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).
In 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.
It’s a 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.
There are 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.
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.
Keep in 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 current value.
• 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.
As with any electrical equipment, amplifiers are high voltage devices. They can cause serious injuries or even death if mishandled. Always use common sense and caution when handling high-voltage devices. If you are unsure of what you’re doing or are uncomfortable biasing amplifiers yourself, let the qualified technician do the job.