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? After all, tremolo is 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 is 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 am very curious how this will 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 some more positions to the “brigh” and cascade the two pre-amplifiers, so there is a lot of extra gain, controlled by the volume control of the “clean” channel. 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.
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.
It was time to power up and test.