TentLabs Negative Bias Supply


Designed in co-operation with Menno Vanderveen, we are proud to announce a new module

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.

This new module fundamentally differs from existing (commercial) solutions, hence solves commonly found problems like bias drift - read on!


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

Customer feedback

"Just finished installing the negative bias boards in my Dynaco ST-70 amp. I put the controller up ontop where the bias pots used to be. At a glance I can see the green LEDs telling me bias is good on all four tubes. It actually sounds better & I don't have to put a meter on the amp every couple of days. Thank you for having this, the instructions were spot on. "

Measured results

Courtesy Satoru Kobayashi (Japan)
- Improved distortion due to the use of Tentlabs bias supply
- Current balance in a PP output stage: Tentlabs versus traditional bias
-Time domain measured results and theory about the advantages of using these new modules

Still in doubt about the qualities of this new module ?


The module contains an integrated negative bias supply, 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. A servo circuit generates the negative voltage for each tube.

The maximum output voltage is -160V, enabling the use of virtually any tube up to a 211 or 845. Every tube can be biased at 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 sense circuit and a 45s 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. In case the high voltage fails, the bias regulator immediately swithes all 4 outputs to full negative.

Application & mounting

Download the Application Note.

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: 200V to 500V


• Total size 100*100*40mm (W*L*H, standoffs excluded), can be separated in 2 equally sized boards
• 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. The module is available from stock

Orders and information

For questions please contact Info@TentLabs.com. Go to the order page for more information.


As this is a true DIY product, so no additional support can be given, other than already available at the Tentlabs website.


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 takes NO responsibility for accidents resulting from improper usage or handling.