That’s more than enough to affect sound quality. But when we switched tubes, the circuit voltage went from 130V to 111V.

Both pairs were relatively new and both were 12au7. To test whether a fixed bias can lead to significant variations in voltage and current depending on the brand of tube used, we used a typical preamp that doesn’t adjust bias, and replaced a pair of Chinese 12au7 tubes with a pair of Russian-made JJ tubes. So they apply a fixed bias, which is set at the factory. But pre-amplifier manufacturers assume that any tube of the correct type for that preamp (12ax7, 6922, 12at7, etc.) will allow the unit to run at a voltage and a current that are “close enough” to what the designer intended. Tubed power amplifiers always allow for tube bias adjustment. How Do Other Manufacturers Handle Tube Bias in Preamps?īefore we talk about preamps, let’s look at power amps. These incorrect operating parameters can affect sound quality. If your tubes are not biased correctly, your amp or preamp will run at incorrect parameters - specifically, a voltage or a current that is different from what the designer intended (or both).

The result of our automatic bias adjustment: a circuit that brings out the very best from every tube you insert. Why is this important? Each pair of tubes requires slightly different bias. Our Rhythm, Rhumba Extreme, and Rhumba Xphono preamps automatically and continuously adjust the bias that is applied to your tubes, ensuring correct bias at all times.Īdjustment is automatic, requiring no action on the user’s part.

How to bias guitar amp tubes