Hawes Amplifier Archive by James T. Hawes, AA9DT
FET Preamplifier FAQs, Part 3

Resistor Questions

QUESTION. I can't find the right values of metal film resistors at Radio Shack. Can I use carbon film or carbon composition resistors?

ANSWER. Yes, carbon film or carbon composition resistors are fine. Metal film resistors produce less noise than carbon composition resistors, though. Also, metal film resistors seem to have tighter tolerances than the usual carbon resistors. Your next choice after metal film is carbon film resistors. Carbon composition resistors should be your last choice.

Unless you're very particular about noise, don't let the resistor type bother you. If you're using a vintage amplifier, it probably has carbon composition resistors. Adding a few more shouldn't be a big problem. On the other hand, the input preamp is the most sensitive point in your amplifier. If you can buy the metal film resistors, definitely do it.

Unfortunately, Radio Shack has been reducing its parts stock. Every year or two, more resistor values go missing. We realize that resistors are low-margin items. Still, without the right resistor values, you can't program a transistor or JFET to work properly.

These vendors all sell metal film resistors...

Mouser Digi-Key Allied

QUESTION. Can I substitute for some resistor values that I can't find?

ANSWER. Some of the resistor values aren't critical. For example, you can use 1M instead of the 3.3M gate resistor. You can use 180K instead of the 220K pulldown resistor. At least at the start, don't substitute for the source and drain resistors (560 and 1.5K ohms). By the way, the 560-ohm bias resistor is only a typical value. Some JFETs require a different value. You must determine this value by testing. The best value gives you the most headroom and the least distortion. The specs for the MPF102 device are broad, and don't allow me to predict your results. The source resistor could be either smaller or larger than 560 ohms. I picked a value that works well with typical MPF102 devices. See my discussion at... Source Resistor.

QUESTION. I can't find a 3.3M resistor at Radio Shack. What should I do?

ANSWER. Use a 1M (one megohm) resistor instead. You'll lose a little sensitivity, but you'll gain lower-noise performance. Radio Shack sells a 500-pack of carbon-film resistors. Carbon film resistors are noisier than metal-film resistors, but carbon film parts will work. The Radio Shack resistor pack includes these circuit values...

  • 3.3M gate resistor

  • 1.5K drain resistor

  • 560-ohm source resistor

The 220K pulldown resistor isn't there. Still, you can substitute either a 180K resistor or a 270K resistor. Both are in the pack.

QUESTION. Why do some players prefer carbon resistors to metal film resistors? Aren't carbon resistors noisier than metal film resistors?

ANSWER. Metal film resistors produce less noise than either carbon composition or carbon film resistors. Yet what's "noise" to some is "ambience" or "presence" to others. The "noise" that a resistor produces is a natural sound. It could add a rough, sizzling, wild, rustic edge to a solo. If the others all use metal film resistors, maybe they all sound antiseptically pure. Maybe the guy with carbon resistors is the one with true grit. Or maybe he's really on fire. In the world of rock and blues music, being on fire can be as good as barbecue.

Just like the sound of water rushing over stones, or wind stirring through fir trees, "resistor sound" is real sound. Some scientists believe that the incomparable tone of a Stradivarius violin stems from shellac. The shellac gives the instrument's voice a natural ambience. If a musician can use the ambience of a shellac, then why not use the ambience of a carbon resistor? The choice is yours.

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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

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