Biasing:
Whenever any output valves are changed, the amplifier will need to be re-biased. This must be done by
a qualified or competent person as there are lethal voltages inside the RD1 and getting killed by a guitar
amplifier is a rubbish way to die. Also take care as valves may still be very hot from use. Use a cloth to
remove valves if possible. Take care when removing any valve as the pins can easily be bent. We
recommend a slow circular motion known a ‘dweezling’ while lifting the valves from their bases as this
will free them with the least amount of effort and stress. Ensure that as you remove them that they are
never more than a few degrees from vertical to avoid bending or breaking pins etc. Please always try to
buy matched pairs of output valves as they will be easier to Bias and give longer service.
Unplug the RD1 from the mains and remove the bottom chassis plate using a No.2 Posidrive screwdriver.
Note that the rubber feet fixings are also used to secure the base plate to the chassis. Remove 4 x M4
screws from the side of the top grill using a No. 2 Posidrive and remove it. Carefully place the RD1 on a
clear and secure surface upside down so all its soft bits are exposed. Ensure that nothing is plugged onto
the input socket and all controls are turned to zero to avoid any signal or circuit noise at all interfering
with the Bias setup.
Plug the amp back into the mains; ensure a speaker or dummy load is connected to the correct speaker
output and switch the amp on from the front panel switch. After 30 seconds, switch the front panel
switch from Standby to High. Do not be tempted to poke fingers anywhere inside the amp; lethal
voltages are present.
The 22K BIAS pre-set resistor is found in the power supply section on the PCB, (yellow circle on photo).
The Bias voltage measurement is performed between the chassis, (use the unpainted corner bush as
