Adaptive AutoBias

Adaptive AutoBias™ – by PrimaLuna

What you are about to read  is critically important. If you own, or are considering buying a tube amplifier, you need to know the facts about tube life. Your tubes can last many many years armed with this information.

Think of biasing like adjusting the engine idle speed on your car. It’s very much the same. Most cars idle around 900 RPM. If the idle is too low, the car can sputter or stall. If the idle is too high, you will waste gas, cause premature engine wear, and early failure. 


“Cathode Bias” is commonly used, cheap to make, and often found in guitar amps. But also found in $20,00 high end audio amps. This is what most companies call “Auto Bias”, which they CAN, but they don’t tell you the whole story. In fact, they won’t even tell you it’s cathode bias unless you ask specifically. Then they get uncomfortable. 

Cathode Bias works – but at a big cost.     

This is the truth. An amp that has 70 watts when you use “Adjustable Bias” will typically give you 40 watts cathode bias. Most of the power goes up in heat through a huge resistor. 

Most cathode bias amps run the tubes very hard. Real hard. They need to so they can squeeze out the power. That means short life and failures via “shorts”. They will say to you that they run the tubes “Within the maximum ratings” for that tube. This is like saying your car usually idles at 900 RPM, the redline is 6500 RPM, so it’s okay to set the idle at 5500 RPM. It’s below redline, but would you do it? What would happen to fuel costs (tube life) and engine failure (tube shorts) if someone did that? 

And they won’t tell you how hard they run the tubes. Ask them for the plate and screen voltages, and how the tubes idle. They don’t want you to know. And they will tell you it’s “proprietary” which is silly. But PrimaLuna will because we run our tubes so easy you may never change them.              

ResistorOur competitors’ “AutoBias” which consists entirely of “space-heater” sized resistors.

“Adjustable Bias” is 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 desirable). If there is one bias adjustment for each pair, you buy two so you have a matched pair. One adjustment for four tubes, you need a matched quad. This is expensive and a hassle. 

But even if you buy matched sets, a new set of problems arise:

Tubes age: You must use a meter, or a built in LED, to check bias every six months.   

Tubes are not linear: As you raise volume, they do not stay “matched”. Distortion increases and performance suffers in every way.

There had to be a better way, and our engineers in Holland solved the problem.  

Real Auto Bias requires a level of engineering that is very advanced. The costs to develop and build are very high. As far as we know, there are only three companies that have a real “Auto Bias”. The first was PrimaLuna, which introduced it in 2003. The other is Mystere, a sister company to PrimaLuna. The third is VAC (Valve Amplification Company), who offers it only on their Statement 450 IQ model, selling for $112,000 a pair.


How We Did It: There are sensors that constantly monitor the performance of each individual power tube in real-time. That data is supplied to active circuitry that adapts instantly and continuously to keep the tube in its linear operating position, regardless of the age or condition of the tube.     

This means the tubes always perform where they are most effective, resulting in the lowest possible distortion, and dramatically improving sound. It also means that you get every last minute of usable life from the tubes, and that saves you money for years to come. That’s why people never sell their PrimaLuna systems. They use them every day without fear of the clock ticking. After all, you don’t buy components just to look at them, right?

Our newest Adaptive AutoBias systems in ProLogue Premium and DiaLogue Premium have a new feature. We figured, since we are monitoring the tubes, we will let you know when a tube needs replacing with a red LED in front of each power tube. No guessing what has happened. When a tube fails on some amps, you will blow a fuse or even have to solder in a new resistor. Our latest offerings are fully self protecting. The amp goes into protection until you change the faulty tube. NOBODY ELSE HAS THIS. 

Adaptive AutoBias is passive: It is NOT in the signal path. Period. There are only benefits in tube life and improved sound.


PrimaLuna’s Adaptive AutoBias – Note the smart-logic and lack of “space-heater” resistors.



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.


To get more power from an amp, you must run the tubes harder. It’s simple math. Put your hand over a PrimaLuna amp and it’s not nearly as hot as you might expect. We’ve told customers that this will equate to longer life from wearing out (loss of emissions) and shorts. But we didn’t know how much.

In May of 2014, something interesting happened. . .

A customer named Les S. from Southern California bought a whole PrimaLuna system, including a preamp and monoblock power amps. A friend told him equipment sounds better if left on 24 hours a day. Les left his system on continuously for six months, and then called us to see if it was okay. We told him it was not. Six months with the amps on 24 hours a day is over 4600 hours. Our manual states that power tubes are typically good for 2500-3000 hours.

We shipped Les a new set of power tubes, since his tubes were either worn out or near it, and told him that we would test his old tubes to see if there was anything left to salvage.

When we got back his old tubes we were shocked. We had the original measurements, and after over 4600 hours, the tubes all tested at 90% of new! This is conclusive proof of our advanced engineering working to make PrimaLuna a product you can use every day.