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Re: Dsl Biasing Question
The volume controls do not matter. Bias is the "idle current" that your output tubes are consuming with no signal passing. The volume controls have no impact on this voltage.
There is no "right" bias for any amp. The factory may suggest one, but there isn't one bias that is right and all other points are wrong. You can actually adjust the bias to run the tubes higher or lower depending on the sound you are looking for, what brand/type your tubes are, and how long you want your tubes to last. If you adjust your tubes to consume more current, you get more volume, but more heat, shorter tube life, and you run the risk of imploding your tubes. Some recently manufactured tubes are less resistant to this than tubes manufactured in the 50s 60s and 70s. This is why a lot of the new old stock tubes from back then are more valuable. Many older amps use huge plate voltages that some modern tubes don't handle as well as others.
Improper bias can cause crossover distortion, which is where the output tubes are not amplifying the entire waveform, instead they are ignoring the lowest voltage parts of the AC signal, around the zero crossings. Because each set of tubes is different, you should bias your amp every time you install new tubes, and check it periodically throughout the life of the tubes. It is not necessary, but it is really best to have an oscilloscope to set the bias.
Tim
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