The FX Loop also incorporates a Mix control which allows you to vary the amount of effects signal that is
returned to the amplifier from any external pedals or effects processors plugged into the FX Loop. This
can be varied between ‘Dry’ (no effects in the signal) and ‘Wet’ (fully effected signal).
The RETURN socket is for connection to the output of effects units. When not used it is internally
connected to the SEND, therefore the EFFECTS LOOP can be ignored if not in use. It can also be used as a
small signal ‘Slave’ input from another amplifier but DO NOT plug an amplifier’s speaker output into the
Return socket or very bad things will happen.
External switching jacks, (Footswitch sockets)
Crunch: Connect the supplied 1 way latching footswitch here to engage/disengage the Pull Crunch
function on the Clean Channel. The footswitch is independent of the Pull Switch on the front so it
doesn’t matter whether the Pull Switch is in or out for the Footswitch to function.
Due to the nature of the pull-crunch feature, the red LED on the single way footswitch will be on when
the crunch is off and will go out when the crunch is activated.
Channel 1/2 Boost:
Connect the supplied 2 way latching footswitch here to change Channels and
engage/disengage the overdrive channel Boost function.
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 Silverback 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.
Please always try to buy matched sets of output valves as they will be easier to Bias and give longer
service. Ensure the toggle switch on the top of the chassis is in the correct position for the selected
output valves, (either 6L6s or EL34s). The Silverback ships with EL34s as these are Rob’s preferred valves.
Toggle away from the output valves for EL34s.
Unplug the amp from the mains; remove the rear grill and the 4 retaining bolts on the underside.
Carefully slide the chassis from the wooden sleeve and place it on a clear and secure surface upside
down so all its soft bits are exposed. Take care not to damage the output valves when turning the amp
over, (prop up the transformer at one end if necessary).
The BIAS pre-set is found on the power supply PCB, (see yellow circle on photo). The best place to take a
Bias voltage measurement is across each one of the two 1 Ohm resistors that are fixed between the
chassis and Pins 1&8 on the output valve ceramic bases (see yellow squares on photo).
Using a multimeter set on the 200mV range, measure across each of these resistors in turn and adjust
the BIAS preset so the meter reads between 32 and 40mV. This translates into 32-40mA of current per
output valve so each valve is biased at between 32 & 40mA, (34mA is a good figure to aim for). Check
both resistors a few times and try to balance the Bias voltage so it is as close as possible between the 2.
We don’t recommend a difference of greater than 6mV between each valve.
