TentLabs Negative Bias Supply
When using fixed bias in your amps, the current needs constant monitoring, and readjustment, to compensate for voltage variations and tube aging. Designed in co-operation with Menno Vanderveen, TentLabs produced this module, based on novel techniques, with major advantages over existing solutions. This new Negative Bias Supply module fundamentally differs from existing (commercial) solutions, hence solves commonly found problems like bias drift.
Purpose
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. However, they are fundamentally flawed because the bias is allowed to drift/change with the music being played (which is undesirable–for lowest distortion, bias should stay rock solid as the music is playing).
The new Tentlabs/Vanderveen Negative Bias Supply offers rock solid bias, independent of tube aging, musical signal or mains power voltage changes. The modules biases up to 4 tubes at equal current, 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 soundstage due to lower hum and noise level
• 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
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
Technology
The Negative Bias Supply module contains a fully contained bias supply, which is properly filtered and stabilized to achieve a clean bias voltage. A section for each tube measures the actual current around a very small window, where the bias is not affected by any music signal.
The maximum output voltage is -160V, enabling the use of virtually any tube up to very high voltage DHT, like 845, 211, etc… Every tube can be biased up to a maximum of 175mA. The outputs of the module provide linear, low output impedance, to avoid interaction with the preceding driver stage. 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 45 second power-on delay. After 45 seconds, the output tube current gently increases, after the heaters or filaments have had their chance to heat the cathode. This avoids clicks and pops, and increases tube life.
Measured results and theory about the advantages of using these new modules.
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.
Specifications
Electrical
• 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
Specifications
Mechanical
• 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
Pricing & warrantee
The module is warranted for 5 years. The price is € 150.42, not including shipping. Mockingbird attempts to use the current exchange rate, adding the appropriate shipping fees to the United States (usually about 10% of the retail price). If there is a large discrepancy between the price shown on TentLabs site, versus the price here, please contact me.
Building monoblocks? Apply for a discount of 10% when ordering a pair.
Support
As this is a true DIY product, no additional support is available from TentLabs, other than the directions provided. Mockingbird offers a service to install this module in the United States. The price depends on the difficulty of the retrofit. Some chassis are not large enough. Some amplifiers are inappropriate. For consultation, please contact me about the specific amp needing to be modified.
Safety
Though this is a product which if handled properly is safe, it is surrounded by high voltages. Working with high voltages requires safety measures and professional behavior. Tentlabs / Vanderveen / Mockingbird take NO responsibility for accidents resulting from improper usage or handling. Keep one hand in your pocket, and if in doubt, let a professional get electrocuted.