stewmac.com
55
©2019 StewMac
STEP 164
Old method of setting
the bias: doing it by ear
If you have a bias meter, skip this step
and go on to Step 165 .
The old-school way of adjusting the
bias is by ear: after setting the bias
voltage to a recommended starting
point, you make tiny adjustments
to the bias pot while listening for
the sound you want . This inexact
method can shorten the life of your
power tubes .
You roughed-in the bias setting in
the previous step by setting the bias
voltage at -35V .
Danger: set your guitar aside
before you adjust the bias pot.
Do not touch any amp circuitry while
holding your guitar . Doing so would
create an electricution hazard!
Use a screwdriver to make a tiny
adjustment to the bias pot . Turn
clockwise to increase the current
and counter-clockwise to decrease
it . Then set the screwdriver aside and
play to hear to the result .
Watch out for red-plating
You especially want to avoid too
much current turning the gray plates
to glowing red . If your tubes start
red plating, shut the amp down and
let it cool before setting the bias to a
lower level .
STEP 165
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 cal-
culation 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 6V6 power tubes in
this amp, that maximum is 14 watts .
Exceeding the 14-watt limit will dam-
age the tube, and operating at the
full 14 watts will shorten its life . Our
goal is to operate the tube at 60% of
the 14-watt limit, or 8 .4 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
the number you found in Step 163 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 cal-
culation gives a bias recommendation
of .020 Amps . Since bias is typically
measured in milliamps, this measure-
ment is written as 20mA .
Calculate your bias current setting
Example:
Your plate voltage from Step 165
60% of max. plate dissapation
÷
Your bias current setting
=
8.4
BOX 2
BOX 1
8.4
÷
=
With the amp unplugged allow the
power tubes to cool . Once the power
tubes have cooled, remove them from
their sockets .
Plug the tubes in to the bias probe
sockets, which are like tube sockets .
Plug the probe sockets with tubes
into the power tube sockets .
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 calcu-
lated (20mA 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 in to
the amp's tube sockets without the
probes attached .
