The PP-18 is a simple amplifier, with an easy-to-follow circuit. The block diagram (above) shows the
PP-18's basic 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 colour-coded section of the amp.|
First amplification stage
The guitar input jacks are shown on the far left, and they feed into the light green shaded section.
This is the first stage of the amplifier and it uses a 12AX7 preamp valve to amplify the low-level
guitar signal. Depending on which of the two input jacks you plug into, the amp uses either one half
of the valve to amplify the signal (as in the original classic Marshall 18W circuit) or both halves
(for a very slight increase in gain and a slightly different tone).
This part of the circuit also includes the Volume pot, which controls how much of
this boosted signal is fed into the phase inverter. It also includes the Tone control, which is the same
as that used in the original Marshall's Tremolo channel.
Phase inverter section
The guitar signal - still fairly clean at this point - is fed into the amp's phase inverter circuit
(shaded in light yellow in the block diagram). This uses two sides of a 12AX7 valve to create two signals
for the amp's power valves - one out of phase with the other (see Phase explained, right).
The way that this stage reacts to the signal dictates a lot of the tonal
characteristics of the amp. If the amp's Volume control is set above about 2, the incoming signal will
start to overdrive the phase inverter valve to create some mild preamp distortion.
The pink shaded section shows the power stage, which includes the EL84 power valves and the
output transformer. One valve takes the in-phase signal from the phase inverter and the other takes
the out-of-phase signal from the other side of the phase inverter. Together, the valves operate in
what's known as "push-pull Class AB" mode (a few people mistakenly believe that the 18Wer and
the Vox AC-30 are Class A amps - this is not true; they're both Class AB amps). Typically, the
push-pull circuit creates a guitar tone with a lot of even harmonics, making the amp's distortion tone
sound warm and full.
The power valves work with high voltages and low currents, so the output
transformer coverts this into a low voltage, high current signal that's suitable for driving the
speakers (connected at the speaker jack sockets at the top right).
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 (it heats the valve's
cathode which allows electrons to flow to make the circuit work). 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 a connection that helps 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
connects to the next part of the power supply - the rectifier.
Four diodes work together as a bridge rectifier, turning the AC voltage into the
positive DC voltage required by the valves. The Standby switch sits in this part of the amp circuit
and controls whether or not the DC voltage is fed into the large capacitors and power supply resistors.
These work together to filter out mains hum and pass various DC voltages 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 socket to the amp's metal chassis.
|The signal from the guitar's pickup is a voltage that varies with time. As the string vibrates
around its centre line, it creates a small voltage in the pickup windings that varies around 0V
(zero volts) - the electronic equivalent of the string's centre line. This is shown here in simplified
form - a sine wave centred on the 0V line.|
The result of the PP-18's first amplification stage is a signal that's out-of-phase
with the input but much higher in terms of the swing around the 0V line. To keep things simple, let's
assume that the amplifier is also totally clean. The result is shown here (yellow trace, below): as the
signal from the guitar's pickups is rising, the output signal of the first stage is falling, and vice
One half of the phase inverter valve takes the signal from the first stage and
amplifies it further. In doing so, it reverses the phase once more (red trace, below); the signal at
this point is now back in-phase with the signal from the guitar pickup. The other half of the phase
inverter is connected in such a way that its output remains in-phase with the input, which makes it
out-of-phase (blue trace, bottom) compared to the guitar signal. The result is that you get two
versions of the guitar signal both at a much higher voltage than the original, and with one being
the 'mirror image' of the other.
These out-of-phase signals are fed into the power valves where they are amplified yet
again, and the phase of each side is reversed once more. Finally, these outputs feed into opposite
sides of the primary winding of the output transformer where they are combined. The result is a single
signal sent to the guitar speaker through the output jack.
Note: Don't worry if you don't understand this - it's mainly background. You
don't need to know anything about it to get your amplifier working.
The two EL84s contribute most of the overdrive to the tone of the 18W circuit