The WF-55 (like the Champ 5F1) has about as simple a circuit as you can get, and
it's very easy-to-follow. The block diagram above shows the stripped-down schematic (a
diagram that shows how the electronic components are connected as a circuit). This block diagram
is a good place to start if this is your first amplifier build. Here's an explanation of each of
the colour-coded sections.|
First amplification stage
The guitar input socket is shown at the top left, and it feeds into the light green shaded section. This
is the first stage of the amplifier and it uses one half of the ECC83 preamp valve to amplify the
low-level guitar signal. The final part of this section of the circuit includes the
Volume pot, which controls how much
of this boosted signal is fed into the second stage.
Second amplification stage
The guitar signal - still fairly clean so far - is fed into the second part of the amp (shaded
in light yellow in the block diagram). This uses the other half of the ECC83 preamp valve. If the Volume
control is set high, the incoming signal will overdrive the second amplification stage to create
some mild preamp distortion (see How the amp distorts box, right). The output of this
stage then runs straight through to the input of the power valve.
The pink shaded section shows the power stage, which includes the 6V6 power valve and the output
transformer. The valve operates in what's known as "single-ended Class A" mode. Typically, this
creates a guitar tone with a fast attack.
The power valve works with high voltages and low currents, and the output transformer
transforms this into a low voltage, high current signal that's suitable for driving the speaker
(which is connected to the jack socket at the top right of the diagram).
Just like the vintage Champ, this circuit uses negative feedback: a small amount
of the power amplifier's output signal is fed back into the preamp. This negative feedback is taken
from the 4-ohm tap on the output transformer and runs (via connection point D on the diagram) to
the cathode of the second amplification stage. It's in the opposite phase to the signal
that comes from the amplifier's Volume control and that acts to lower distortion in the output stage.
Valve heater supply
All valves used in popular guitar amps need a heater supply (the heater is the part of the valve's
internal electrodes that glows dull-orange when an amplifier is switched on). The light orange section
of the block diagram shows this part of the circuit. It uses one of the windings on the power
transformer - the one that provides approximately 6.3V - and there's a simple connection to
minimise hum levels.
High voltage supply
The power transformer provides a high voltage in addition to the low voltage supply that heats the
valves. This high voltage winding is shown in the light blue section of the block diagram. It feeds
into four rectifier diodes which work together to turn the AC voltage into the DC voltage required
by the valves. The fifth diode, a type known as zener, acts to lower slightly the supply voltage to get it
into the desired range.
The large power supply capacitors and resistors filter out mains hum and pass the DC
voltage on to each of the amplification stages (shown by the dotted vertical lines).
Finally, the light grey section at the bottom right shows the part of the amp that operates directly
from the mains supply. It includes the fuse, On/Off switch and a neon indicator. Just as important
is the connection of the Earth wire from the IEC lead to the amp's metal chassis.
|How the amp distorts|
|To generate distortion in the signal that reaches the speaker, the amplifier is designed so that
each stage can be overdriven by the previous stage. So, when the Volume control is at maximum, the
output of the first amplification stage is 'too hot' for the second stage to reproduce accurately. The
result is that the guitar signal is amplified and clipped at the same time, and it's this clipping that
produces the harmonics that fatten up the tone.|
Distortion also comes from the power valve - because the already fattened-up signal
is also too hot for the 6V6 valve to reproduce accurately. So it also distorts while trying to amplify
the signal, which leads to an even richer distortion tone.
The ECC83 preamp valve's two identical triodes are cascaded one into the other