I ran across this circuit on an old Sunn schematic. It's a servo to set the operating point of a JFET so you don't need a pot. Resistors R1 and R2 form a voltage divider, the voltage at the center will be the Drain voltage. The voltage divider will track changes in supply voltage and the opamp will adjust the Source voltage to bring the Drain voltage back.

R3 needs to be low enough that the opamp can still supply the Drain current and not have to swing it's output too close to ground. If the JFET needs a source voltage too close to ground, add a resistor across the 47uF cap, lower than needed to bias the JFET and let the opamp pull it positive to reduce Source current. If you want to add a treble peaking R/C or Fetzer resistor in the Source, add it between the 47uF cap and the Source.

I built this circuit on a proto board with a J201. When I plugged in a MPF102 the bias adjusted itself and the gain was very close to where it was with the J201. The bias adjusts perfectly changing the supply from 9 to 32V. The bias can drift when the output is clipping. You'll get a little jump in bias when the signal cuts off. A slight adjustment to the R1/R2 ratio will fix this.

This is a different take on the emulating triode tube distortion theme. First, a few acknowledgements are in order. Findeton's thread, http://www.ssguitar.com/index.php?topic=1532.0 is some fine work but I find it a little complicated. Certainly the math gives me a headache. I didn't look at Dimitri Danjuk's paper, I just figured it would be more mind numbing math. The Runoff Groove article http://runoffgroove.com/fetzervalve.html is interesting but I need to see waveforms, not just equasions. The one problem I found in the article was the statement "The first valve stage of a Fender amp can withstand input voltages up to +/- 2.5V without noticeable clipping." We shall see that isn't true. Steve Conner's thread http://www.ssguitar.com/index.php?topic=2039.0 is more my style. I know Steve from Ampage, he is very knowledgeabe on tubes and solid state design. I hope to be able to present pictures as good as Steve's

The first method I will present is the opamp with diodes in the feedback path. As I mentioned at the end of Findeton's thread, the clipping stage of the TS-9 does not come close because in the non-inverting configuration, the input signal gets added to the distorted signal due to the +1 term in the gain equasion. That leaves inverting opamps and shunt clipping. A significant problem is that diodes clip at pretty low voltages so, to keep from having to attenuate the signal, you would need lots of diodes in series. I have overcome this obstacle with a circuit I call the Diode Multiplier.

When I was in high school, I was told that a silicon diode drop is 0.6 or 0.7 volts. That's true at around one milliamp. With a 100K input resistor, +/- 2.5V input signal, we would like to know what the voltage drop is at +/- 25 microamperes. I have curve tracer photos of a 1N4148, a germanium diode, and the gate of a J201. The germanium diode is interesting because it starts conducting right at zero volts. The J201 gate stays off for a significantly larger voltage than the 1N4148, that could be useful. Low voltage zener diodes are another surprise. They start conducting around 0.4V and don't reach full zener voltage until the test current of 20mA is reached. The 1N5221B is a 500mW zener rated at 2.4V (at 20mA).

The first circuit uses a TL072. One trimpot sets the gain, the other sets the amount of diode multiplication by attenuating the feedback appled to the diodes. This circuit does not address the grid current of a triode issue, that will be added later. The 10K trimpot was set to divide by roughly 17, 10.26K on the top portion and 724 ohms on the bottom portion.