Saturday, May 24, 2008

Stompboxes II — Stupid FET Trick

I started out wondering: "How variable are JFETs? How many parameters must be measured when selecting them?" I ended up with a grungy overdrive/distortion: the Stupid FET Trick.

Looking at the 2N5457 datasheet from Fairchild, you can see that drain saturation current (IDSS) and gate cutoff voltage (VGS(OFF)) are correlated: I've seen other methods on the web for selecting FETs to use in audio, and they generally involve selecting for gate cutoff voltage, since this affects the operating point. IDSS is much easier to measure, or at least to select for. In my case, I was designing a circuit using the stock CircuitMaker model, which has an IDSS of 3.859 mA at 15V. (The range of values in the datasheet are 1.0 mA to 5.0 mA.) The model's behavior was determined with this little test circuit simulation:The device test circuit is not much more than that. It needs a supply voltage somewhat greater than the reference voltage in the datasheet (I have an 18V regulated supply), has gate and source tied together, and has a drain resistor set to drop the supply voltage to about the reference voltage with saturation current flowing:
You don't really have to have a close-tolerance on the resistor; just measure its actual resistance and use that to select the range of voltages it will drop for the range of currents within your design tolerance. I needed five 2N5457s that were within 5% of the Circuitmaker model. I bought 100 (for 8 cents apiece at Mouser), and I tested 40 before I found six that were within my range. Most of the others were under and a few were over (not surprising since the selected value was in the upper portion of the range). My circuit was nothing earth-shaking, just a few ideas taken from typical tube guitar preamp design with the JFETs substituted in, and component values adjusted to get a meaningful operating point at battery voltages. I didn't do a thorough scientific verification of the gain in the real circuit, but the measured operating points were all within 10%. (Even though I selected the JFETs at 5%, the resistors were 10%). It doesn't really sound like a tube amp, but it is better than the last distortion unit I built (some 25 years ago), the Tube Sound Fuzz from Craig Anderton's Electronic Projects for Musicians. I continued my tradition of natural aluminum enclosures, and aside from the name of the unit, I only used pictograms for the legend (Skull & crossbones = gain; heart = tone; quaver = volume). I'll have to design the next JFET circuit around a device with a lower IDSS (and VGS(OFF)) to use up the ones I have left.

Coming soon: Interstellar Overdrive!

Saturday, May 17, 2008

Stompboxes I — The Buffalo Box B175

While performing inventory of my parts bins last fall, I came across a couple ancient-looking TO-1 metal cans. I have no recollection of their origin, but the leads were rather short, so I presume I cut them out of a junked radio or similar item, probably when I was a pre-teen. At first I presumed I acquired them at the same time, but it turns out they're from different manufacturers, so maybe not. One was labeled B175, the other was B186. A bit of searching showed they are both PNP germanium transistors intended for audio amplifier use. Word on the street was that should try one in a Rangemaster clone.

I breadboarded the crazy-simple circuit, and the B175 had a higher gain when plugged in, so I chose it for the finished project. I also added a bypass switch and an "anti-pop" 1MΩ input resistor. I assembled it on a Radio Shack "Bug" PCB (276-159) and crammed it in a Bud econobox enclosure. A "vintage" knob controls the gain. The unadorned "natural aluminum" look I used with the LK8SE continues, and is even refined in my choice of functional legend on the unit.

I've never encountered a real Rangemaster, so I don't know if it sounds like one. It does work nicely with the LK8SE for "screaming solo" work. You need to crank up the bass control on the amp to get a crunchy sound, though.

Saturday, May 10, 2008

Eric Barbour or Fred Nachbaur?

Recently, I mentioned in on an online forum my idea for a 15-ish-watt amp based only on Compactrons with a 6AS11 preamp and a pair of 6LU8s for the power amp and phase-inverter (each tube contains a beam tetrode and a medium-μ triode). One response was: "You should get a job with Metasonix". I suppose I could have taken offense at the comparison: While I think there is a place (and probably a need) in the world for a guy (Eric Barbour) who makes an amp called the F*%@ing F*%@er [grawlixes mine], that's not me or my style. Also, amusing as it is, I don't think I could bear to build an amplifier that uses 800 watts of power for just 30 watts of output.

I think the Metasonix comparison really came from the idea of using new-old-stock tubes for other than their original purpose. It wasn't Eric Barbour that gave me the idea, though; it was the circuits and writings of the late Fred Nachbaur. Fred's pages were one of my key resources when I decided to move from repairing & tweaking amps (like a Kalamazoo model 1 and a "Blackface" Fender Bassman) to designing & building from scratch. His design philosophy was mostly rooted in classic methods, but he loved to use oddball tubes, especially ones with atypical heater voltages arranged into series/parallel combinations. That approach seemed to border on obsession in some cases: Just look at the power supply for his "Spunky" guitar amp. It's a pretty cool arrangement that eliminates the need for a transformer with a heater winding. A bit of goofiness though is seen in the two 10-watt power resistors needed to set the voltages right, and the fact that there's an entire separate transformer for the bias supply. I guess it works if you have the parts on hand.

In any case, the all-compactron amp is not going to happen, at least not right away... Since I'm on the waiting list for a new Danelectro reissued '63 Baritone guitar, which will need an amp, I'm going back to a previous idea for the 15-ish watt amp. The power section will be a pair of Electro-Harmonix 6973s. Just like when I chose the JJ 7591S as the power tube for the Lama Kazu 8SE, (7591s are rarely seen in single-ended amps— they mostly turn up in old Ampegs) I couldn't bear to use the typical EL84 or 6V6 power tubes that are more common in amps of this size. More to come on that project.

Saturday, May 3, 2008

Amplifier Tweaks: Lama Kazu 8SE

The Lama Kazu 8SE was revised last fall; I never got around to mentioning it here until now. The revised schematic is at right. Most of the changes were to reduce the gain, smooth out the overdrive character and clear up some blocking distortion.
  • V2 was changed to an ECC832, (JJ's equivalent to 12Dw7) with the high-μ triode at the input and low-μ triode at the driver stage.
  • V1 was changed to an ECC81/12AT7.
  • The volume potentiometer was changed to 1MΩ and the attenuator resistor was removed.
  • Coupling capacitors into the tone stack and power amp were reduced to 22nF to drop some of the "mega bass" muddiness.
  • Cathode bias resistors were changed to 4.7kΩ at the second stage and 3.9kΩ at the driver stage.
  • Load resistor on the driver stage was reduced to 39kΩ. This, combined with coupling and valve changes really reduces the output impedance so that grid clipping at the power stage is more graceful.
  • The 68kΩ dropping resistor in the power supply was reduced to 39kΩ, since the second preamp stage is using more current with the ECC81.
Sound files will be posted eventually. This amp works really well with my Rangemaster clone, known as the "Buffalo Box" and based on an old B175 PNP Germanium transistor I found in my junk bin.