LND150 Boost

I’ve recently come across this very cool website, FET Preamplifier, Part 1 which is authored by James T. Hawes. What intrigued me about this site is the wealth of information on using JFET preamps to get tube like tone. JFETs and MOSFETs have some characteristics that make them a better choice to use in audio circuits vs. ordinary bipolar junction transistors such as the 2N2222A. JFETs and MOSFETs can do a very good job of providing tube like warmth and soft distortion characteristics. JFETs are also a lot easier to bias than BJT’s. The basic JFET circuit that James uses in low voltage applications is based on the very available MPF102 JFET. James has a very good FAQ section and I posed a question to him about how to get more power out of the amp. His answer brought up the use of a Supertex LND150 MOSFET. The Supertex LND150 (LND150 Datasheet) is a high voltage depletion mode device that can deliver more power to a subsequent stage than a 12AX7. It’s almost as simple as dropping in the LND150 in place of a 12AX7. The gate is analogous to the grid, drain to the anode, and source to cathode. In the circuit that James has tested, he uses a 100K drain resistor and a 330 ohm source resistor. You typically see 100K anode (plate) resistors on 12AX7 preamps. James provides instructions on how to tailor the source resistor to your particular amp. Now, LND150 curves look just like those of a pentode. This might be a problem to some of you who are stuck on triode curves, but there are a few highly thought of amps out there that use the EF86 pentode as a preamp. I recently built James’ circuit and I can tell you that it sounds awesome. I used the circuit as a boost in my AA764 Vibrochamp clone. The boost was temporarily wired in right before the first 12AX7 stage. I’ve been playing guitar since I was 13 years old and play professionally…so you’ll have to take my word on the sound. Roll back on the gain and you can get great blues distortion. Crank up the gain and you’ll get Billy Gibbons tone and more. I like this circuit so much that I’m thinking about building it into every tube amp I build from this point on. My circuit is different from James only in the fact that at the input, I use a 1megohm resistor instead of a 3.3megohm resistor. I will install the LND150 circuit as a switchable boost in the Vibrochamp. Here’s the link to my circuit…LND150Pre-Boost. You’ll have to make adjustments according to the voltages present on your power supply. The key is to pay attention to the recommendations from James regarding voltages and current. The picture above is of the finished PCB that I etched using the laser printer/iron on method. It’s connected to the Vibrochamp via aligator clips and leads. I left a few spots for experimenting with diodes to bias the MOSFET, and adding a cap from gate to ground to roll off high frequencies. If you have a question email me at bcharbour at bcharbour78 at yahoo.com

1 year ago  /  0 notes

Single Ended El84 Amp with LND150 Mosfet Preamp

I’m calling this little amp the Pilot 150.  It resulted from my desire to try a high voltage LND150 mosfet in the role of preamp.  Many thanks to James Hawes for helping me with the design.  You can check out his article “FET preamp runs on tube B+” on his site (URL at the end of this post).  Below is the schematic as built.

The above schematic comes from LTSpice and the actual voltages pretty much agreed with the simulation.  I love the sound of this little amp and it looks ’cute’ with only one tube sticking up between the transformers. 

There’s no real trick to building the amp with the exception of biasing both the LND150 and the EL84.  I’ll add information later on how to bias the EL84.  Information on biasing the LND150 is on James Hawes site.  Both potentiometers are 1 megohm audio taper (linear taper will work if that’s all you have in your junkbox).

Plate voltage is 249v,

Voltage at the top of the screen resistor 247v, bottom 243 (4v drop),

Screen current 4/1000=4mA,

Cathode voltage 6.99v, cathode current 6.99/160=47mA,

Plate current 43-4=39mA,

Dissipation (249-6.99)*.039=9.4 watts (keep it under 12 watts),

LND150 drain voltage was 116v (121v was what I was shooting for).

Things I might try:

Changing the tone control.  The current configuration does work, but it seems to me that it should have more effect. 

James Hawes website is http://www.hawestv.com/

If you have any question, or build this circuit, please let me know.

1 year ago  /  0 notes

MIB - Marshall In a Box

MIB1

I recently built the Condor cabinet emulator off of the Runoffgroove (ROG) site. The Condor is an excellent circuit and I built mine using the values to produce a Marshall cabinet response. You can run guitar directly into the Condor, or put a stompbox in front of it. But either way, I felt the sound lacked a little realism when the output was connected to a PA. To me it seemed that an amp-emulator/tone-shaper in front of the Condor was the ticket…enter the MIB. I also wanted to give the ROG Fetzer Valve a try. The fetzer valve is supposed to re-produce the frequency response and output curves of a tridode preamp tube.

I designed this circuit in LTSpice and it’s based loosely on the Fender Bassman and Marshall JCM Lead Series circuits. It uses all J201 JFETS (because I had a bunch of them in my parts box), and uses the Low Pass Filter (LPF) borrowed from the ROG Thunderchief. You can see the ROG Fetzer Valve at J1 and J3.

The “Marshallesque” tone control circuit uses fixed resistors and capacitors that were available in my parts bin and are a close approximation to the maxed out values of the original potentiometers. The tone stack is fed with the signal from a low impedance source follower JFET. The result is the scooped mids frequency slope that is very close to that of the Marshall tone stack.

I wanted the design to produce clean as well as overdriven sounds so there’s not a ton of gain in this circuit. A TubeScreamer or my ROG Thor pedal (built from a kit available at Officially Licensed Circuits) will supply the boost for overdriven sounds.

As for the sound (into the Condor)…I really am impressed. I especially love how clean notes are articulated. You can hear each string very plainly with a nice full tone.

Potential modifications to this circuit;

• Remove the LPF and run the signal straight to the 100K potentiometer before the last buffer stage. This will create a larger signal at the output.

• Add a switchable boost section like the AMZ Mini-Booster (JFET mu-amp).

• Replace J201’s with MPF102’s and up the Vcc to 15 or 18v for more clean headroom.

• For an adjustable tone section, replace the fixed resistors with potentiometers as in the Marshall tone section.

• Build the preamps (J1 and J3) per James Hawes using MPF102’s. I have built this fine little circuit…sounds great.

Web Sites mentioned in this article:

• Condor Cabinet Simulator, Fetzer Valve, Thor Pedal, and Thunderchief pedal…http://www.runoffgroove.com

• Hawes MPF102 JFET Preamp…http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp1.htm

• AMZ Mini-Booster…http://www.muzique.com/amz/mini.htm

If you build this circuit, please let me know how it works for you.

1 year ago  /  1 note