What should I know about BIAS? Bias is a simple DC (negative) voltage that sets, what is called, the "operating point" of the tube. For our purposes it "fine tunes" the current going through the tube. We want each of the 4 tubes per channel to be equally sharing the load. This gives the amplifier's lowest distortion and the tube's optimum life. When we set "bias" we are adjusting that voltage and measuring the current at a pin called the cathode. The cathode is connected with a 10 ohm 3 watt resistor that allows the more convenient voltage measurements from terminals right on the top panel. These 10 ohm resistors also have a second job. They are fuses in case of a really nasty tube failure. Sometimes a tube will die in a way that burns up this resistor (with a little puff of smoke) and then with a new tube you will only measure zero at the terminal for that tube. The resistor should be replaced by a technician, but it should only take ten minutes. Can you solder?
How often should I check these BIAS points? Some people never do. It really should be done when you change a tube and it is very likely that a little adjustment will be needed. We suggest that you check every 3 months - that way you are assured that the amp is running optimally and will spot a tube on its way out. Some check every month, once a week is getting a little obsessive.
What tools do I need? Just two. A small flat screwdriver (insulated handle) to adjust the trimmers. You also need a voltmeter or
What do I do? The amp should be well warmed up (30 min.) and no music playing. Set the meter to read DC volts. The meter has two test leads, red and black. Put the black lead in the black terminal on the top marked TPG (ground) or touch the metal point to the steel chassis (but not a painted or anodized part). Put the red lead in the first red terminal marked TP1. The meter should read either
.25 V or 250 mV(same thing). If it doesn't you need to adjust BIAS 1 trimmer until it does. It doesn't need to be exact
What if the reading is drifting up and down? Some drift is normal. It is caused by the AC mains changing and by a little very low frequency noise in the tube. If one drifts above .3 or below .2 then you may be seeing a tube begin to go bad. Watch it for a while or check it every few weeks noting changes. This tube should be replaced probably. With mains changes all the tubes drift together and you may see more long term drift. Another cause is that you forgot to stop the music.
What if I can't trim a tube into range? If all you get is zero or near zero on one terminal it means either the tube or 10 ohm resistor is dead and needs replacing. If it is too high then it probably is the tube. If you look at the tubes, that one will probably be cherry red. Get a new tube. If all the terminals read zero on one channel, then the B+ fuse is likely blown on that side. If all the terminals read zero on both channels but the amp still plays, then you have a problem with the meter.
Is there any advantages to biasing higher or lower than .250? This is the optimum
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