A simple EL84SE guitar amp

Finally some updates to this website. What do I have been doing up to now? Well, university isn’t easy.
A friend of mine asked to design him a simple guitar amplifier, just to practice at home. He’s now building it, I thought it would be good to post here the schematics, in case someone is interested.

It’s simple, probably lightweight if carefully built, and could go in a small combo amp with a 8″ or 10″ loudspeaker.
It has a “gain” control (R22) and a “volume” control (R12). Here’s the amp schematics:

(click on the image to enlarge)

And this is the power supply schematic:

(click on the image to enlarge)
Do not forget a 1A slow blow fuse in series the primary of the power transformer!!!

About the sound of the amp:
Most of the sound of a typical guitar amp derive from the speakers, the cabinet, and from the tone shaping of a preamplifier circuit, before the output tubes. Then, minor things help to shape the sound, such as power supply impedance, output transformer, the brand of the tubes…
Where are the “critical” components in the amp to tweak the sound? Well, you can go after the sound you want in two ways:
  • Use a “guitar amp” output transformer, particular speakers in particular boxes; this way you get the sound of your OPT, if it’s, for example, too small to give full output power without distortion. Or you’ll get the sound of your speaker if it’s not an average sounding guitar speaker.
  • Use a “quasi hi-fi” output transformer and average speaker box. This way the possibilities are much broader, and you can get many different sounds. Some people prefer one kind of sound, perhaps the sound of smallish OPTs that tend to saturate, I personally suggest that you try to modify capacitor and resistor values, instead of trying to shape the sound by “misusing” of the output transformer.
So? Well, let’s start from the end. Please note that ALL the components (capacitors, resistors…) that I won’t mention in these notes, should be left with the value I show on the schematic. Of course you can get higher voltage capacitors, higher power resistors, different dielectric material, etc etc… but the value should remain the same.
Instead, there are some components that can be tweaked to change the sound.
  • C7, starting from the input. It defines an high-pass filter in conjunction with R14 and cathode impedance. Choose low values (< 1uF) if you like hard rock distortion and tight bass. This is to eliminate some low end signal before distortion, to limit bad sounding intermodulation products. Instead, use an electrolytic (10-22uF) to get full bandwidth, to play blues or light rock, where little distortion is required.
  • C8 should be left that value. Tweak it only if you have an oscilloscope available and can monitor the output of the amp for possible ultrasonic oscillations. If it oscillates, increase C8. If it doesn’t, try to reduce its value. Higher values will give a mellower sound, cutting the highs.
  • C9 (and C2): it makes another high pass filter. Maybe it’s one of the last components to trim: I think 220nF is a good all-around value. If you have too much bass, or farty distortion, take a look at C7 first.
  • C11: this is important. If the amp oscillates remove it. It bypasses the divider formed by R19 and R22, making an high-pass filter, centered in the upper mids. Tweak the value to go from a mellow overdrive to edged shred.
  • C6: same as C7. If you use the “two channels” switch, when you’re in high gain mode the low pass is domined by C7, and when you’re in low gain mode the low frequency response is dependent from C6. So tweak this value for a suitable sound in the “clean” channel. I prefer to use big caps in this position – at least 22uF, to get clean full bandwidth sound.
  • C1: should be left 100uF. Reduce its size to reduce bass response or “farty” sound, but don’t exceed. Also making it bigger could somewhat “slower” the transient response of the amp, making for a “slower” sound when you hit the strings hard.
  • C4: screen bypass capacitor. I usually use an average valued electrolytic, you can increase it getting less “sag” when the strings are strum hard, or you can decrease it to get higher “sag”. Also increasing R25 (and also R23 – R24 in the PSU schematic) can give more “sagging”.
  • C13: it is a good idea to bypass it with 1uF 400V polypropilene, or 100nF 1kV ceramic. This tends to cut background noise of the amp, maybe giving the sensation of a sweeter sound. Also I would suggest to use UF4007 diodes in place of 1N4007s.
Wiring issues, building, layout…
A guitar amp is very similar to a phono stage: it amplifies tiny signals, shapes them with various nonlinear stages (in this case, both in amplitude and in frequency response), and most of the time is subject to and noise and oscillations.So use phono-stage style building and layouting: shielded wires, star grounds, maybe DC heaters. Feel free to use grid stopper resistors as a powerful weapon against oscillation, just observe that they are almost 10x the values you would find in a typical hi-fi tube amplifier, and you’ll experiment (if you build the amp) that there is a reason for this 😉
Also shielded tube sockets do help. Place input tubes away from the power transformer but also from the output transformer, use metal sheets to shield input circuits, and do not use a metal chassis as a ground return. This guitar amp makes a very suitable project for beginners, but getting good results in terms of noise and hum could be tricky.