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 bias 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 pentode, single-ended and push-pull.
Result areas are:
• better imaging and stage due to lower hum land 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
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.
The maximum output voltage is -160V, enabling the use of virtually any tube up to a 211 or an 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 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.
• 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
As this is a true DIY product, so no additional support can be given, more information can be found in Tentlabs website.