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The JFET Bender Preamp

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This thread is not about a distortion pedal called the Tone Bender.
It is about a preamp that emulates a Blackface Fender tube preamp.

In a previous thread I explored ways of getting opamps to act more like a 12AX7. Link: This is a continuation of the same idea except a JFET is used in the feedback network. A JFET is ideal for this circuit because it doesn't have a dead zone where it stops conducting current if the bias is set right. I hope some of you will try this idea in some exsisting designs to add some low level even order distortion. I call it the JFET Bender because it bends the straight line input-output relationship an opamp normally produces.

The JFET is used as a variable resistor in a region where it's resistance changes at different current levels. MPF102 JFETs were used because they had a bendable resistance in the right range. I originally tried 2N4391-3 but the pot setting was way too touchy. I had to install multi turn pots. The MPF102s have an Idss usually between 8 and 12mA. The Curve Tracer shot shows how Voltage and Current behave around zero volts. When the Voltage swings negative, the curve is fairly linear but as it swings positive, the JFET starts to go into constant current mode and the current levels out. Adjusting the Gate Voltage lets you adjust the amount of bend and thus the amount of distortion.

The distortion generated is mostly pure 2nd order at low output levels. Some 3rd order creeps in as the output of the stage reaches 10V pk-pk. The distortion rises to about 10% (this is adjustable via the pot) when the output of the opamp starts to clip. Because the gain is not the same on plus and minus halves of the waveform, one side clips before the other. To adjust the pot, set the pot near zero Volts. That's where maximum gain is. Apply a signal and adjust the pot until the gain is reduced to about 80% (-2dB) the max value. Or just adjust it by ear for a little gain reduction. If the pot is set too far negative, the JFET cuts off and the gain falls to 1. You can find a place where there is severe clipping on one side of the waveform if desired.

For those paying attention, the gain reduction caused by an N channel JFET in the first experiment is backwards compared to what it's like in the first stage of a tube guitar amp. What is really needed is max gain on positive inputs (closer to vertical line on the curve tracer) and a slow reduction in gain for negative inputs (the curve of the JFET lays over horizontally). I played guitar through a couple of stages setup backwards like this with a tone stack in between and it didn't sound right. It just didn't have the Fendery clean tone I was looking for.

Next I tried a J201 connected differently to invert the distortion. The Drain is grounded. When the Gate and Source swing positive, you have the bulk resistance Rds(on) in parallel with the Gate to channel diode. This is fairly linear until the diode starts to conduct. When the Gate and Source are pulled negative, you get the Idss curve and it lays over (resistance increases) in the third quadrant. The disadvantage of this circuit is that there is no adjustment and the resistors in the feedback network have to be adjusted to get the current and voltage in the right range for the J201. This circuit sounded better as a first stage but there was not enough distortion at the gain I wanted for the first stage. Distortion was in the range of 1% to 4% at 1Vrms input depending on the impedance in the feedback network. Too big a resistance in series with the JFET in the shunt arm of the feedback network limits the amount of distortion the JFET can generate.

Lastly I looked for a P-channel JFET that had favorable characteristics. J174 and J175 were nowhere close. The 2N5462 looked good. It is a near compliment to the MPF102. This worked in the original circuit, all that was needed was to tweek the pot for positive bias on the Gate. A 2N4343 was also used, it worked about the same.

Since a tube preamp has mostly inverting stages, a non-inverting opamp preamp will have to flip the distortion on alternate stages. I think not doing this is why the first try didn't sound very good. So a P-channel JFET is in the first stage and an N-channel JFET is used in the second stage. The second stage needs a gain around 50.

When the gain is high, there is not much Voltage available for the JFET to distort. On the second stage the feedback resistors set the gain at 100 and the pot is adjusted to get 6dB of gain reduction. This gets the distortion up to around 10% before the opamp starts to clip.

The preamp is configured to mimic the Fender Blackface preamp. The first stage has low gain to provide about the same headroom as a tube, the diodes on the input clip the incoming signal so as to be pedal friendly and protect the opamp. The diode on the second stage is inverted because the input stage opamp is non-inverting.

To test the circuit out, I used a Peavey Mark III Standard from the 80's. It has two opamp based preamp channels, one with a FMV tone stack, and a handy power amp input on the front panel. The power amp is a 260D (130W at 4 Ohms) with Voltage only feedback. Without the capacitors in series with the JFETs, there was too much subsonic feeding through to the power amp so the caps were added to limit subsonic response.The amp gives a very convincing Blackface tone for both bass and guitar. I especially like it for bass, but the caps could be increased slightly to give a little bit more low end.

Thank you for a full and illuminating report on your investigations.  This moves things forward.


Hello Loudthud.

Glad to have found someone who has ideas and research close to what I have done for many years.

I'm not a master in the art (as mr. KMG), but I noticed a parallel between our work.

Now I saw this and its other posts will follow them, and occasionally make some suggestions


I thought I would provide a little background on how the JFET Bender emulates a triode. Some of this has been covered before in this thread:

The first scope shot is an X-Y display of a Chinese 12AX7 with a 100K plate load to a 300V supply. Input is the horizontal axis, the plate is the vertical axis. As the input moves negative (to the left) there is a reduction in gain. This is where the JFET needs to become a higher impedance to reduce the gain. The next photo shows the JFET Bender. To make a clearer display on the scope, the 1uF capacitor in series with the feedback resistor is shorted out. To better match the 12AX7, the input clamp was reduced to three diodes as shown in the schematic. The last scope photo shows what happends when the bias is reduced on the JFET taking it out of it's nonlinear region. The input clamp is still working on the right side of the input.

how critical are the FETs? Is it the gate voltage that is the important factor?
If I dont have MPF102's will other FETs suffice - eg:2sk170, 2n3819, 2sk117 etc?



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