The Bulbamp: 1 Watt guitar class A amplifier with a light bulb

1 Watt class A guitar amplifier with a light bulb

I've started this project with the idea of designing a solid state 1 Watt guitar amplifier. There are a few projects with similar objectives on the web but I didn't want a IC amp, for instance using the well known LM386. I was looking for some discrete circuit that I could modified to my needs. I also wanted an amp that worked at 9V, so that I could use my pedals power source. After looking or googling about it I didn't find any that would please me.

The basic inspiration came from the amplifiers designed by Nelson Pass and namely the De Lite amplifier. And so I ended up with this 3 resistors, 3 capacitors and a mosfet in a class A configuration and, of course, a light bulb.

Here is the circuit.

So there you have it! It couldn't be simpler.

It has about 500K of input impedance (you can change this by increasing the values of R1 and R2) and you can use what ever pre-amp you like, even you favorite booster. The only catch with this design is that it needs a very well regulated power supply. It consumes about 200mA but you should feed it with no less than a 1A power source. The IRF610 should be mounted on a small heat sink. The theoretical output power of this amp is about 0.86 Watts. One of the great things about it is that the light bulb reacts to your picking and playing. Glowing strong if you hit the guitar hard and stays dimly otherwise. This light variation is accompanied with a variation of the resistance of the light bulb and thus introduces a kind of compression on the sound of your guitar.

I've also recorded some sound samples, and please don't pay any attention to my playing :D These were recorded with a portable mp3 recorder at 1 meter from the speakers. I've also test the Bulbamp with the speakers of an old VOX AC-30, with a Chandler tube driver as a pre-amp. It sounded amazing.

If you can't find a light bulb that makes this amp work you can substitute the bulb with a 47Ohm 10W resistor. For my version of the Bulbamp I have used a car lamp. If you live in Portugal you can use the car bulb lights made by Continente. Adjust the input capacitor too a smaller value if the amp sounds to muddy.

You could also build two of these and plug it to your sound card computer.

Another thing that you could try is to put a pot instead of the voltage divider and adjust the bias voltage accordingly to the mosfet that you use.

Please check the forum for further details.

I've also made two versions: a stereo and a mono. Here are the pics:

Please give me some feedback if you build the Bulbamp.


Palavras chave/keywords: guitar, amplifier, lamp, diy

Última actualização/Last updated: 2014-02-20 [14:40]

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