FETs That Swing Like Tubes
You want to overdrive that output section. Maybe it's a 6V6, 6L6 or even a 6BQ5. A JFET pedal sounds like a
good idea, except for one problem: It doesn't put
out a big voltage swing. And the swing is what your output section needs. You can jack up the preamp's power voltage to
maybe 18 volts. But even an 18-volt supply doesn't provide enough of a swing. You need to hit that final stage hard.
The question remains: How can you get the overdrive preamp to kick out the voltage you need? If only you could find a JFET
that operates at tube power voltages!
Now you can. But you'll have to give up on that JFET and switch to a depletion MOSFET. JFETs are wonderful devices. Yet finding one that operates at tube-type voltages just isn't easy. With depletion MOSFETs, the whole picture changes. Depletion MOSFETs that handle tube voltages are rare, but available. Mouser stocks the excellent
Supertex® LND150N3-G, a device that takes care of business! Check Mouser or your favorite vendor and arrange to order half a
dozen. You'll need a few for spares and experimenting.
Depletion Mode. The LND150 differs from most MOSFETs that you'll find in stores. Like a JFET or a tube, this depletion MOSFET is a normally-on device. You can bias it just as you'd bias a tube. We'll operate the device in its saturation region. Here, the gain is
better than what you'd expect from a 12AX7. And of course, the LND150 operates from a supply that would normally power
tubes like the 12AX7.
Linear. Don't let the LND150's pentode-like curves trouble you. Cathode or self-bias linearizes the FET's operation.
You can improve the linearity even more by deleting the optional source bypass capacitor C2. The cost of operation without the capacitor
is a decrease in gain. The LND150 has plenty of gain to spare. Also, despite the pentode curves, you don't need to wire up a screen and suppressor grid. And the FET requires no filament power source. You'll save gobs of power too, because there's no filament to
use it. (You'd swear that the filament was there. This device is that good.) And don't forget: Surrounding
components will run cool and last longer. In normal operation and with correct design, preamp FETs don't get very hot.
Variable gain. The 3.3-megohm gate resistor R2 offers you several choices. You can substitute a potentiometer, allowing
you to vary the stage sensitivity. The wiper of this potentiometer would connect to the LND150 gate. The pot value is also a
matter of choice. Useful standard values are 500K, 1M, 5M and 10M. If you use a fixed resistor, you may follow the schematic
or select your own value. Smaller values make the stage less sensitive. Larger values make the stage
more sensitive, but also more susceptible to noise pickup. The 3.3M resistor in the schematic seems to work well with most
guitars and microphones.
Built-In Overdrive. An LND150 project isn't really a stompbox. That is, unless you can provide a portable, 200-volt power
source. By the way, I don't recommend a series string of nine-volt batteries. Imagine the size! Imagine the replacement cost!
Also please don't string 200-volt power lines across the stage. A DC-to-DC converter inside the effect could
source the power. Yet a homemade preamp and converter might not be very small. What's the alternative? Add the LND150 overdrive
as a new input channel on your guitar amp. Breadboard the circuit. It's tiny. Then patch the power leads into your amp's power
supply. The output should drive your power tubes. (I can't say for sure. Circuits differ.)
FET Preamp Schematic
||Capacitor, 0.02 μF, 600 volts, Mylar
||Capacitor, 150 μF, electrolytic
||Capacitor, 10 μF, electrolytic
||MOSFET, type LND150N3-G, N-channel, TO-92 package
||Resistor, 68K, 1/2-watt, 5%, metal film
||Resistor, 3.3M, 1/2-watt, 5%, metal film
||Resistor, 100K, 1/2-watt, 5%, metal film
||Resistor, 330Ω, 1/2-watt, 5%, metal film
||Diode, 18 V, 1-watt zener
LND150 Drain Curves
Above, the saturation region starts at the knees on the left. We're interested in curves for negative
gate voltages (depletion mode). Note that this device also operates in enhancement mode (with positive
Flexibility. The construction method for this preamp is quite flexible. You can use point-to-point wiring with
terminal strips, perfboard, or a PC board. I recommend perfboard, such as Vectorbord®. Look for the type without copper traces. Use the general-purpose type with holes that are 0.042-inches in diameter. These holes should have 0.1-inch spacing.
Building the circuit.Unless you must fit the circuit into a cramped space,
don't be stingy with the board. Be sure to allow for mounting space around the project. This circuit should
then fit on a two-by-two or two-by-three inch board. Run bus wires across the top and bottom of the board's backside. Loop the
ends of the wire through the last two holes in the board. The loops should poke out the top or component side of the board. While testing, you can connect these loops to your power source. Connect off-board jumpers to push-in stakes. (Vector® makes these stake terminals. I suggest part T42-1/C. Such hardware is available from Mouser.) Components mount on the top, with leads poking through
the holes to the back of the board. Most of the wiring runs across the back of the board. Otherwise, build the circuit as nearly a literal copy of the schematic. That is, the power bus becomes the top of the board. Mount the FET halfway between the ground and power buses. Tack solder resistors between either power bus and the FET as appropriate. You can mount the board on standoff bushings. Most
hardware stores stock such bushings.
Slip the tiny PC board into a quiet, cool corner of the amp cabinet. Mount the board on standoffs. Or screw it to a
wall of the cabinet. Just keep the power lines short. Also, separate the input from your power transformer.
Prototype. On June 7, 2010, I breadboarded the active part of this preamp on a plugboard. An old
Conar® tube power supply provided the B-plus power voltage. (The 234 volts comes off a divider after
the supply's 5U4 rectifier.)
Source Resistor. A 330-ohm source resistor (as in other circuits on the Web) seems to be the best value. I
bracketed that by trying a 390 and a 270-ohm resistor (standard values). Each of these produces close to the desired
drain voltage. The 390-ohm resistor gives me 130 volts, which seems too high. The 270 gives me 114 volts on the drain, the
closest to my goal of 117. Yet I want to allow for the source drop and the FET's internal resistance. Considering those
voltage drops, I think that the 330-ohm resistor is the best choice. This resistor gives me 122 volts. Particular FETs and
resistors vary. For best performance, your circuit might require a value different than 330 ohms.
Capacitors. On the schematic, I've drawn in optional capacitors C2 and C3. Some experiments with and without
these parts will help you to tailor the preamp to your power amp. Capacitor C2 is a source resistor bypass capacitor. This
part will increase gain, especially for high frequencies. I base the value on capacitors that Gibson® used in the
GA-1RT practice amp. My value is proportional to the Gibson value. For more bass, you can increase the C2 value. For more
treble, decrease the C2 value. Before you add C2, try out the preamp. You might not need the added gain. Capacitor C3 is an
extra power supply filter. Again, try the amp without this capacitor. Do you hear any extra hum? If the power lines are
long, you might need this capacitor.
Gate resistors. Without substantially affecting performance, you can change the values of the gate resistors. For
example, you can use a 75K or a 47K resistor at R1. For R2, values from 100K to 10M will work just fine. Remember that
smaller values add a load to the input circuit (guitar). Larger values may let noise into your power amp. Also, larger
values themselves might become a noticeable noise source. The gate is a good place to use low-noise resistors such as
Zener Diodes. The optional zener diodes protect the input from large signals that would damage the gate. If
you're brave, you may leave off these diodes. The LND150 can't sustain transients of more than 20 volts peak-to-peak.
Above: My solid-state breadboard looks odd beside a tube power supply!
Above: Closeup of the LND150 circuit.
Why I like the LND150
The LND150 offers both similarities to, and advantages over tubes...
- FETs are easy to work with, small, portable, durable, and cool.
- Many characteristics parallel the best features of tubes: The high-impedance input. The ability to use quality film capacitors. The depletion mode of operation.
- FETs don't take a lot of power. And FETs don't overheat and dry out nearby components.
- While FETs are very sensitive, they're quieter than are triodes.
- At last! Depletion FETs work at tube-like voltages, but without filament or screen connections.
- Soldering and testing FETs requires no unusual equipment.
- If you drop a FET, it usually doesn't break.
- FETs aren't microphonic.
- FETs aren't immortal, but they last a long time. (JFETs probably outlive MOSFETs.)
We get mail. If we can believe my correspondents, FET preamps sound very pleasing. We can expect the LND150 to share the mellow tone of the MPF102 FET.
Save Space. A depletion FET takes up less space than an tube does. All you need fits onto a little piece of perfboard. Average
power drain is about 1.2 mA. You can "borrow" power for the circuit from your guitar amp power supply. Just make sure
that you keep the power voltage down to 234 volts or so.
Try it. Duplicating tube amplifiers isn't my goal. Unique, attractive, seductive tone is my goal. Music engineers haven't yet
invented all "beautiful tones." A FET preamp might well not sound "exactly like a tube," but what if the FET sounds really good? Then you'd try it, and so would others. Django Reinhardt, Charlie Christian, Robert Johnson, Les Paul, and Jimi Hendrix were each first with a trademark tone. They shook the earth. Thanks to them, it's still rocking. Have what it takes? Then what's keeping you from being the the next original? Lead the band. Rock the world. Try an LND150 booster today!
At last! The LND150 preamp achieves incomparable sound without parallel. Listen for yourself and believe.
Visit Bill Harbour's page about the fabulous LND150 preamp!
- Click here for a genuine LND150 sound
sample. In this sample, Bill demonstrates the substantial overdrive capabilities of this subminiature
- Bill describes how he uses the LND150 in two amplifiers with tube outputs. Click...
Bill Harbour's Project Blog.
Trouble finding resistors?
Click... About Resistors.
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