What happens if you put a resistor in serie with the clipping diode on a TS?

[Hardtailed]

Well the subject says it all (we are talking about the diodes in the feedback path of the clipping amp in a TS-style circuit here). I'm interested in doing this to get a softer clipping. The idea is that, once the diode "opens" the signal will go through the resistor thus instead of ending up with an almost zero resistance (which simply limits gain to 1), you could instead reduce the gain, giving more of a compressed sound than a clipped one.

But I'm just wondering if the resistor could have any other side-effect. As long as the diode is "closed", it is an infinite resistance, so the whole voltage accross this section is "dissipated" in the diode. However, as the diode "opens", the resistance goes down to almost zero, meaning that now the resistance is now seeing the biggest potential difference (voltage if you will). Can this affect the functionning of the diode itself? Or should it work exactly as I want?

I know the answer should be "try it out, you'll see", but I don't have much prototyping stuff around here (I've only built a clone of my Guv'nor so far) so I thought it was worth asking!

[Mark Hammer]

Small value resistors will have the effect of "softening" the clipping.  If placed in series with one diode of a back to back pair, it will also introduce some asymmetry.  I stuck a 330R resistor in series with one diode on a TS clone and it sounds terrific.  On another I installed a 1k (or maybe it was even 5k, I forget) pot to vary the asymmetry, which produce substantial differences in dynamic responsiveness.

[Johan]

  I stuck a 330R resistor in series with one diode on a TS clone and it sounds terrific.  On another I installed a 1k (or maybe it was even 5k, I forget) pot to vary the asymmetry, which produce substantial differences in dynamic responsiveness.

..and if you then bypass that resistor ( or diod ) with a cap, you can have the symetri/asymetri shift over different frequencys..
I made a diod/resistor/cap-combo for a box, consisting of a LED in one direction and a resistor and a 1N914, each bypassed indevidualy with caps, in the other direction. the resistor I had to try different values and look at a oscilloscope to sellect. but in the end, I got something that clipped at the same level ( with a squarewave put through it), but with different frequency curves..tried to mimic how a single tubestage LOOKs when clipping and got somewhere close...but there is so much more to SOUNDING tube, so I lost interest...

johan

[WGTP]

Using the AMZ Warp set up, I used a 10K pot to pan between a Ge diode and LED.  It appear that the Ge diode plus resistance gets to around 7K before the clipping of the LED starts to take over.  This is in a Rat, rather than in the feedback loop.   

[Mark Hammer]

It's not so much trying to sound "tube-ey", but rather the manner in which any clipping gets introduced in response to picking dynamics.  We had some discussion a ways back about this particular circuit from Bernie Hutchins: http://img.photobucket.com/albums/v474/mhammer/EN132.gif  It's an interesting arrangement of diode number, type and location, which is intended to introduce clipping in a graded manner.  Requires a higher level input signal (it was originally aimed squarely at synth processing), but interesting nonetheless.

[Hardtailed]

It's not so much that I'm trying to do a tube emulation, more that I'm looking for an overdrive/boost circuit that would simply sound like an additionnal gain stage on my Marshall (typical clean boost don't do it for me, the mid-boosting of an OD is part of the magic, but most stock pedals are too harsh for my taste).

I'm trying to come up with a one knob circuit that would have a very high-output yet have a typical "mid-boosting" flavor as you crank the gain, that I could tune to my tastes by experimenting with different caps. I was thinking about a classic TS circuit, well actually more like a Son of Screamer, with no volume pot (replace the pot with a 100k resistor, with output just before it) and no tone stack (the desired tone would be hardwired in the circuit). So it's more like a Grandson of Screamer! And I thought that replacing the diodes with LEDs in the feedback loop would allow much more output while retaining some clipping when driven hard (and allowing me to provide ample gain without fear of overdriving the opamp too easily). I was thinking about putting each LED in serie with a resistor which would be bypassed by a silicon diode. Thus we have soft clipping until the diode opens then we have a harder clipping, a bit like tubes...

The idea of LEDs in a TS is nothing new (some simply remove the diodes completely). What I had never seen before was the resistor in serie with the LEDs, and mostly the silicon diode bypassing this resistor (for "two stage clipping").
The real challenge though will be carefully carving the frequency response of the circuit. Good thing resistors and capacitors are cheap.

[JHS]

Simple answer, it will alter the sound of the TS to fuzz, with undefined bass.
Some commercial IC-based fuzzes are ODs modded in that way to sound like a fuzz.

JHS