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| Tube bias theory |
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| jeffjmr:
I understand there is an ideal bias voltage for tubes. But besides the difficulty in accurately measuring the current to match the particular tube’s ideal spec, I am put off by some who recommend “set it to what sounds best”. I’m curious to learn what exactly happens to the tube’s output when slightly underbiased and slightly overbiased, ignoring for now tube life differences. I’d like to err on the side of caution to extend tube life by underbiasing, but I can’t hear the difference in my amp, and would like to know what is actually happening when I depart from the ideal. Thanks, Jeff |
| Cubdriver:
It depends on how the tube is being used. If class A, you want to set the bias point to put it in the center of its most linear region to maximize the output swing on either side of the bias point. If you bias it too low, you'll start to get distortion on negative input swings as it starts to cut off. If too high, it will begin clipping on positive input swings as it approaches saturation. If in class B (or AB) - push-pull - biasing the tubes too low will cause crossover distortion because there will be a 'dead' zone on either side of zero where the first will turn off before the other side turns on. What is the application? -Pat |
| Gyro:
To do it properly, you need the tube operating curves from the datasheet and then (as Cubdriver says) chose the most linear operating point. Under or over biassing from that operating point is going to increase distortion - the people who recommend 'setting for what sounds best' are choosing increased 2nd harmonic distortion (or just don't know how to work out operating points and read datasheets). |
| Circlotron:
--- Quote from: Gyro on February 14, 2021, 11:12:34 am ---the people who recommend 'setting for what sounds best' are choosing increased 2nd harmonic distortion --- End quote --- Which may actually be a valid approach for a musical instrument amplifier, but not for hifi. |
| bob91343:
There is no substitute for knowing what you are doing and using decent test equipment. Everything else is a compromise with widely varying results. |
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