The bias of a tube amp
is referring to the idle current of the tube. The
reason for this is that all tubes have different
tolerances. That's why it's generally a good idea to
buy matched sets of tubes, as they tend to have
pretty much the same characteristics. If you under
bias a valve (idle current too high), the signal will
tend to distort quicker, and thus the tube valve life
will be shortened. Over biased valves (low idle
current), the signal will deteriorate too quickly and
tend to sound "sterile" and
"cold". To help set the bias current, the
grid voltage is adjusted (which is a negative
voltage), which in turn sets the current. You're
adjusting the voltage of the grid which in turn
controls the degree to which the output tubes overlap
in conduction.
Under
biased: Is when there is too little
voltage, which means too much current flowing.
Over biased: Is when there is too
little currently flowing, therefore the voltage
is too high.
Note that under and
over bias is referring to the voltage setting, which
is the reverse in transistor type amps.
Amplifier
Operating Classes:
There are three useful configurations of amplifier.
Class A, AB1 and AB2.
Class
A:
The bias of the output tubes are set so that both
tubes are always conducting, even when the signal
reaches maximum. This configuration has the least
crossover distortion between the swing of the signal
from +ve to -ve. As such, class A amplifiers consume
a lot of current, even when there is no signal
applied. Types of amps that use this would be your
vox amps.
Class
A/B:
In both types of AB class, the bias is set so that on
a maximum peak of signal (in either direction), the
opposite tube will turn off. The only difference is
that class AB1, there is no grid current flowing into
the grid of the tube. In class AB2, some grid current
flows into the grid. Because the tubes can turn off,
power consumption is greatly reduced, which also
prolongs tube life. Typical amps that would use this
would be Fender and Marshall's.
Class
B:
Both tubes never conduct at the same time, only
alternately. However because of the crossover
distortion, it sounds awful which makes this
configuration unusable.
The
Relationship Between Bias and Classes:
If there is a lot of bias, the grid voltage very
negative, which makes it a class B amplifier. If you
reduce the negative voltage, allowing both tubes to
conduct a small amount, crossover distortion is
reduced making it a class AB2. Reducing the voltage
further, they are conducting more and you get class
AB1 operation. Reducing even further so that both
tubes are always on, and you're operating it in class
A mode. What you're looking for when adjusting the
bias is to have a low cross over distortion is a
small amount of power dissipation. Because these are
inverse to each other, reducing the amount of
crossover distortion will increase the amount power
used. Reducing the power used, increases the
crossover distortion, so a compromise is often used
when setting amps where there is a reasonable amount
of power being used, and an acceptable (or
non-audiable) crossover distortion.
Remember, running amps in class A means less
crossover distortion, more power consumption, more
heat, shorter lasting tubes. Class A/B, better power
consumption, more tube life, more crossover
distortion.
A tube amplifier only needs biasing when you change
the tube set. Pre-amp tubes are self biasing, so it's
only the power tubes that need biasing. Some tube
manufacturers, code their tubes according to scales,
which means that they have the same characteristics.
So if you replace your old tubes with the same coded
tubes, you may get away by not biasing the tubes.
However it's a good idea to have it checked by an amp
technician every time you change tubes. Some newer
amps also have self biasing power tube circuitry, so
biasing of power tubes is not needed. Check with the
amp manufacturer if you're unsure.
How
To Do It:
There are a few methods of biasing amplifiers. Before
starting, remove any signal sources from the amp and
make sure that the amp has some kind of load on the
output (whether it be a passive load or the speaker).
Also make sure that the amp has warmed up before
taking any measurements.
Shunting
The Output Transformer:
Connect an ammeter from the plate (anode) across half
the primary of the output transformer. Because of the
low impedance of your meter, all of the current flows
through the meter. Be careful as the voltage on your
meter will be a hundreds of volts! Now adjust the
bias pot until it's within the spec for your amp.
Keep in mind that the current you read will be a
multiple of how many tubes are working per side (i.e.
if you read 60 mA and there are two tubes, then each
has 30 mA going through them). If you can't get a
proper reading (the current will be too low and the
tubes will most likely be glowing red), then your
meters shunt resistance is too high. In that case,
use the method below.
Cathode
Resistor:
If the amp has a resistor from the cathode to ground
you can use this. Hook up your meter across this
resistor and use ohms law (V=I*R) to calculate the
current (E.g. if your cathode resistor is 1 ohm and
the voltage reading is 30 mV, then your current is 30
mV (V/R). If however there is no resistor to ground,
you can put in a small value between the cathode and
ground (1 ohm / 1 Watt). You'll most likely need to
compensate the reading you get by about 5-10mA
because of leakage currents. With this method there
are no high voltages to deal with, which is much
safer!
Below are the typical
values for running an amp in class AB1. Note that
amps running in class A will have higher current,
however are also designed to run at higher currents.
Trying to run an amp that was designed for class A/B
in class A configuration, it will over heat, blow
fuses and reduce tube life (if not blowing something
else with it). Class A amps normally run at lower
plate voltages.
