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Biasing a tube amp11/9/2023 ![]() If you look at the schematic, you should see that the only thing between the meter leads at this point should be 1/2 of the output transformer's primary winding (or the entire winding if you have a single-ended circuit). Next, attach a second alligator clip between the black lead and one power tube plate pin (usually pin 3, consult schematic or datasheet to be sure). You'll need the schematic or the assistance of someone familiar with tube amps to figure out where the center tap of the output transformer is. With the amp off, put the meter in current mode (mA range) and attach one end of an alligator clip to the red lead and the other end to the center tap of the output transformer's primary (tube side) winding. If that didn't scare you off, then we'll proceed. Worse is that you might connect 500V to yourself, someone else, or parts of the amp that cannot withstand high voltage. This normally results in the fuse blowing along with a big spark that will probably leave a mark and may even damage your test lead. This means that if you touch 500V with your red lead, and then touch ground (0V) with your black lead, you will be shorting that voltage and sending the 500V to ground. It is very important to understand that in this mode your meter leads are basically connected directly, especially as far as high voltage is concerned. You will actually use your meter's mA setting in this method. This method requires use to probe the open amplifier while the amplifier is on, which is dangerous and not recommended to anyone unsure as to what they are doing! Only attempt this with alligator clips. Usually we only work on our amps with the unit unplugged from the wall, and the filter capacitors drained of any stored voltage. Be extra careful if attempting this method. The last method is dangerous, but quite handy. ![]() ![]() However, once your ear is trained to what crossover distortion "sounds" like, you can get adequate results by simply listening to the output and adjusting "by ear," only needing a simple meter to ensure the final bias setting doesn't run the amp too hot (see above). Most technicians would also use a "dummy load" resistor so as to not actually drive a speaker with the annoying 1kHz tone. This method is the most test equipment heavy, requiring both an oscilloscope and an oscillator. During the flat spots the output devices are all "off." Increasing the idle current should decrease the flat spots until a continuous sine wave is observed at the output. If you have a variable bias control, turning the amp maximally cold should result in the output sine wave developing flat spots where the wave crosses the zero voltage point. 1,000Hz or 1kHz is often selected as a test tone as it is a rough midpoint for the human hearing range, and it is a nice round number. The input is connected to a low distortion sine wave at a fixed audio frequency. ![]() To observe crossover distortion, the output is connected to an oscilloscope. If your output sounds especially distorted at low volumes, or during the decay of a sound, it is likely crossover distortion or a faulty speaker. If you have a class AB amplifier, you have to watch out for crossover distortion. ![]()
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