Power valves in valve amplifiers need a good controlled and stable quiescent current for optimal operation. There are two standard methods to do so: trimming the negative control grid voltage or the use of a heavy cathode resistor. Both methods have the disadvantage of drifting of the quiescent currents through valve aging and or variation in supply voltages. Traditional servo circuits might solve this. However, they can’t cope with the raising of the cathode currents at higher output levels. Such traditional circuits change the quiescent current, which should not be the case.
In the new circuit of Vanderveen and Tentlabs, the quiescent currents are absolutely constant under aging and changes in supply voltages and power levels. The condition of operation is not of any influence anymore, like class A, AB, B or single ended or push pull. The circuit uses the amplified music itself to permanently measure only in a very small window around the selected quiescent current. Any current variations outside this window are neglected. Therefore changes of power levels and supply voltages have no influence anymore.
The new module has its own integrated power supply including negative control grid voltages and four sections to measure and regulate four power valves. It can deliver negative control grid voltages up to -160 V, while the quiescent currents can be set with only one trim pot between 0 to 175 mA.
Mains Input Voltage : 115 or 230 Vac
Range negative bias : 2-80 Vdc or 2-160 Vdc
B+ voltage sensing : from 200 to 500 Vdc
Adjust quiescent current : from 0 to 175 mA
Power save : switch for half quiescent current
Inrush-time : adjustable
Accuracy : 4 currents deviation < 1 %
Dimensions : 100 x 100 x 40 mm
Mass : 375 gram
Download manuals: “Automatic valve bias module”; and “Auto-bias module improved”;
Download application (courtesy Saturo Kobayashi): Valve measurements -1 en -2