Guitar amplifier (pre-amplifier)

A few months after getting the power amplifier to work, I started with the pre-amplifier. Building it took more time than I expected. Actually, it is quite a complex amplifier. First, I spent a few evenings to plan and design the lay-out. Joining the assembled pre-amplifier to the power amp took some more time. Then, final check, power-up and test...

The design

The intricate pre-amplifier is one of the reasons why I chose the VOX AC15 as an example. It actually contains two pre-amplifiers, each one connected to an input of the power amplifier.

First, there is the “clean” channel consisting of a single EF86 AF penthode. This stage should give a clear and bright sound.

The other channel is the tremolo/vibrato channel. First I thought: tremelo, ok, but vibrato? Tremolo is easy, rapidly changing volume of the signal. Many guitar amps contain tremolo units. But vibrato, changing the pitch of the instrument? I studied the circuit to see what was happening. On the right, there is a VLF phase shift oscillator that produces the driving signal for tremolo/vibrato. The first stage is easy: just a triode amplifier. After the input stage, there is a paraphase stage, producing two opposite output signals. The outputs are connected to two filter stages, each with two inputs, having different time constants. The phase shift of these filters with the 180 degrees phase shift of the paraphase stage will produce interesting results when adding both signals. I expect a kind of comb filter will result. The third stage is a mixer controlled by two opposite signals from a paraphase splitter that is connected to the VLF oscillator. The controlled mixer stage switches between two differently phase shifted signals and some frequencies will be cancelled when the signals are equally strong. This may resemble a vibrato. A switch labeled “vibrato on/off” cancels one half of the mixer stage, which leaves the tremelo but takes away the phasing and comb filter. After the mixer there is a 5-stage high pass filter to filter out the VLF driving signal. I was very curious how this would sound. It must be good, it is one of the reasons why people love the AC15.

The tremelo unit has a “depth” trimpot that is hidden inside. I decided to move it to the front panel. We'll find out if this is a useful control.

My son inquired whether the amplifier would deliver groovy distortion. To produce more distortion, I decided to add an extra position to the “bright” channel that will cascade the two pre-amplifiers, so there is a lot of extra gain, controlled by the volume control of the “clean” channel. This will cause the amplifiers to clip. We'll hear what that sounds like.

Originally, all four inputs are at the left of the front panel. I decided to make some changes to the front panel lay-out to group the inputs and controls per channel.

Finally, I designed the DC power supply for the heaters of the pre-amplifier valves. The power transformer has a 36 V winding that was originally intended to power the heaters of two 12AX7 valves. I put the EF86 in series with the 12AX7 and 12AT7, adding 6.3 V to the heater string. Because the EF86 needs 200 mA, the two 12.6 V valves need a bypass resistor of 250 Ω. I used a 12V regulator to control the voltage for the 12AX7, thus regulating the current through the other two valves.

Building up the pre-amp

I had a u-shaped chassis made from 1.5 mm aluminium sheet. I drilled the holes and removed the burrs.

Then I populated the two tagboards and the DC heater power supply.

I collected all the pots and rotary switches and trimmed the shafts to size. I mounted the four valve sockets and the two tagboards. Then I started to add the power and ground wires. I tied the wires together in a cable harness. Then I mounted the inputs and controls and soldered the remaining components in place. I checked the wiring against the circuit diagram and admired my work.

Putting it all together

Now was the time to join the pre-amplifier chassis to the power amplifier chassis. I threaded the power wires and signal input leads from the power amp through the holes in the chassis and soldered them to the pre-amplifier circuits. I extended the two wires from the 36 V winding and soldered them to the heater supply board. Added the wiring for the high-cut control and the power switch and pilot light. Then I tied the wires together in a cable harness.

It was time to power up and test.