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WF-55 Block diagram

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.

Power stage

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).

Mains circuit

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

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