What you are about to read is critically important. If a manufacturer states that they have auto-bias, or that it isn't important, is simply not telling the truth.
Biasing four tubes is much like synchronizing four carburetors on a car. In either case you get the same result. Potential roughness, decreased performance, lack of power, and shortening of parts life
The Current Situation and Challenges
Generally, there are two ways of output tube biasing:
"Cathode biasing" This method consumes a lot of power and is very inefficient, resulting in most of the amplifier's power going up in heat. It’s clearly far from an optimal. Some companies will go so far as to call this "automatic biasing" when clearly it is not.
"Fixed biasing" is really a manual bias adjustment for each tube or each set of tubes. If the amplifier has a bias adjustment for each tube, then a matched set is not required (though still desireable). If there is one bias adjustment for each quad, then a matched quad of tubes is required.
But even if you buy matched sets, a new set of problems arise:
Tubes age: The matched set you bought may not be matched in six months or a year. If you have a bias adjustment for each tube, re-biasing is needed every few months.
Tubes are not linear: As you raise volume, you lower the negative grid voltage to the power tube. This is similar to pressing on gas pedal on your car. Imagine if you had to synchronize four carberators to work together; not just when the engine is idling, but when you press down on the accelerator. Distortion increases and performance suffers in every way.
The Solution: At PrimaLuna, 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, a bold new approach would be needed without regard to engineering and build costs.
PrimaLuna Adaptive AutoBias.
How We Did It: There are several "pickup points" that constantly monitor the temperature, voltage and audio signal of the individual power 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 effective, resulting in the lowest possible distortion. The system does not use time intervals or "sampling" so there are no gaps between adjustments. It's an ongoing, seamless process done in real-time.
Shown below is the Adaptive AutoBias board installed in a PrimaLuna ProLogue One.
This type of system also accounts for tube aging and eliminates the problems caused by differences between tubes. Tubes can be used right up to the end of their life and tube replacement will probably be done because you feel it's time, not because performance suffers .
Adaptive AutoBias is passive:
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.
Adaptive AutoBias allows the amp to sound better:
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.
Best of all: The musicality of the amp with Adaptive Autobias is far superior to prior versions without this innovative circuit.