Similarly to Brian May who uses his Deacy amp with a treble booster in front, I wanted to build something similar that can be used with my germanium radios converted into guitar amps. All those amplifier circuits in radios are designed to amplify wider range of frequencies and they expect lower impedance signal at the input, so plugging the guitar straight into the amplifier may not be ideal. As soon as you roll down the volume on guitar from 10 to 9, output volume drops significantly, because of the higher impedance of pickups. Having some sort of a treble booster at the front helps shape the sound of the guitar and reduces issues with impedance mismatch. I chose Rangemaster because it’s a legendary circuit and I have a bunch of germanium transistors that I could try and pick the one I like the most.
The circuit is very simple, but effective. It can pump out a respectable 24db boost in higher frequencies and push the amplifier into overdrive. Two components are crucial for the effect – input capacitor and transistor. Input capacitor forms a high pass filter and depending on the value it will let more or less of middle and lower frequencies.It’s best to experiment and find the right value for the particular guitar and or amp, but I found that anything above 22nF makes it less of a treble booster and more of a full range booster. Feel free to experiment with wider range of capacitance, but I recommend starting with values between 5nF and 15nF. The transistor should be a PNP Germanium (or even silicon). It affects boost level, but also the sound. Rangemaster is not a squeaky clean booster, it will overdrive when pushed hard and transistor will affect the character of overdrive. R.G. Keen from GeoFex.com wrote a great article dissecting the Rangemaster circuit. He recommends a low-leakage Germanium transistors with gains between 75 and 100. In addition to the choice of resistor by gain, we also need to worry about the bias. According to R.G. Keen, the best sounds can be had with collector voltage around -7V.
For more versatility, input capacitor can be omitted from the circuit board and placed on a switch, so we can have switchable capacitor values. One of many ways to accomplish this uses an ON-OFF-ON toggle switch with 5nF capacitor across the middle lugs. It’s always in the circuit. Outer lugs can have two capacitors that can be switched in parallel with the existing 5nF capacitor. Diagram below shows a setup that allows switching between 5nF, 10nF and 15nF capacitors for a wide range of sounds.
I drew a simple perfboard layout that can be used on 1×1″ protoboards that have 81 holes. It’s drawn with a trimmer for bias, although I put a fixed resistor in there and experimented with several transistors until I found the one that biases perfectly. Also, there’s room if you want to install sockets in parallel with input cap to add more capacitance and shift the frequency response curve to the left.
Being simple circuits, most boosters can be squeezed onto small boards, so I used one those awesome little 1×1″ proto boards with 3-per-pad plated-through holes. It’s very sturdy and easy to work with and I love using them for simple builds like this. I started from Dallas Rangemaster schematic and changed a few things – added socket in parallel with the input capacitor so I can add more capacitance and change the low-end/midrange response of the booster, replaced 47uF emitter capacitor with 22uF solid electrolytic (shouldn’t affect the sound, even 22uF is plenty), replaced the 10nF output capacitor with 22nF (there’s another input capacitor at the amp input, so I didn’t want to cut too much bass by having two capacitors in series).
As always, I used transistor socket to allow playing with different types of transistors and tried pretty much everything I had unfortunately, no OC44). The one I liked the most was GT308V that came out of the bin labeled 100-110hfe and gives a perfect collector voltage of -7V with stock biasing resistors.