Guitar amplifier (combo)

For years I had been wanting to build a valve amplifier. Finally my son brought the occasion to build this guitar combo. It was inspired by the VOX AC15 from the early sixties. After a first prototype using vintage Amroh transformers, I built a new power amplifier using vintage Geloso transformers. Then I assembled the pre-amp, joined the two and tested the amplifier. This amp really sounds good. It is finished, just leaving a few cosmetic details to improve.

The Plan

One day my son said he wanted to play the guitar. Electric, of course. I jumped on the occasion and proffered to build him a real groovy sounding valve amplifier. I wanted to build an amplifier that does not only sound great and look good, but that is also sturdy and safe.

After some deliberation I decided to build a 10-15 W amplifier, roughly based upon the VOX AC15 design. My Elektor valve amplifier book pinpointed it as the amplifier being loved by many guitar players for having shaped the sound of the Brit pop in the sixties. I also got a lot of inspiration from Jheijers site who describes his VOX AC-15 clone in great detail, mechanical as well as electrical. The VOX AC15 dates from 1960, when the use of distortion and guitar effects was just beginning to catch on. It has 2 separate pre-amplifiers: one clear channel and one with a tremelo/vibrato unit. It has a characteristic sound. Some people call it bright, others somewhat tinny. My version certainly answers to that description.

Building the amplifier

After building a first prototype, I decided to use different transformers and make a new chassis. I built the new power amplifier and tested it. Then it was time to assemble the pre-amplifier and join the two to form the L-shaped amplifier chassis.

I mounted the chassis in my “service brackets” so I could handle the chassis and make it stand upside down during measurements and modifications.

Powering up

After joining the two chassis and interconnecting them, I thoroughly checked the circuits against the schematic diagram. First I inserted the 3 pre-amplifier valves to test the DC heater supply. No rectifier valve valve yet, no plate voltage. I connected the power cord and switched on. Monitoring the DC heater voltage, I saw that the DC heater supply was working fine.

Then I connected a load resistor and hooked up my scope. I inserted the rectifier, output valves and phase splitter and switched on again. The voltage on the buffer capacitor rose to 338V and then dropped to 286V. I checked the voltages in the pre-amplifiers and was satisfied to see that these were well within range.

Then I connected the input signal. Both pre-amps were working excellently. The “clean” channel turned out to be much more sensitive than the “tremelo/vibrato” channel. Using the switch position I had added putting the channels in cascade, I was able to overdrive the input of the “tremelo/vibrato” channel and get a fair amount of distortion.

Then I inserted the VLF oscillator valve and saw the output signal started wobbling. I could control the speed and depth. In the “tremelo” position of the “tremelo/vibrato” switch the amplitude variation was more visible than in “vibrato” position. Triggering the time base of my scope to the VLF oscillator at low speed, my scope could visualise the action of tremelo and vibrato. Well, not completely, as the phase shifts are only visible by the image getting a bit hazy.

First time live

Time to make music. I connected the amplifier to a speaker and my son plugged in his guitar. We were not disappointed at all. This amplifier had a nice smooth sound. Quite bright on the clean channel, more veiled on the “tremelo/vibrato” channel that already has a really interesting sound with the “tremelo/vibrato” function switched off. The vibrato and tremelo functions give something extra to slow songs. My son liked the distortion caused by the two pre-amps at the “cascade” position of the clean channel. The distortion does not make chords unrecognisable, you still hear the music. Compared to it, the Bedrock sounds a bit harsh.

When I switched off the lights, the inner beauty of the valves became visible: the glow of the cathode and heater and the blue haze within the output valves.

Then it was time to start building the cabinet.

After finishing the shriner’s work I covered the cabinet with shiny snakeskin Tolex. Then I bolted the speaker to its baffle board, mounted the amplifier on its carrier board and put it in place. We connected the guitar and tried the amplifier. It was sounding really fine. Again, a clean and bright sound. A pretty veiled sound from the tremelo channel and deep tremelo action. After a while, we really became quite fond of the “vibrato” function, that actually does sound like you are bending the tone of your strings with a vibrating motion.

A few months later I picked up the project and covered the baffle board with speaker cloth. Before that, I replaced the simple wood screws that held the speaker by stronger M6 screws going through the baffle.

You can see two of these screws on the picture that shows how the chassis and its carrying board are mounted in the cabinet.

At this point, I also added corner pieces to cover the corners.