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Vintage threads from the first ten years

Re: Let's do something fun

4/3/1998 6:54 PM
Steve A.	Re: Let's do something fun John: Isn't there a difference between an 'overdrive' pedal vs. a 'distortion' pedal? Good point, but my question is whether the end result is to be a pedal that could be plugged into a clean ss amp for the desired sounds, or are we to factor in the effects of running it into a nice tube amp. Steve Ahola P.S. Does 'distortion' pedal include "fuzz boxes", or is that a third category?

4/3/1998 6:54 PM

Steve A.

Re: Let's do something fun

John:

Isn't there a difference between an 'overdrive' pedal vs. a 'distortion' pedal?

Good point, but my question is whether the end result is to be a pedal that could be plugged into a clean ss amp for the desired sounds, or are we to factor in the effects of running it into a nice tube amp.

Steve Ahola

P.S. Does 'distortion' pedal include "fuzz boxes", or is that a third category?

4/3/1998 9:35 PM
Ever	Good point, but my question is whether the end result is to be a pedal that could be plugged into a clean ss amp for the desired sounds, or are we to factor in the effects of running it into a nice tube amp. Exactly why I said this should be an iterative process. Now is not the time to think about this, there is a specific goal to be reached. How it can be modified later to serve such purposes will come afterwards. The point is to reduce the objective to the fundamentals, then start thinking about the 'tweaks'. The end result may be the whole thing gets redesigned, but many useful things may fall out between now and then. Let's reduce things as far as we can first, then build from there. 'K? -E

4/3/1998 9:35 PM

Ever

Good point, but my question is whether the end result is to be a pedal that could be plugged into a clean ss amp for the desired sounds, or are we to factor in the effects of running it into a nice tube amp.

Exactly why I said this should be an iterative process. Now is not the time to think about this, there is a *specific* goal to be reached. How it can be modified later to serve such purposes will come afterwards. The point is to reduce the objective to the fundamentals, then start thinking about the 'tweaks'. The end result may be the whole thing gets redesigned, but many useful things may fall out between now and then.

Let's reduce things as far as we can first, then build from there. 'K?

-E

4/3/1998 3:26 AM
MHelin	So, step one would be what kind of distortion do we want? I'll start with two possibilities: preamp distortion or power amp distortion. preamp distortion will be primarily asymetrical while power amp distortion will be more symmetrical type clipping. We want both. Preamp distortion of tube amp is not primary asymmetrical. Each stage clips asymmetrically and inverts the phase, so the signal is symmetrically clipped in whole. I think Jack Orman's FET-Muff with diodes is a good tube preamp emulator. There should be some level control between the stages, and the tone control between the stages as well. Power amp clips symmetrically, but it should also emulate power supply sag. This can be done with JFETS whose supply voltages are are lower than that od the previous stage's, controlled by trim pots, sag created by caps. There should also be the EQ, which emulates output transformer by limiting the bandwidth when signal level is high. -Mikko

4/3/1998 3:26 AM

MHelin

So, step one would be what kind of distortion do we want? I'll start with two possibilities: preamp distortion or power amp distortion. preamp distortion will be primarily asymetrical while power amp distortion will be more symmetrical type clipping.

We want both. Preamp distortion of tube amp is not primary asymmetrical. Each stage clips asymmetrically and inverts the phase, so the signal is symmetrically clipped in whole. I think Jack Orman's FET-Muff with diodes is a good tube preamp emulator. There should be some level control between the stages, and the tone control between the stages as well.

Power amp clips symmetrically, but it should also emulate power supply sag. This can be done with JFETS whose supply voltages are are lower than that od the previous stage's, controlled by trim pots, sag created by caps.

There should also be the EQ, which emulates output transformer by limiting the bandwidth when signal level is high.

-Mikko

4/3/1998 9:20 AM
Mark Buckingham	>There should also be the EQ, which emulates >output transformer by limiting the >bandwidth when signal level is high. Why not put some sort of small transformer on it? There ought to be something that would work.

4/6/1998 5:21 AM
MHelin	Why not put some sort of small transformer on it? There ought to be something that would work. That could work provided there were an appropriate load. To emulate output amp saturation there should be quite large current. Maybe those simple line trasformers used on modems could work, they are usually 600ohm:600ohm (1:1) transformers, and could be used between collector and power line of a power transistor. Simple IC bandpass filter with large bandwidth is simpler option and allows some tweaking. -Mikko

4/6/1998 5:21 AM

MHelin

Why not put some sort of small transformer on it? There ought to be something that would work.
That could work provided there were an appropriate load. To emulate output amp saturation there should be quite large current. Maybe those simple line trasformers used on modems could work, they are usually 600ohm:600ohm (1:1) transformers, and could be used between collector and power line of a power transistor. Simple IC bandpass filter with large bandwidth is simpler option and allows some tweaking.

-Mikko

4/3/1998 1:03 PM
John Greene	We want both. Preamp distortion of tube amp is not primary asymmetrical. Each stage clips asymmetrically and inverts the phase, so the signal is symmetrically clipped in whole. I just looked at the schematics here on Ampage and this can't happen on any of the 'vintage' amps. There are only two preamp stages before the phase-splitter. The input tube, which can't be overdriven without additional gain in front, and a stage immediately after the volume control and/or tone stack. So, I think, this would be the only stage that could be overdriven. The Marshalls are a different story but I don't know if they land in the proper time frame. --johng

4/3/1998 1:03 PM

John Greene

We want both. Preamp distortion of tube amp is not primary asymmetrical. Each stage clips asymmetrically and inverts the phase, so the signal is symmetrically clipped in whole.

I just looked at the schematics here on Ampage and this can't happen on any of the 'vintage' amps. There are only two preamp stages before the phase-splitter. The input tube, which can't be overdriven without additional gain in front, and a stage immediately after the volume control and/or tone stack. So, I think, this would be the only stage that could be overdriven.

The Marshalls are a different story but I don't know if they land in the proper time frame.

--johng

4/6/1998 5:42 AM
MHelin	There are only two preamp stages before the phase-splitter. The input tube, which can't be overdriven... .. and it may be that if there's odd number of overdriven stages the preamp creates more even order harmonics. When you look into Jack's diode clipping FFTs at DMZ (http://members.aol.com/jorman3/fft.html) for one diode, it doesn't seem to generate dominant second-order distortion, 3rd and 5th harmonics are well above 2nd. The 2nd harmonic is in fact a sound an octave above fundamental tone. The 3rd is nicely produced with two clipping diodes. In tube amps the 2nd and 3rd are dominant, there are less higher order harmonics. So what I mean with this is that asymmetric clipping does not in fact produce enough 2nd order harmonics, and there are too much higher order harmonics which should be avoided. In fact the same FFT's show that back-to-back LED's produce more 2nd harmonics than single clipping diode. If just there were a way to cut out the 5th and above harmonics... Anyway, it would be wise to try (and build) different pre-amp and power-amp emulators, separately of each other. Maybe we should try the power amp first. I vote for for a push-pull power amp emulator... -Mikko

4/6/1998 5:42 AM

MHelin

There are only two preamp stages before the phase-splitter. The input tube, which can't be overdriven...

.. and it may be that if there's odd number of overdriven stages the preamp creates more even order harmonics. When you look into Jack's diode clipping FFTs at DMZ
(http://members.aol.com/jorman3/fft.html)
for one diode, it doesn't seem to generate dominant second-order distortion, 3rd and 5th harmonics are well above 2nd. The 2nd harmonic is in fact a sound an octave above fundamental tone. The 3rd is nicely produced with two clipping diodes. In tube amps the 2nd and 3rd are dominant, there are less higher order harmonics. So what I mean with this is that asymmetric clipping does not in fact produce enough 2nd order harmonics, and there are too much higher order harmonics which should be avoided. In fact the same FFT's show that back-to-back LED's produce more 2nd harmonics than single clipping diode. If just there were a way to cut out the 5th and above harmonics... Anyway, it would be wise to try (and build) different pre-amp and power-amp emulators, separately of each other. Maybe we should try the power amp first. I vote for for a push-pull power amp emulator...

-Mikko

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