Beware of Fake AutoBias
Think of biasing like adjusting the engine idle speed on your car. 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.
More important is, how do you keep tubes perfectly synchronized when you “accelerate”, and what do you do about aging?
Before PrimaLuna, there were two ways to bias tubes:
1) Cathode Bias: Commonly used, cheaper to make, and often found in guitar amps. But also found in $20,000 high-end audio amps. This is what some companies call "Auto Bias", which is NOT correct. The correct term is “Self Bias”. 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. An amp that has 35 watts when you use "Adjustable Bias" (see below) will typically give you 20 watts cathode bias. Over 33% of the amp’s power goes up in heat through a huge resistor.
Also, most cathode-bias amps run the tubes hard. Real hard. They run in class A which sounds like a good idea unless you know what that really means. It means shorter life and failures via "shorts".
Manufacturers will say to you that they run the tubes “within the maximum ratings" for that tube. That is like saying your car can be set to idle at 5,500 RPM instead of the usual 900 RPM, because the rev limit is 6,500 RPM. It's below redline, but would you do it? What would happen to fuel costs (tube life) and engine failure rates (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. They will tell you it's "proprietary" which is silly. But PrimaLuna will tell you all this, because we run our tubes so easy you may never change them. We give you 42 watts from a pair of KT120’s. Others give you 100 watts. There’s no magic way to do that except to run them hotter.
Our competitors' "AutoBias" which consists entirely of "space-heater" sized resistors.
2) Adjustable Bias - Technically called fixed bias, this is a manual bias adjustment for each tube or each set of tubes. If the idle adjustment is for each tube, then a matched set is not required (though still desirable). However, if the amp has only one bias adjustment to set the idle for each pair of tubes, you have to buy a perfectly matched pair. If there’s one adjustment to set the idle for four tubes (and some amps do have this), you need a perfectly matched quad. This is expensive and a hassle.
But even if you buy matched sets, a new set of problems arise. The biggest issue is that tubes don’t age in perfect sync. You must use a meter, or a built-in LED, to check bias every six months, and if a set of tubes with just one bias adjustment is aging unevenly, you have to either buy a new set too early, or try and find a compromised bias setting.
A handful of manufacturers has employed a small chip near bias adjustments to maintain the same “idle current” once it is set. However, bias still needs to be re-adjusted when you replace any of the tubes in that circuit.
Not convenient, but there is a bigger problem. Because tubes age unevenly, you adjust them to the same idle, but distortion will still rise when you turn up the volume.
There had to be a better way, and our engineers in Holland solved the problem.
True automatic biasing requires a very advanced level of engineering. The costs to develop and build this circuit are very high. As far as we know, there are only three companies that have a true “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.
PrimaLuna Adaptive AutoBias
PrimaLuna’s ground-breaking real-time autobiasing uses sensors to constantly monitor the performance of each power tube. That data is supplied to active circuitry that adapts instantly and continuously to keep the tube within its linear operating tolerances, 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?
To make life even easier, the latest Adaptive AutoBias systems let you know when a tube needs replacing with a red LED in front of each power tube. This is standard in the EVO Series, and in the discontinued DiaLogue Premium and ProLogue Premium amps. 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. WIth PrimaLuna, a relay opens, and the amp goes into protection, automatically coming back when you change the faulty tube. NOBODY ELSE HAS THIS.
Adaptive AutoBias is passive: It is NOT in the signal path. There are only benefits in tube life and improved sound.
PrimaLuna's Adaptive AutoBias - Note the smart-logic and lack of "space-heater" resistors
ADAPTIVE AUTOBIAS LOWERS DISTORTION AND IMPROVES SOUND
The audible advantages of 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.
Is it really possible that your tubes may outlast you?
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 4,600 hours. Our manual states that power tubes are typically good for 2,500-3,000 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 4,600 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.
