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Finding Your Own Replacements

Check the Data Sheet. What if you need a replacement JFET, but none of the parts I've mentioned is available? If you can find datasheets online, you can make educated guesses about which parts will work. This page explains how I chose four parts from Radio Shack and Mouser that tested pretty close to an MPF102.

What to Look For

Research is the name of the game. If you know what to look for, a nice substitute is fairly easy to find. Start by checking out catalog pages at Mouser or another vendor. Go to the vendor's catalog page for the JFET type that you want to investigate. Check for a match on the target device's I_DSS spec. If you can't find this spec, download the device datasheet. Or go to the manufacturer's page. Again, find and download the device datasheet. Here are some JFET manufacturers (foundries) and vendors to check...

Find the best match. A few parameters should match as well as possible to the specs for the target device...

• No bipolar transistors. Select a JFET or depletion MOSFET. A 2N3904 won't work in the circuit.

• Only choose a depletion device. Enhancement FETs such as the BS170 and 2N7000 won't work in the circuit.

• Maximum operating voltage. Using 9-volt power? A 25 to 30-volt part (or better) is what you want.

• I_DSS should match as closely as possible. Minimum I_DSS is more important than maximum.

• G_M or G_FS should in the ballpark. (A minimum I_DSS match is more important.)

Can't find the G_M? Take the reciprocal of R_DS(on). If R_DS(on) is in ohms (typical), multiply the reciprocal by 1,000, giving mS (millisiemens). If the datasheet gives a range for R_DS(on), use the highest value. For most JFETs, operating at 9VDC favors a higher R_DS(on) (lower transconductance).

The target I_DSS is 2 minimum to 20 maximum. These are the figures from the MPF102 datasheet. Again, the minimum is more important than the maximum. Minimum I_DSS can be less than 2, but must not be greater than 2. The maximum I_DSS must exceed your circuit's peak drain current. Peak current is ususally about twice the average current. (In our example MPF102 circuit, the peak current is 4 mA.)

Test the device. Insert the device into the Hawes preamp circuit. Measure the no-signal, DC drain voltage (drain to ground). A typical drain voltage is somewhere in the range 4.5 to 5.1 volts. If your measurement is way outside this range (say, 2 volts or 8 volts), you need to adjust the source resistor. If you don't, the device could clip. Then your tone will suffer.

Need to adjust the drain voltage? Follow the guidelines at Getting the Best Tone.

Voltage gain. After adjusting the source resistor for best I_DSS, compare your source and drain resistors. Do they have the same value, or is the drain resistor smaller? With the same value, you have a gain of one (unity) or below. If the drain resistor is smaller, then the circuit is attenuating, not amplifying your signal! The JFET's internal resistance, R_DS(on), equals your source resistor value. The R_DS(on) adds to your source resistor. (Remember: The R_DS(on) in ohms is the inverse of the device transconductance in siemens.) Here is the voltage gain formula...

Attenuator. What? Your V_G figure is one or a fraction? Then your amplifier is an attenuator. Yet all is not lost! To cope with this situation, wire a bypass capacitor across your source resistor. I suggest a 47 µF, 35-volt part. You can experiment with this value. Smaller values of capacitance emphasize treble notes. Use a larger value if the circuit is for bass guitar. Use of any capacitor causes gain to vary with frequency. Don't worry. Bypass capacitors are common in desirable tube guitar amplifiers. (Word to the wise: You can bypass the external R_S, but not the internal R_S.) Here's the bypass capacitor formula...

Index

• Replacements for the MPF102

• Replacement Table

• Getting the Best Tone

• Finding Your Own Replacements

• What to Look For

♦ WARNING. This is your project. Your achievement is entirely yours. I assume no responsibility for your success in using methods on these pages. If you fail, the same is true. I neither make nor imply any warranty. I don't guarantee the accuracy or effectiveness of these methods. Parts, skill and assembly methods vary. So will your results. Proceed at your own risk.

♦ WARNING. Electronic projects can pose hazards. Soldering irons can burn you. Chassis paint and solder are poisons. Even with battery projects, wiring mistakes can start fires. If the schematic and description on this page baffle you, this project is too advanced. Try something else. Again, damages, injuries and errors are your responsibility. — The Webmaster

Copyright © 2015 by James T. Hawes. All rights reserved.

•URL: http://www.hawestv.com/amp_projects/fet_preamp/fet-mpf102-sub2.htm •Webmaster: James T. Hawes
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