To get a new opinion
Get a little work done
The next day, I headed off to the home improvements stores, hoping to find some 3/8 thick dimensional lumber to build a box with roughly 800 enclosed cubic inches and dimensions of 5.75x9.25x15. The closest I found was some plywood and tongue-and-groove wainscot material. I'll take gluing up tongue-and-groove pine boards over cutting plywood any day, so the decision for me was simple.
Satisfied that things worked as expected, I built a grille panel from another piece of oak veneer panel board and some jute cloth. It's a nice snug fit and the grille panel can be easily removed with a flat blade screwdriver or similar tool if I ever need to service the speakers.
While waiting for the clear coat to dry on the cabinet, I began focusing on the sound and circuitry. I had built something like this for my son a few years ago, from an old 2-way bookshelf speaker and the same pre-assembled LM386 audio amplifier, and it sounded about the same as this one did. The main problem was the amplifier circuit would distort when the volume control was at or above 3/4 of maximum volume. Second, there was no tone control. For this to be a useful guitar amplifier, I had to address both of these issues while getting as much volume out of it as possible.
The tone control was the simplest of these problems to solve. I already had a Big Muff Pi tone circuit on my breadboard, so I put it in the signal path. between the guitar and amplifier. The addition of this passive circuit between the guitar and amp input jack almost took care of it all, as it cut the incoming signal strength.
There was just a little bit of distortion with the amp and guitar volume all the way up, but using my son's Epi LP Junior with a humbucker pickup resulted in too much distortion at higher guitar and amp volume settings. What I needed was a preamp stage that would allow me to manage the gain as well as a way to reduce the signal level into the amp enough to keep from clipping the LM386. That also meant the addition of control knobs to the cabinet. That being the easier part of this build for me, I decided on the knob locations and spacing and came up with a way to mount the tone and gain control potentiometers.
Back to the gain and output issues, after some experimentation, I came upon a combination of two simple jfet circuits, an Alembic Stratoblaster Booster and a Fetzer Valve 1.1 Booster.
In order to optimize this circuit for a variety of guitar pickups, I found I could vary the value of Rs=10k, between the J201 transistor Source and ground. I found three particular fixed resistor values that worked well with single-wound pickups, humbucker pickups and active pickup on-board circuits typical in acoustic-electric guitars, that were also compatible with one setting on the 100k trim pot at the preamp output, setting the signal level into the LM386 amp.
The selector switch in the preamp circuit allowed choosing an overall gain range to match the guitar pickup output level, the B100k gain control potentiometer allowed for a gain adjustment that includes a tube-like overdriven effect at the upper range of the control, and the 100k trim pot kept the preamp output low enough to avoid clipping distortion in the LM386 amp.
I found a few things to maximize the volume of this little 1/2 watt power amplifier in the schematic notes and technical data for my power amp. The notes indicated removing the on-board jumper would provide additional gain. The datasheet also indicated this, and included instructions on adding a resistor at the jumper contacts would produce different levels of gain. Unfortunately, taking off the jumper resulted in a large loss of volume as well as large reductions in the overdriven ranges. Trying a 1k pot in place of the amp jumper also resulted in lower volume levels as well as reductions in overdriven ranges. Switching the gain control from a B100k pot to a B50k, as drawn in the Alembic Stratoblaster circuit, provided a smoother set of gain ranges, but did nothing to allow for any more volume. Seeing no other ways to optimize this circuit, I decided to set it aside for now and try some other preamp circuits with this 1/2 watt amp and speaker setup.
Since jfet preamp circuits typically have a warm tone than their NPN and PNP counterparts, I thought there might be a way to use a jfet stage to take the edge off of the LPB-1. Feeding the output of a Fetzer Valve stage through a trim pot into the LPB-1 yielded some nice tone, so I replaced the first stage trim pot with a B100k potentiometer. After turning through a rather large clean range, the upper range of this control allowed the first stage to overdrive the second stage. Turning up the drive control all the way allowed for using the guitar volume as a gain control with a very small clean range, as well as a HUGE amount of sustain with the upper end of the guitar volume control. But, this came at the cost of some muddiness in the harmonics that detracted from the overdriven sound.
Finally, I swapped the two preamp stages, putting the LPB-1 in front of the Fetzer Valve with the B100k drive control in the middle. Knowing the gain of the NPN stage could easily overdrive the jfet stage, I was pleased at how well this combination worked. With two stages of amplification in this preamp, I expected to have to keep the Fetzer Valve trim output very low to keep from overdriving the amp, and was pleased to find the power amp could handle the preamp output up to about half of its 100k range with a single pickup guitar. I also found a good amount of usable Drive control settings that produced clean sounds when turning the guitar volume control down. This left plenty of knob rotation on both the Drive control and guitar volume control for inducing a wide range of tasty jfet overdrive, all the way from mild after-buzz to all-out, long-sustaining, mega buzzing sounds, full of rich harmonics. To my surprise, the humbucker on my son's LP Junior did not significantly alter the Drive control ranges or require me to reduce the output level into the amp. This circuit even worked with the preamps on my acoustic-electrics. Considering how versatile this setup was with just the three controls and without the need for a gain range selector switch, I settled on this preamp circuit for the LeSpork Practice Amp. All that was left then was to solder the rest of the circuit to the board, bias the J201 transistor, reassemble the amp, add some insulation to the inside of the cabinet, and reattach the rear cabinet panel.
Time to stick a Spork in this project and call it done, and play some of These Old Guitars!