Our competitors' "AutoBias" which consists entirely of "space-heater" sized resistors.Ģ) Adjustable Bias - Technically called fixed bias, this is a manual bias adjustment for each tube or each set of tubes. There’s no magic way to do that except to run them hotter. We give you 42 watts from a pair of KT120’s. But PrimaLuna will tell you all this, because we run our tubes so easy you may never change them. They will tell you it's "proprietary" which is silly. Ask them for the plate and screen voltages, and how the tubes idle. 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?Īnd they won't tell you how hard they run the tubes. 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. Manufacturers will say to you that they run the tubes “within the maximum ratings" for that tube. It means shorter life and failures via "shorts". They run in class A which sounds like a good idea unless you know what that really means. Over 33% of the amp’s power goes up in heat through a huge resistor.Īlso, most cathode-bias amps run the tubes hard. An amp that has 35 watts when you use "Adjustable Bias" (see below) will typically give you 20 watts cathode bias. Then they get uncomfortable.Ĭathode bias works - but at a big cost.
In fact, they won't even tell you it's cathode bias unless you ask specifically. This is what some companies call "Auto Bias", which is NOT correct. But also found in $20,000 high-end audio amps.
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:ġ) Cathode Bias: Commonly used, cheaper to make, and often found in guitar amps. If the idle is too high, you will waste gas, cause premature engine wear, and early failure. If the idle is too low, the car can sputter or stall. Think of biasing like adjusting the engine idle speed on your car.
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