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P a g e
Biasing
The Sheriff 22 is a fixed Bias amplifier and requires the output valve idle current to be
pre-set with a variable resistor.
Biasing needs to be done each time the output valves are replaced and should be checked
periodically to make sure they are working at their optimum for sound quality & valve
life. To set the Bias on the 22, you need a multimeter set to the 200mV DC range. This is
carried out externally so no need to remove the base plate from the amp. Output valves
can be replaced by removing the top cage, (2 x M4 screws per side). The valves can then
be pulled out using a slow rotational movement to ease them from their sockets,
(dweezling). Ensure that the pins on the replacement valves line up correctly with the pin
holes as they are easily bent if not.
We recommend always buying matched pair of output valves or Biasing may be difficult if
not impossible.
To Bias the new valves, make sure the amplifier is connected to a speaker cabinet, (or
dummy load resistor). Turn all controls to zero and remove the guitar input as any signals
may interfere with the Bias settings. Switch to High power and then into standby mode,
wait 60 seconds and then switch to ON.
Now put the black meter probe, (-ve) into the GND Bias socket and the red meter probe,
(+ve) into the left hand V5+ Bias socket. Using a small flat blade screwdriver into the Bias
adjustment POT, turn this so you get 22mV on the multimeter.
This equates to 22mA of current per valve as we are measuring the voltage across a 1
Ohm resistor.
Now put the red meter probe into the right hand V6+ Bias socket and measure the
voltage. Try to balance the 2 measurements around 22mV with a maximum of 4mV
between them. For example, an extreme but perfectly acceptable case would be 20mV
for V5+ & 24mV for V6+.
FULL POWER SPEAKER OUTPUTS
PLEASE NOTE: The lightning flash with arrowhead symbol, within an equilateral triangle,
is intended to alert the user to the presence of uninsulated ‘dangerous voltage’ within the
product’s enclosure that may be of sufficient magnitude to constitute a risk of electric
shock. Terminals labelled as “Speaker Outputs” must be connected to a speaker cabinet
of the designated load rating using an un-shielded two conductor cable for speaker use at
all times during operation. Never use a guitar cable to connect the amplifier to a speaker
as this presents the amplifier with a ‘capacitive load’. This can cause instability or
oscillation which may seriously damage valves and/or the expensive output transformer.
Always ensure a speaker is connected to the amplifier before powering up or damage to
the output transformer may result. Never unplug a speaker when the amplifier is ON as
this is even more likely to damage the transformer and the output valves.
