Negative Bias Supply

When using fixed bias in your amps, the currents need constant re-adjustment to compensate for mains variations and tube aging. TentLabs now introduces a new module, based on novel techniques, with major advantages over existing solutions.

Tentlabs Negative bias supply

Building monoblocks ? Apply for a discount of 5 % when ordering a pair!

€ 156.20
€ 189.00

Application info

Negative Bias Supply

Power tubes need to be biased at a constant current, to achieve optimal performance of the amplifier. Both cathode and fixed bias methods suffer from the fact that the current depends on tube age and mains variations, not to mention the aging of other passive components.
As a solution, servo circuits have been proposed in many publications, but the bias these supply all depends on the music signal, hence the bias current drifts with the music played, which is unwanted.

The new Tentlabs / Vanderveen circuit offers rock solid bias, independent of tube aging, music signal or mains power voltage changes. The modules biases up to 4 tubes at exactly equal currents, with the setting of only 1 trimmer. All classes of output stages can be biased, A, AB or B, triode, tetrode or penthode, single ended and push-pull.

Result areas are:
• better imaging and stage due to lower hum land noise evel
• better bass due to near perfect (<1% error) current matching in push-pull stages
• Better resolution and imaging due to optimized left / right channel matching

The module contains an integrated negative bias supply (including power transformer), which is properly filtered and stabilized to achieve a clean bias voltage. A measurement section for each tube measures the actual current around a very small window around the bias hence is not affected by any music signal.

The maximum output voltage is -160V, enabling the use of virtually any tube up to a 211 or a 845. Every tube can be biased up to a maximum of 175mA. The outputs of the module provide linear and low output impedance to avoid interaction with the preceding driver stage driving the power tubes. Each of the 4 outputs can sink and source 1,5mA, allowing the use of relatively low grid resistors, if required.

The module has an integrated high voltage detection circuit and a 45sec switch on delay whereafter the output tube current gently increases, after the heaters or filaments have had their chance to heat the cathode. This avoids clicks and plops, and enhances tube life.

Application info
Download the Application Note (pdf)

Application examples
Menno Vanderveen describes the application of the automatic bias module in the
UL40-S2 amplifier at his website

• Output voltage range to grids -2V to -85V or -2V to -160V
• Output sinc & source current max 1.5mA
• Input mains voltage 115Vac or 230Vac (+6% / -10%)
• Input range high voltage sense: 100V to 500V

• Total size 100*100*40mm (W*L*H, standoffs excluded)
• Hole pitch 91mm * 91mm (symmetrical)
• Total size separated boards 49*100*40mm (W*L*H, each board, standoffs excluded)
• Hole pitch separated boards 91mm * 40mm (symmetrical, each board)
• Hole size 3.5mm
• Weight 375 gram

The module is warranted for 5 years.

Though this is a product which if handled properly is save, it is surrounded by high voltages. Working with high voltages requires safety measures and professional behavior. Tentlabs / Vanderveen take NO responsibility for accidents resulting from improper usage or handling.