Valves and Biasing
Like the instruments played through them, amplifiers themselves require occasional tune-ups for optimal performance. In the case of amplifiers, the output valves specifically require adjustment to properly set their biasing (or the optimum amount of voltage running through them) which directly controls power output and signal distortion. Biasing is a special voltage applied to amplifier valves to set them at their best operating point. The optimal bias is specified by tube manufacturers and is the point where the valves amplify most effectively. Valves can't actually be 100 percent linear, but their bias is set to get as close as possible. In fact, the slight non-linearity that exists in valve amplifiers contributes greatly to their sound.
Cathode biasing is the simplest biasing method. In using this method, bias is derived directly from the valve itself via a cathode resistor. It has the advantage of its simplicity and the capability to automatically adjust itself. As current drawn by the valve increases, so does the bias produced, thereby bringing the plate current back in line to its optimal point. Conversely, as the current drawn by the valve decreases, so does the bias, thereby increasing the plate current back to its optimal point. The drawback of cathode biasing is that it is limited for low- to medium-power amplifiers not exceeding 25 watts output. Cathode biasing is also used in all low-level stages of valve amplifiers. You'll find cathode biasing in the classic amps popularized in the earliest days of the British Invasion as well as some of the earliest American tweed amps.
When higher power is needed, amplifiers will use fixed biasing. In this case, a negative voltage derived from a simple power supply is applied to the power valves. Fixed biasing has the advantage of allowing more power to be drawn from the same power valves. However, as valves age, their bias requirements change and to keep an amplifier in optimal operating condition, the fixed bias needs to be adjusted periodically (every 6 months or so) or whenever new power valves are installed. Optimal bias is different for every set of valves and must be set manually for fixed bias amplifiers. This ensures that an amplifier will operate in a way that is sonically pleasing and safe for the valves themselves. Fixed biasing is found in many classic high-watt amplifiers first produced in the 50s and early 60s.
Bias in an amplifier is a lot like the lean/rich settings on the carburetor of older gasoline engines from a generation ago. The optimal carburetor setting allows for maximum fuel efficiency, better performance and minimal emissions just as the optimal bias setting results in a nice sounding amplifier, efficient operation and prolonged output valve life.
- If the bias setting is too high (overbiased) the valves will run cool and produce low power and sound unpleasantly distorted. This will not harm the valves, but the amp will sound really bad. This is similar to having a carburetor set too rich and having the engine run sluggishly with incomplete combustion, belching out black smoke.
- If the bias setting is too low (underbiased), the valves will run hot, have a loud background hum and also sound unpleasantly distorted. The valves will self-destruct in short order (within minutes, or seconds if the bias is really low or absent) if this condition is left unchecked. This is similar to having a carburetor set to lean, producing less power and having the engine run hot, resulting in it being susceptible to bearing damage.
Fixed biasing comes in two flavors: adjustable (as found in BUGERA amps) and non-adjustable.
- With adjustable fixed-bias amplifiers, as the name implies, there is a small potentiometer that allows adjustment of the bias to its optimal level. This is very convenient since it allows the use of many different valves with different biasing requirements to be used in your amplifier. For example, 6L6GC valves come in a spectrum of bias ranges. You can buy valves that need -30 volts, -35 volts or -40 volts to achieve the same plate current. Sonically, these valves will sound different. Low bias valves will put out less power and will sound cleaner. High bias valves will put out more power and sound more distorted. With a BUGERA amp, the choice is always yours.
- With non-adjustable fixed bias amplifiers, as the name implies, there is no potentiometer to adjust of the bias to its optimal level. This is very inconvenient since you have to buy a set of valves that are optimized for a specific bias point.
Bias adjustment can be done one of two ways: Statically and Dynamically
- Statically, the amplifier is on but no signal is applied. The current of each individual power valve is measured in the amplifier and is checked for two things. Firstly, bias is adjusted to ensure plate current is within an optimal range (say 35 to 45 milliamperes) and secondly, that the plate current is close in value to that of the other power valves in the amplifier. For example, an amplifier with four output valves with plate currents of 36, 35.5, 36 and 37 ma would be said to have a set of closely matched valves (within 5% of each other) in proper bias range. An amplifier with four output valves with plate currents of 32, 42, 37, 35 milliamperes would be said to have a set of badly matched valves.
- Dynamically, the amplifier (with a matched set of valves) is fed a signal (usually a sine wave) and terminated in a dummy load with an oscilloscope monitoring the output waveform. The bias is first checked to see if it is within optimal range and then adjusted to produce the cleanest output signal possible.
The best method is actually the combination of both methods, although it is time consuming. Nonetheless, it allows you to statically bias the amplifier and then to fine-tune it with the dynamic biasing approach. In conclusion, having your bias checked frequently and adjusted by your technician is an excellent way to prolong the life of your valves and amplifier, as well as optimize your tone.