7 |
P a g e
OUTPUT STAGE
The VC35 uses a combination of ‘Cathode’ and ‘Fixed’ biasing to
supply the
optimum current for the output valves. In LOW power mode, which uses lower
plate voltages, the amplifier is running entirely in Cathode Bias which is effectively
Class A operation. This mode of output stage produces very natural and sparkly
tones reminiscent of the early British guitar amplifiers. In HIGH power mode, the
output stage is running partially in Cathode Bias but also in Fixed Bias where we set
a maximum idle or Bias current through the use of a variable potentiometer, (Bias
Adjust). This limits the maximum valve plate dissipation to ensure longevity &
reliability and to avoid the thermal issues associated with Cathode Biasing at high
output levels. Many early EL84 design guitar amplifiers exceeded the recommended
plate dissipation and this can lead to early valve failure and the output stages
running extremely hot, even when they are not being played.
BIAS ADJUSTMENT:
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. The VC35 uses 4 x EL84 output valves.
To set the Bias on the VC35 you need a multimeter set to the 20V DC range. Biasing
is carried out externally so no need to remove the base making the whole process
very safe. Output valves can be replaced by simply removing the top cage and
carefully removing the spring retainers, (lift them up and over the top nipples on
the valves/tubes). We recommend the use of a soft cloth to remove valves if
possible. The valves can then be pulled out using a slow rotational movement,
(known as dweezling) to ease them from their sockets. This will free them with the
least amount of effort and stress. Ensure that the pins on the replacement valves
line up with the socket holes in the valve base before pushing them firmly home.
Take care when removing any valve as the pins can easily be bent. Make sure that
they are never more than a few degrees from vertical to avoid bending or breaking
pins etc.
WARNING
–
Output valves can get extremely hot so ensure they have cooled
sufficiently before removing or replacing them.
Always buy matched quartets, (set of 4) of output valves or Biasing may be difficult
if not impossible. Poorly matched valves will also wear quicker as one will be
drawing significantly more current than the other causing an imbalance and
potential premature failure.
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