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.
Adjustment is automatic, requiring no action on the user’s part.
Why is this important? Each pair of tubes requires slightly different bias. The result of our automatic bias adjustment: a circuit that brings out the very best from every tube you insert.
Why Does Bias Matter?
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). These incorrect operating parameters can affect sound quality.
How Do Other Manufacturers Handle Tube Bias in Preamps?
Before we talk about preamps, let’s look at power amps. Tubed power amplifiers always allow for tube bias adjustment. In fact, they require it. 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. So they apply a fixed bias, which is set at the factory. 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. Both pairs were relatively new and both were 12au7. But when we switched tubes, the circuit voltage went from 130V to 111V. That’s more than enough to affect sound quality. It wasn’t the tubes’ fault — they just required a different bias.
However, when we tested these tubes in a Rhythm preamp, both pairs caused the unit to run at the exact same voltage (the voltage specified by designer Bob Backert). That’s our automatic bias adjustment in action.
So Preamps Require Tube Biasing?
Actually, no. None of the other tube preamps on the market today, as far as we know, adjusts bias. And they work just fine. In fact our own Rhumba preamp does not adjust tube bias. And it sounds great. This is because the tube bias, set at the factory, works perfectly with the stock tubes that we supply.
When bias adjustment matters is when you experiment with other brands of 12au7 tubes. Or tubes that are compatible with 12au7, such as 5814A, or CV4003. Some of these will have unusual bias requirements, which can make them sound “not their best” if those unusual bias requirements are not met.
So if you are the kind of music lover who will never bother to change the stock tubes in your preamp to see what other kinds of sound can be obtained, you don’t really need to be concerned about adjustable tube bias in your preamp.
The Best From Every Tube
Some tube brands sound better than others. And clearly, some really are better. But something else is happening. As our test showed, inserting different tube brands in a typical preamp can change the voltage and current that were intended by the designer, possibly by a lot. So in a typical preamp, a truly great pair of tubes might sound merely “OK”, because bias isn’t adjusted correctly for them.
Because our Rhythm, Rhumba Extreme and Rhumba Xphono preamps automatically adjust bias, a newly inserted tube will never sound worse merely because it throws off the unit’s operating parameters. As a result, with Backert Labs’ automatically customized bias, you will hear each tube perform at its very best.