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To add to Roscoe's comments:

Fixed Bias -
Advantages:
1. Allows adjustment of bias level to a specific plate current. Usually, this can be performed on individual tubes, or a group of tubes with DC balance pot between pairs of tubes.
2. Does not require matched pairs of tubes.
3. Voltage across tube is constant regardless of bias setting. Only adjusting plate current.
4. Lower input impedance into output transformer since there is no cathode resistor in series with plate resistor.
5. More power for a given B+ voltage since voltage is not being wasted across cathode resistor.
6. Allows one to re-bias tubes as they age.
7. Can allow for individual replacement of output tubes upon failure of an tube (just replace and re-bias).
8. Generally, a tighter sound (better bass) due to lower impedance.
Disadvantages:
1. More complicated - as you noted.
2. Failure of bias supply can result in damage to tubes and output transformer due to excessive current (redplate). Bias supply need to be reliable in its design and component selection.
3. Noise in negative bias supply is injected into grid of output tube. Usually not much of an issue, most bias supplies are very low current and consist of a half-wave rectifier with a cap-input filter. However do not want to use a regulated supply since that will upset the bias with any change in B+, and increases complexity (more failure-prone).

Self (Cathode bias)
Advantages:
1. Very simple (cathode resistor)
2. Very reliable - failsafe
Disadvantages:
1. Requires careful matching of tubes. Cannot replace tubes individually upon failure without impacting balance.
2. Cannot adjust bias levels (one can install a rheostat or adjustable wirewound resistor for limited bias adjustment, but impacts both voltage across tube as well as current.
2. Lower power output due to voltage drop across cathode resistor
3. Higher impedance (due to cathode resistor)
4. Cannot adjust tubes as they age (though "Blumlein garters" can help in maintaining balance).

You cannot use self bias and expect to have the same sound if it was originally a fixed bias amp. It a matter of debate on which is preferred.

Just to make thing complicated, a combination of self and fixed bias can be a good compromise arrangement, which provides the failsafe advantage of the self bias with the adjustability of fixed bias.

David

Doesn't self bias require some sort of small cathode resistor to allow actually measuring plate current?

Mark

That's a good idea. I don't have the equipment for measuring low resistance values with high accuracy, so I could use a small value precision cathode resistor to establish an accurate plate current, measure the corresponding opt primary voltage drop and then remove the resistor.

Mark

Traditional self bias requires the amp to draw more or less constant power regardless of the signal level. This implies class A operation. If you try to violate this by driving the tubes harder, the bias voltage will be unstable. Fixed bias allows Class A-B or B, and much greater power output for the same operating conditions for the tubes.

David