches when in clean mode. The small amount of crossover distortion they add is not heard in the lead chanel, but would be noticed in clean.

Noise is typically a very low level signal in the background of your music. What these diodes do is get in the way of that. The signal has to go over the half a volt the diode needs before it can pass. And since the signal goes both ways, we have two diodes cross wired. If you have say 30v of signal, yuou won;t miss that half a volt. But since the noise signal is typically lower than that half volt level, it cannot make it past the diodes. SO the noise is trimmed out of the signal.

That does make more sense. Usually when you see back to back diodes on an input it's a clipper. Well shut my mouth, lol.

[Enzo]

Yes, that is true, but the only way they can clip is if they are across the signal path, not along it. The exception to this description would be if the diodes were in the feedback loop of an op amp.

[olddawg]

Yes, that is true, but the only way they can clip is if they are across the signal path, not along it. The exception to this description would be if the diodes were in the feedback loop of an op amp.

You are absolutely correct as usual. The road to hell is paved with assumptions. I assumed there was a ground path somewhere.

[exclamationmark]

I believe these are 'coring' diodes which basically gates noise below their turn off voltage (hence taking the core out of the signal, when above the threshold). They probably add a miniscule amount of crossover distortion too, but compared to the voltages in the preamp, it's negligible. Impossible to notice on a scope.

I also once saw another interesting circuit that used diodes (I think it was in a diezel amp). It was basically a voltage divider that had a diode across one of the resistors (much like a bright cap). The effect of this was to engage the voltage divider on one half of the sine wave, but bypass it on the other half - you can get huge amounts of asymmetrical distortion this way.