10/3/2023 0 Comments Biasing a tube amp![]() The bias current going through pin 8 is routed through a 10 ohm resistor (some amps use 1 ohm to ease calculation). It demonstrates what is already included in this amplifier. The bias current is "converted" to voltage for measurements.Ībove, the diagram shows how a bias probe plug works: a special kind of socket you insert between the tube and its amp socket, for amplifiers without bias measurement accommodation built-in. If the voltages are within 10% of recommended 470mv and within 10% of each other, I wouldn't bother tweaking with the adjustments, as the amp is probably shipped with the pots matched, and you'll instead be chasing bias every time you switch around tubes or retest aging tubes.Ĭlick to expand.We are not directly measuring the current in this amp (in fact, we can't truly measure current - we can only measure its effects, and also can't observe the system without changing it). Record your results and move on to the next. Don't accidentally short the leads to the chassis while inserting them. Measure the voltage between the ground test point and the corresponding + tube test point on that side. Press the manual range button until it shows mV (the lowest range, up to 600mV - or 0.60V). Use the two probes connected to COM and INPUT on the multimeter. You will be measuring DC voltage, so put the multimeter in V= mode. ![]() And after being tweaked to within the recommended range, the bias stayed rock-solid over a period of many weeks. In my listening room, the initial bias voltage checks conducted after an hour warmup were in the range of 0.50V to 0.53V. ![]() The recommended voltage range for each test point is 0.46V to 0.48V, which implies an idle current of 46 to 48mA. The test points allow you to measure the voltage drop by the idle current across a 10-ohm cathode resistor, a very common test scheme. And unless you have asbestos fingers, I would caution you to use a cotton glove to protect your hand from accidental contact with a hot tube. An insulated Phillips head screwdriver should be used to adjust the corresponding potentiometer as needed. There are three test points for each channel, with the center point being ground, while the left and right test points are for the left and right KT88s. ![]() I would recommend a multimeter with long leads capable of measuring DC voltage below 1V. Although this amp does not require frequent bias checks, it is good practice to check and tweak the bias after the initial installation since the bias may vary with the local AC mains voltage, which in many localities runs higher than nominal. However, seasoned audiophiles who are accustomed to biasing power tubes should have no problem with this. Hence, no biasing instructions are included in the owner’s manual. Luxman apparently did not intend these adjustments to be performed by anyone other than a qualified technician. Bias adjustment pots and test points corresponding to each output tube are located in front of the output transformers. The output stage uses fixed bias obtained from a dedicated power supply. Here's from an Absolute Sound review, since the procedure and load sense resistor is not documented: ![]()
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