If you're like most guitarists you probably own a tube amp or two. At one time your amp may have sounded good but you may not be too impressed now. You think you need new tubes, and buy expensive matched power tubes and pop them in your amp but it still sounds like crap. You heard others say, “You need to have your amp re-biased”. So you take their advice and take your amp to a tech. The tech charges you $50.00 - $100.00 and you have no idea what he did to your amp, but your amp suddenly sounds better.
What on earth is biasing? Think of it as "tune-up" for your amp. Every tube is unique and draws a different amount of bias current. The “matched” tubes are tubes that draw about the same amount of bias current and are grouped together. Every time you install new sets of tubes, or an electronics component in your amp has been changed, or your current tubes are just plain old and have had hundreds of hours of workout, you’ll want to re-adjust the current that flows in the cathode of the tubes (bias current).
addition, the electronics properties of the tubes can drift over time and need
to be readjusted. Without actually
measuring the bias current you have no idea where it’s set. It could be set
outside of the recommended range and you wonder why your amp sounds like crap,
and if the current is set too high you may be risking the tubes to begin the
melt-down process and take out the output transformer.
If you’re a
serious guitarist and own several tube amps like me, you’ll want to learn how
to bias your amps. In the guitar amps it
will alter the tonal characteristics, and with the right bias setting the tubes
will last a long time and they will sound their best.
simple bias adjustment you can perform yourself instead of taking your amps to
a tech. This simple tool should be in
the possession of all musicians with tube amplifiers.
several methods in biasing amps. Some
methods involve making measurements in the high voltage sections of amplifier
circuits and are not recommended for most of us, and should be performed only
by qualified technicians with full electrical knowledge of amplifiers.
Cathode Bias The safest
method is “cathode bias”. In this method
a current flow through the tube’s cathode to the ground (low voltage) is
measured. The value is very small as in
milli-amperes (1/1,000 of an ampere), and is the “bias current”. This method involves breaking (cutting wire)
the circuit path from the tubes’ cathode to ground and solder in a 1 ohm
precision shunt resistor. A voltage across
the resistor is measured to obtain the current value (bias current) through
it. This needs to be repeated on all
power tubes in the amplifier. After that
everything needs to be put back (removing resistor and re-soldering wire). This method is still not preferable for most
of us for the obvious reasons.
Tube Adapter To The Rescue A special
tube adapter was developed to make the cathode biasing easier and convenient
without actually getting into the amplifier circuit. The adapter is inserted between the
amplifiers’ tube socket and the tube, and the adapter cable is connected to a
voltmeter to take the cathode current measurements – that’s it. No more fussing with taking the amp chassis
out, locating cathode pins, cutting wires, soldering resistors, and undoing and
putting everything back together.
In order to
have a true “cathode bias” a shunt resistor must be built in or at the adapter
to minimize the current path distance from the tubes’ cathode pin to
ground. That’s the way my BisaPro
products are built.
The circuit path is
right at the tube, and the path is never broken, and without any added stray
capacitance and resistance to it. The
measurement is taken right at the shunt resistor, and the tube never sees the
rest of the probe cable.
No Shunt Resistor At The Adapter? It's A Bad Idea Be aware,
some adapters sold today are built without the shunt resistor at the adapter
simply because it’s easier and quicker to manufacture. Without the shunt resistor at the adapter, an
altered (false) cathode current measurement is taken at the end of the probe
cable. The current value displayed on
your voltmeter is not what the tube actually sees.
mind that the cathode current value is very small as in milli-amps. Any added stray capacitance and resistance
(from excess cable length) to the tubes’ cathode to ground will alter the true
• A typical adapter cable length is
roughly two feet. Connecting a voltmeter
to it will result in an increase of the cathode to ground circuit path length
to four feet (incoming and outgoing).
This certainly will add stray capacitance and resistance to the circuit
and would result in a false cathode current reading. Would you want your amp having four feet of
excess wire dangling at the tubes’ cathode pin?
• The above approach has another
negative effect. The tubes’ circuit is
broken when the adapter cable becomes disconnects or is not connected to a
voltmeter. When that happens it may
result in damage to the amplifier. Would
you pull and reinsert the tubes while the amplifier is powered on?
• Some adapters have shunt resistors
located at the end of the cable, or in a switch box. Again, it’s another convenient and easier way
to manufacture the probe cables. This
approach will have the same negative effect as ones without the shunt resistor
and will result in a false current reading.
Again, four feet of excess wire dangling at the tubes’ cathode pin.
• With these adapter construction
approaches, it is known not to work with some older Marshall amps as they
oscillate and can cause damages to the output transformer.
Do Your Research There are
several articles written and are available on the Internet about the “cathode
bias” method and the importance of shunt resistor. I encourage you to do some research and come
to your own informative and “biased” conclusion.