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Plug the amp back in and flip the
power switch ON . Give the tubes 30-
60 seconds to warm up, then flip the
standby switch ON .
Watch the bias meter as the power
tubes heat up . Both needles should
rise at an equal rate and settle at the
same time .
Adjust your bias pot until the bias
meter displays the reading you
calculated (36mA in our example) .
Let the amp idle for a few minutes to
make sure that the bias doesn’t drift .
When you see a constant readout
of your bias setting, turn the power
switch OFF .
After a few moments flip the standby
switch OFF and unplug the amp from
the wall .
Once the tubes are cool, remove
them and the bias probes from the
amp and plug the tubes back into
the amp's tube sockets without the
probes attached .
After removing the bias probes and
taking it for a test drive, if the amp
is stable and your tests match the
voltages specified, feel free to let it rip!
After playing for a few minutes and
testing all inputs, if everything seems
to be operating normally, go ahead
and turn the amp off .
If there are any strange oscillations,
squeals, or the amp seems at all
unstable, use a wooden chopstick to
begin probing for loose connections:
n
from the input jacks to the
turret board
n
from the tube sockets to the
turret board
n
from the tube sockets to the
front panel controls .
It usually takes just a minor wiring
adjustment, perhaps resoldering
a loose joint, to correct this sort of
distortion .
After removing the bias probes and
taking it for a test drive, if the amp
is stable and your tests match the
voltages specified, feel free to let it rip!
STEP 106
Improved method of setting the
bias: using a bias meter
The more accurate way to set the bias
is by using a bias meter, such as the
VHT Bias Tester (StewMac #1580) .
This method starts with a simple
calculation to find your bias setting,
then you adjust the bias pot until your
bias meter displays that reading .
The calculation uses two numbers,
the tube’s maximum plate dissipation
and its plate voltage .
Plate dissipation . Every power tube
has a specified plate dissipation—
the maximum wattage the tube can
handle . For the EL34 power tubes in
this amp, that maximum is 25 watts .
Exceeding the 25-watt limit will
damage the tube, and operating at
the full 25 watts will shorten its life .
Our goal is to operate the tube at 60%
of the 25-watt limit, or 15 watts . We've
entered this number for you in Box 1
of the calculation .
Plate voltage . You already have the
plate voltage for these tubes: it’s the
number you found in Step 104 and
wrote in Box 2 below .
Divide Box 1 by Box 2 . The result is your
recommended bias setting for use in
the next step . Our example calculation
gives a bias recommendation of .0357
Amps . Since bias is typically measured
in milliamps, this measurement is
written as 36mA .
With the amp unplugged allow the
power tubes to cool . Once the power
tubes have cooled, remove them from
their sockets .
Plug the tubes into the bias probe
sockets, which are like tube sockets .
Plug the probe sockets with tubes
into the power tube sockets .
Testing and troubleshooting
Example:
Your plate voltage from Step 165
60% of max. plate dissapation
÷
Your bias current setting
=
15
BOX 2
BOX 1
15
÷ =
