BIAS Switch: FIXED or CATHODE
The Sheriff 44 has the option of Fixed or Cathode Bias. Fixed Bias gives more power, (see
below for all the available modes) and requires the output valve idle current to be pre-set
with a variable resistor, (potentiometer in this case). Cathode Biasing sounds and feels
more urgent and dynamic at lower volumes but compresses quickly as the volume goes
up. Fixed Bias gives maximum headroom and dynamic attack at higher volumes.
Biasing
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 44, you need a multimeter set to the 200mV DC range. Biasing
is now carried out externally so no need to remove the amp from the wooden sleeve.
Output valves can be replaced from the rear of the amp by simply removing the back grill
and carefully pushing down on either side of the retaining butterfly clips. The valves can
then be pulled out using a slow rotational movement, (dweezling) to ease them from
their sockets. Ensure that the key on the replacement valves lines up with the keyway on
the socket before carefully pushing them home.
Always buy matched pairs 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). The Bias switch should be set to ‘FIXED’. 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 Black centre GND Bias socket and the red
meter probe, (+ve) into the left hand V5+ Bias socket. Using a small flat blade screwdriver
in the Bias adjustment POT, turn this so you get 34mV on the multimeter. This equates to
34mA 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 34mV with a maximum of 6mV
between them. For example, an extreme but perfectly acceptable case would be 31mV
for V5+ & 37mV for V6+.
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.
