Hawes Amplifier Archive by James T. Hawes, AA9DT
FET Preamplifier, Part 7

Troubleshooting

SYMPTOM CAUSE WHAT TO DO
No output signal - A Battery problem
  1. Check for reversed battery leads.
  2. Leads are fine: Replace battery with one that you know is good.
  3. Still no output signal: Measure battery current. It must not exceed 3 mA. If you read over 3 mA, you have a wiring error or solder bridge.
  4. Repair the faulty circuit.
No output signal - B Mis-wired FET
  1. Check your PC board against pinout diagram.
  2. Unsolder FET.
  3. Reposition FET.
  4. Solder FET into circuit.
No output signal - C FET is loose in its socket.
  1. Never use FET sockets. They add capacitance, resistance and inductance to your circuit. They can also become intermittent. Remove FET from socket.
  2. Unsolder socket.
  3. Solder FET into circuit.
No output signal - D Reversed zener diode
  1. Make sure that zener cathode (striped side) connects to JFET gate.
  2. ZENER WIRING IS FINE: Go to next cause.
  3. FOUND REVERSED ZENER: Reverse zener and retest amplifier
  4. STILL DOESN'T WORK: Replace zener and retest amplifier.
  5. STILL DOESN'T WORK: Go to next cause.
No output signal - E Wiring error
  1. Check PC board connections against schematic.
  2. With colored pencil, go over each correct connection on schematic.
  3. Check component values against schematic.
  4. On schematic, circle correct values.
  5. Repair circuit errors.
  6. STILL DOESN'T WORK: Use ohmmeter to check resistors for damage. If you can't get good reading in circuit, remove resistor from circuit and retest it.
  7. Replace damaged parts.
No output signal - F Reversed battery
  1. Check to see that positive battery lead connects to 1.5K resistor.
  2. Check to see that negative battery lead connects to 560Ω resistor.
  3. YES TO 1 & 2, BUT CIRCUIT DOESN'T WORK: Go to next cause.
  4. NO TO 1 OR 2: Repair wiring & retest circuit.
No output signal - G Dead FET
  1. Temporarily replace FET with part that you know is good.
  2. Retest circuit.
  3. CIRCUIT STILL DOESN'T WORK: Go to next cause.
  4. CIRCUIT WORKS: Discard the old FET.
FET makes my FM or AM radio whistle. Or: FET picks up radio. Oscillating FET
  1. Solder 0.01 μF ceramic or mylar capacitor from gate lead to ground.
  2. Retest circuit.
  3. Still oscillates: Solder 0.01 μF ceramic or mylar capacitor from drain lead to ground.
  4. Retest circuit.
  5. Still oscillates: Slip ferrite bead over gate lead.
  6. Retest circuit.
Touching amplifier case makes noise. Floating case.
  1. Solder ground lead between metal case and signal ground.
  2. Solder ground lead between metal case and earth ground.
  3. Retest circuit.
Amplifier distorts signal. Too much gain
  1. Solder 100K resistor in series with input.
  2. Still distorts: Replace gate resistor with 1M pot. Wiper goes to gate. Input goes to top of pot. Third leg of pot goes to ground.
  3. Adjust pot for undistorted signal.
Output is too soft. Not enough gain
  1. Solder 10μF to 1,000μF capacitor across source resistor. Larger capacitors allow amplification of lower signals (more bass). Smaller capacitors (for example, 0.01 uF) turn preamp into high-boost amp.
  2. Still distorts: Replace gate resistor with 1M pot. (Wire pot's center tap to input. Either outer tap to ground.)
  3. Adjust pot for undistorted signal.

FET amplifier links

  • The Tillman amplifier page describes the JFET preamplifier. Also discusses advantages and disadvantages of the Tillman circuit.

  • JFET preamplifier for professional Telefunken U47 microphone.

  • FET project book. Entire text, including details and schematics for 50 projects. I don't know how the webmaster squares this site with the copyright office. Anyway, for now, you can download and read about all these projects. In itself, this site is an education.

  • Replace tubes with FETs. With FETs, Darlingtons and BJTs, a radio amateur redesigned an R392, tube radio. Very clever analysis and reconstruction. Many parts just plug into tube sockets! Afterward, the radio's performance was the same or better. Of course, power consumption dropped. Other advantages that the conversion would accomplish: The retrofit would alleviate heating and its destructive effects on components. Parts replacement costs would drop dramatically. Extremely clever. Read this article for ideas.

  • Professor Tweed. Works with several types of FETs, because pots in the drain lead control the bias. Original design used 12AX7s.

  • Fetzer Valve. Description of one way to design with JFETs. Also discusses replacement of audio tubes with JFETs.

  • FET Matcher. Build this clever circuit that can match key characteristics of different FETs. If you want repeatable performance from your FET circuits, a matcher might be a good idea. When you must replace a FET on the road, you need to know that it can do the job. Match those parts and carry your spares for insurance. Great how-to!

  • Albert Kreuzer's site. Describes a three-FET preamplifier with gain. Kreuzer built the advanced circuit for bass guitar work. The circuit includes an optional equalizer and an LED that indicates clipping. The main preamp is three BF245 JFETs. Two stages provide gain and the third stage is a power amplifier. The circuit requires 15 volts. The source-follower output has 4.7-kilohm, relatively low-Z impedance for a preamp. Some stages (EQ and overload indicator) employ op amps. An advanced builder could easily delete these stages and features. The amp has optional controls for sensitivity, bass cut, ultra-low filter, mute, EQ/bypass, ultra high filter, and volume. The five-range EQ section has separate bass/midrange/treble controls, plus five switchable midrange filters. The clip sensor has a threshold adjustment. The amplifier also provides a follower preamp for the tuner. The site offers a PC board. Kreuzer says the amplifier has a "warm, tube-like sound. Clarity and directness." On overload, the circuit behaves similarly to tubes.

  • British Activator kit. Circuit is a JFET that drives a PNP transistor for low-impedance output. Provides 5 dB of gain. Undistorted output is 5 Vpp. That might be optimistic. Circuit operates on nine volts. Very small, packed PC board. Site provides schematic without parts values. The circuit mounts in the instrument. This is a wired module. Assembly involves installation in the guitar. Six connections. Listen to the blues sound sample.

  • Peppermill FET overdrive circuit. Gives a tube-like ambience to overdriven guitar tone. Not a straight amplifier, but instead a "soft distortion" box.

  • Ready-made FET amplifier that you can order. Includes equalizer.

  • Interesting audio forum with several useful links to articles and schematics.




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WARNING. The author assumes no responsibility for your success or failure in using methods on these pages. Further, the author neither makes nor implies any warranty or guarantee as to the accuracy or effectiveness of these methods. Proceed at your own risk.
 

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