cathode resistor bias versus negative supply biasing

[ND]

Is there a rule of thumb to follow when deciding on the bias scheme for your output tubes?

I have seen amps built with cathode resistors and caps to provide the bias point and others with negative voltage applied to the grid inputs to achieve the bias point. Adjusting a pot is easy when you re-tube to re-set the bias. Then on the other hand it's hard to accidentally change the resistance of a cathode resistor.

Another question, why do most negative grid bias amps only have one bias pot, wouldn't it be beneficial to be able to bias the top and bottom output tubes individually?

[LeftyStrat]

The different bias schemes have a couple of factors. First of all some terminology that was confusing to me when I first started learning about tube amps. The use of negative bias is called 'fixed bias' even though it is usually implemented with a pot. Using a resistor and cap is called cathode bias.

Early amps mostly used cathode bias, mainly because it is cheaper. Later on amps used fixed bias because you can squeeze more power out with fixed bias.

Using a single trimmer or pot for fixed bias is just cost savings. Ultimately having individual pots for each tube allows you to compensate for mismatched tubes.

Lastly, some feel there is a difference in tone or feel between the two biasing methods, that maybe cathode bias has more squish or compression.

One last factor is with cathode bias, you can tailor the response of the amp by choosing a smaller cathode bias capacitor to roll off some low end.