Adaptive AutoBias Technical Info

Some Basics:
Every power output tube, such as an EL34, KT88, 6L6GC, etc., must be set, or "biased" to its optimal operating point.  Deviations from this operating point typically result in increased distortion, compromising performance. The voltage needed to bias the tube to this operating point is called "bias" or "bias voltage.”
Think of bias voltage like the idle setting on a car's carbeurator.

The Current Situation and Challenges:
Until now, there have only been two ways of biasing output tubes:

The first is "cathode biasing," which requires putting a big resistor in the cathode line and using the voltage drop over this resistor as the bias to set the tube at its desired operating point.  Some companies will refer to this method as "auto bias."  While it's true that this method requires no adjustments, it's hardly automatic, and it has a few inherent problems.  First problem:  This method consumes a lot of power and is very inefficient. Much of the power output is dissipated as heat.   This in-turn leads to the second problem: To achieve a higher power output, tubes are run much harder, substantially reducing tube life.  This method is clearly not optimal, but... it's certainly cheap to build!

The second, more common method, is "Fixed biasing," where the customer manually adjusts bias voltage to a specific idle point.  This appears promising, but unfortunately all tubes, even the same type from the same manufacturer, require different bias voltage. More importantly, THE BIAS VOLTAGE LEVEL CHANGES as the tube ages, as audio signal levels fluctuate, and as myriad of other factors change. Therefore, every tube needs to be biased individually, or the tubes must be mached.   The problem? This bias setting must be readjusted constantly to keep it within the optimum range, and, just as important, to keep it within a safe range.  Failure to do so may damage the amplifier and require a trip to the shop. 

Considering the technological advantages we live with today, we found it surprising that these issues had never been addressed properly, and that no one had come up with an adequate solution. Until now.

The Solution: We knew that in order to take the hassle out of tube biasing, make tube equipment more user-friendly and raise the level of performance, we had to come up with a radical solution.
First, it had to be absolutely reliable and it had to be kind to the tubes. Then it had to be "set 'n forget”; factory adjusted for a lifetime. Finally, it had to be 100% sonically transparent; no intrusion in the delicate audio signal would be acceptable.

The answer? Adaptive AutoBias.



 

How We Did It:
Throughout our amplifiers there are several "pickup points" that constantly monitor the temperature, voltage and audio signal of the individual tubes. That data is fed to an active circuitry that determines the correct bias for each one, adapting instantly and continuously to the demand for power and all the other changing variables. This means the tubes always perform where they are most comfortable and effective, giving the lowest possible distortion and highest signal integrity. The system does not use time intervals or "sampling" so there are no gaps between adjustments. It's an ongoing, seamless process.

This type of system also accounts for tube aging and eliminates the problems caused by differences between tubes. Finally, tube start-up is softened; internal tube currents are controlled; and potentially damaging peak currents are eliminated.

Just as important, the system is inherently transparent sonically because it contains no extra active or passive parts in the signal path, other than the usual coupling capacitors and grid resistors. Further, steering signals are kept 5 octaves below the audible region. So here we have a system that takes care of power tube biasing in a radically new fashion: continuously adaptive to the audio signal and power demand, while efficiently accounting for a host of other variables.

How It Works In Mystere Amplifiers:

The audible advantages of the Adaptive AutoBias were far greater than we had anticipated. We foresaw a slight improvement in power efficiency and distortion levels, but the gains exceeded our expectations. Transparency over the entire frequency spectrum improved significantly, even in the bass region where qualities such as power, authority and control surpassed what we heard in amplifiers of much greater power. The amplifier sounds quicker and more responsive but with unexpected power reserves far beyond its rating.

Distortion levels plummeted fifty percent, adding incredible authenticity to vocals and instruments and eliminating harshness when the amp was pushed hard. Tube aging and tube replacing are no longer factors requiring tweaks and adjustments; overall, tube life is increased and the number of tube failures dropped significantly.