How to Bias JFET on Clandestine Preamp PCB?

[alonkka]

Hi! I just finished my Clandestine Preamp. I did some research on how to bias the JFET with the trim pot, but it was pretty over my head. Another thread asked what the optimum voltage was for the JFET and someone replied "My original measures 12.46V, with a Vref of around 22V." The PCB looks like it is set up for idiot proof biasing with a multimeter. Could someone please share with me what exactly to measure to bias this?

 

[BuddytheReow]

To bias you can do 1 of 2 options since there is only 1 jfet here. The lazy method is to do it by ear: simply play and turn the bias knob in either direction until you hit your "sweet spot". You'll know it when you hear it.

The more "correct" approach is to measure the voltage at the drain of the jfet (most left pin in the pic above). With your DMM set your voltage to 20V DC. Red probe to the drain and black to ground. It should be roughly half of the input voltage. I say roughly because some like the voltage higher/lower, but not by much. Looking at the schematic you should be able to find the input voltage at R101. Use the higher number of the two leads on this resistor. That's your input voltage. Without doing the math the jfet drain should be roughly 9v or even 8v. Dial in the bias until it reaches that number.

 

[alonkka]

Thanks very much! I will try this out tonight and see how it goes. This effect is so subtle (but tasty) that I wanted to understand how to get in the ballpark first before trusting my ears. For anyone reading this in the future. Here's how the sweep of the Chase Tone Secret Preamp is described. It might help in dialing in the jfet. When I first plugged mine in unity gain was more like 8-9.

• FULL COUNTER-CLOCKWISE = NO SIGNAL
• 10 to 11 0’clock = UNITY GAIN (Depends upon EQ Setting)
• 1 to 2 o’clock = 3dBs+ MAXIMUM OUTPUT BOOST
• 3 to 5 o’clock = 1dB or 2dBs with FLAT EQ just like a worn, vintage Echoplex

 

[devanbumstead]

I was just able to calibrate my bias based on this helpful thread. My original Secret Preamp measured 13.2V with a 22V Vref.

For anyone else like me who is fully “paint by number” on these builds, here are some pics of how I got the readings I needed to see.

I stuck my ground probe in the output jack ground to hold itself in place and I put the red probe on the upper JFET transistor pad on the Clandestine and the top leg of the Secret Pre.

For the reference voltage readings I took it off of the R101 resistor on the Clandestine and the resistor right above the transistor on the Secret Pre. I tried both sides of the resistors and took the higher reading (as advised by another thread).

*Other notes: the pedal has to be powered. When getting a reading for the reference voltage, I changed my meter to 200 DCV to get a better reading. For the bias readings the meter is set to 20 DCV

Reading bias voltage on Clandestine Preamp:


Reading reference voltage on Clandestine Preamp:


Reading bias voltage on Secret Preamp:


Reading reference voltage on Secret Preamp: