WHAT IS OUTPUT TUBE BIAS?
Tube bias refers to a voltage setting on the control grid of the output tubes which controls the amount of idle current of the output tubes (power tubes). Much like the idle in your car’s engine, a tube amplifier needs to have the output tubes biased properly for optimal performance. If the output tubes are under-biased (idle current is set too HIGH), the audio signal will tend to distort earlier and the tube life will be shortened. This condition is sometimes referred to as biased “too hot”. If the output tubes are over-biased (idle current is
set too LOW), the audio signal will deteriorate more quickly and the amp will sound thin, cold and sterile. Therefore, this scenario is sometimes referred to as biased “too cold”. Typically, a hotter bias will give you louder, punchier and fuller sound; a colder bias will give you a cleaner, thinner sound.
There is no such thing as the “perfect” bias setting that is acceptable for a particular tube type in a specific amp. There are vast differences in output transformers and tubes, varying quality between tube manufacturers and even significant variation between individual tubes of the same brand and type. Bias ranges are also subject to personal taste.
When does bias need to be checked and adjusted? At a minimum, every time you replace the output tubes in your amplifier. Uncle Sams is equipped to test and adjust bias to the optimum performance level as we replace the tubes in your amp. Even if you are not replacing tubes, having us check and adjust your bias setting can result in an improvement in the sound of your amplifier if the bias had not be properly set previously, or had drifted from the original setting. As tubes
age, their characteristics change, and rebiasing to compensate for aging is often necessary. Many customers tell us that they are pleasantly surprised at the difference in sound quality that a bias adjustment can make.
Uncle Sam's Jamms uses the professional Bias King measurement tool for all of our bias adjustments: