Build Your Own Guitar Amp – In this project article, the author shares his guitar tube amplifier, a simple, low-wattage, that any DIYer can easily build. Costas Sarris believes in building simple tube amplifier designs, simple circuits that he finds work well, especially when electrical signals carry musical information, because signal distortion is easily handled. This article was originally published in June 2015.
When a guitarist friend asked me to build a guitar tube amplifier, we agreed that the design of the amplifier should be a simple, low-wattage single-ended amplifier — built like a Swiss Army Knife for everyday use. Inspired by the late 1950s Fender “Princeton” and “Champ” amplifier designs, I decided to build a small but efficient single-ended head amplifier, which any DIYer could easily build (see Photo 1).
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This little amplifier is a medium/high gain head, which means it can be played at room or studio levels. Although it can be quite loud, it can also be played with smooth bass and “quiet drums” on small steps.
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Because of its warm sound, the amplifier is not made for any kind of hard metal music. On the other hand it is ideal for blues, rock, jazz, or funk music. Simplicity is the essence of this amplifier. This makes it different (see photo 2).
Photo 2: Inspired by the Fender “Princeton” and “Champ” amplifier designs of the late 1950s, this simple project was designed for everyday use.
The musical information generated in the form of electrical signals by electric guitar pickups is weak and requires amplification. Guitar pickups have different output voltages depending on the model. Low-output models tend to produce a “clean” sound, while high-output models tend to overdrive the amplifiers and produce a “dirty and aggressive sound”. The output voltage of most pickups varies between 100 mV and 1 VRMS.
This amplifier uses a high gain 12AX7/ECC83 double triode vacuum tube for the preamp stage and a 6L6GC beam power tube for the output stage in a single-ended class-A design.
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Class-A amps are voltage amplifiers. The waveform of the output voltage in a class-A amplifier is the same as the waveform of the signal voltage applied to the grid. In other words, the power tube always operates at full power and this affects the way the output tube distorts. Class-A power amplifiers generally sound warm and natural. When it goes to clip, it sounds dynamic and aggressive. This is why these amplifiers are desirable for many guitarists.
The preamp stage uses a high gain 12AX7/ECC83 dual triode vacuum tube, and operates as a grounded cathode amplifier. The output is taken from the second part of the 12AX7 triode plate to the grid of the power tube, using a high quality coupling capacitor 0.1 μF. The driver tube is cathode biased with 1.5kΩ resistors (R8, R2 shown in Figure 1), and 200V plate voltage (both phases).
The first half of the preamplifier stage is a completely bypassed cathode bias circuit. The input grid-stopper resistor R12 is 33 kΩ. To bypass the cathode resistor in the first stage (first half triode), I used a 25µF/25V audio electrolytic capacitor (C2 shown in Figure 1).
A gain potentiometer (1 MΩ) acts as a variable grid resistor across the grid of the second half of the 12AX7 preamp tube. The tone control section consists of a simple “tone boost control” circuit. C6 is a treble-shunt capacitor, so P2 (the tone control pot) affects both “tone cut” and boost. The amount of boost depends on the setting of the gain pot. By tweaking the gain, you can adjust the amount of distortion you want.
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The second half of the triode stage can be with or without a cathode bypass capacitor. Reducing the size of the cathode bypass capacitor, improves the transient response of the amplifier.
The power tube cathode resistor is completely bypassed (C1, 25 µF/25 V), requiring about 20-V signal amplitude to drive the power stage at full power. In this circuit, the grid resistor is a variable resistor (1 MΩ master volume pot). So if we use a small size (eg, 1, 000 pF) cathode bypass capacitor in the second stage of the preamplifier, the input sensitivity of the amplifier—with the master volume pot fully open (1 MΩ)—is only 15 mV. This means we only need a 15 mV input to overdrive the power stage! By using a high-output guitar pickup, we can easily overdrive the amplifier to produce the “aggressive sound” we want. By increasing or decreasing the cathode bypass capacitance in the second half of the driver stage, we can improve the tone response to our personal taste.
In this project, the 6L6GC power tube is operating at 58 mA plate current when no signal is applied, and 350V plate voltage. Primary impedance of output transformer 4.5 kΩ. The power tube cathode resistor is completely bypassed by a 25 µf/25V capacitor (C1). The input grid-stopper resistor is 1 kΩ (R1). The voltage across the cathode resistor is measured to be 19.5V.
One of the most important components in the audio path is the output transformer. Therefore, you should never compromise on the quality of the output transformer.
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For this amplifier, I chose a custom-made, “E” shaped, high quality single-ended output transformer (see Photo 3). The resonance-free frequency range for this transformer is well over 80Hz to 20kHz (–3 dB). A transformer has two secondary windings of 4 Ω and 8 Ω. I also recommend using Hammond manufacture’s 125ESE or 125FSE output transformers.
When designing a single-ended tube amplifier circuit, the power supply section must be carefully considered. For this project, my custom-built power transformer used the following windings:
12AX7 and 6L6 power tubes and secondary windings for 260 V-0-260 V/120 mA were 6.3 V/1.5 A filaments.
Hammond Manufacturing’s 269AX power transformer is a good choice and is readily available. The rectifier circuit uses a single segment “pi” filter or “capacitor-input filter”. Both filter capacitor C11 and smoothing capacitors C9 and C10 are 47 µF/500V electrolytics. The rectifier bridge circuit in the power supply section consists of two 1N5407 general purpose rectifiers. The filaments are powered by 6.3 VAC for driving and power tubes. For the AC input to the power supply, I used a filtered IEC power input connector (see photo 4).
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Photo 4: I used a filtered IEC power input connector for the AC input to the power supply.
For the construction of the amplifier, I used a ready made 1.2mm thick aluminum chassis. Dimensions were 25 cm × 15.5 cm × 4.5 cm (W × D × H). I made the wooden cover from handcrafted 16mm thick birch plywood (see photo 5). Next, I covered the wood cover with leather. Tolex or textured vinyl can also be used.
Before final assembly, I soldered all the parts to the soldering tag board (see photo 6). All pots are Alpha 24mm full-shaft 1-M linear.
I installed all the basic components in the chassis (tube sockets, output transformer, power transformer, pots, input, and speaker jacks, etc.). Next, I finished the basic wiring starting with the filaments. I used 1.2mm wire for the ground plane line and all critical paths. All soldering under the chassis is done by hand, using a soldering tag board to place the parts, point-to-point wires.
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All power supply parts installed on the left side of the chassis below the power transformer are soldered on a separate tag board. To complete the wiring, I connected all the parts directly from the tube sockets and input/output jacks to the tag board (see photo 7). All tube sockets were made of high quality porcelain (see photo 8). For the final step, I mounted the chassis to the wooden casing.
Photo 7: To complete the wiring, I connected all the parts directly from the tube sockets and input/output jacks to the tag board.
Two guitarists tested the amplifier. One is Elias Zaikos, a legendary guitarist and founder of Blues Wire (www.blueswire.gr), and the architect of the blues in the Greek blues scene. Another guitarist and good friend is Jorge Ligas, owner and designer of Warlord Custom Pedals (www.guitarpedals.eu). He is also the one who asked me to build this amplifier.
Both guitarists agreed that with 10W of output power, this amplifier rocks amazing tones, and sounds great when paired with a good-quality speaker. For our testing, we used the P12R 12″ Alnico speaker.
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“Kostas Sarris was kind enough to let me try one of his little miracles, and I’m so glad he did!” Zaikos said. “I’m a working musician, I play tight and hard, electric blues. A guitar sound needs harmonics, tone, bite and character. His amp allowed me to travel around exploring sweet and bold single notes and double stops and all the points in between, Really good! To cut it short, when you play the blues on the road, you need an amp to “hear” you, feel your touch and express your innermost self…and that’s not easy. Made it feel like playing through a trusty sonic box. I couldn’t ask for more. Clear and slow or rough and growling, it was a fantastic experience, Kostas, thanks for sharing your work!
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