There still is serious discussion over which is the better way to go, fixed or cathode bias.
CATHODE BIAS or Automatic Bias or Self Bias, call it what you will, is and always has been used as a safe, effective and somewhat automatic way of setting the bias point of output tubes, (or any tube for that matter), hence the name Automatic Bias. The tube simply biases itself, due to the amount of current it draws from the plate supply. As the tube ages it cannot draw as much current and so, bias automatically becomes less negative causing the tube to again draw similar current to what it did when new, so in effect performance continues at or about the performance level of a new tube. If, on the other hand, the tube develops a somewhat common flaw known as “grid emission” it may actually start drawing more current than it did when new. Again bias will change and the control grid will be driven more negative, thus blocking the tube’s ability to conduct more and the tube continues to run at or about its original operating point. It should be pointed out that if a tube develops severe grid emission nothing can stop it from failing, not even cathode biasing.
Because a tube has to draw considerable current to bias itself, tubes run warmer and power supplies have to be quite hefty to handle the high idling current of several self biasing output tubes found in any given amplifier. Tube life may be shortened somewhat, but not excessively, when used in a cathode bias circuit, more so if the tube power dissipation ratings are not given proper consideration during circuit design. The higher idling currents are drawn through the amplifier's power supply from the 120VAC line supply and this means that inherently cathode biased amps simply consume more energy than comparable fixed bias amps. Self biasing also requires heavy power resistors, sometimes one for each tube's cathode, which in turn generates considerable heat inside the chassis and can cause overheating of other components if the chassis is not carefully designed to include adequate ventilation. The reader will note that all cathode biased amps should have significant ventilation designed into the chassis to let the heat out, as well as significant bottom and rear ventilation for cool air to enter, a feature not seen on many other power amplifiers.
Cathode bias circuits are simple, foolproof and require no additional bias power supplies. They do not usually develop as much audio power output as equivalent fixed biased circuits because the voltage that a tube “sees” from an equivalent power supply is always less by the amount used to bias the tube. To explain more fully let’s say that a tube requires 40 volts to bias itself and is running on a plate supply of 400 volts. This means that the tube actually has only 360 volts to facilitate driving the output transformer, where if fixed bias were used the full 400 volts is available to drive the output transformer.
Many swear that cathode biased amps simply sound better than their fixed bias relatives. Tests have indicated that some cathode biased amps tend to operate in Class A territory for a portion of their given power output rating. But this opinion on the sonic qualities is perhaps more a matter of personal preference. It is true that cathode biasing does introduce, more as a byproduct of the design rather than skillful engineering a certain amount of frequency sensitive negative feedback which tends to flatten out aberrations of frequency response and so produces what many feel is a mellower, laid back sound. This effect is noticed in some designs but not in others. Critics of cathode biased circuits may equate “laid back” to “muddy” and “vague”.
FIXED BIAS designs use a separate power supply to bias the output tube grids. This makes for a more complicated power supply overall and catastrophe is only a few moments away if the separate bias supply should fail for any reason, as the tubes will have no control voltage and will most certainly self destruct. It should be pointed out that there is no general reason for a bias supply to ever fail if properly designed. The amount of current demanded of the tubes for biasing is extremely small, so small in fact that there is almost no current flowing, and so almost no load on the bias supply, so simply no reason for failure.
Some detractors will point out that fixed bias machines require periodic manual bias adjustments to compensate for tube aging. While in theory this is true, it is also true that fixed biased tubes age very gracefully because idling currents are generally very low in the first place. And age only makes them a safer device under fixed bias conditions. Good quality tubes will still last thousands of hours before requiring any form of compensational adjustments and at that point can be replaced very cost effectively, keeping in mind that one has realized perhaps thousands of hours of listening time anyway. The only real effect of advanced age in a fixed bias circuit will be slightly reduced power output and not necessarily increased distortion. As most fixed bias circuits produce more power output initially than comparative cathode biased machines it is a very long time before output power ever falls to even par with the cathode bias machines.
Fixed bias machines are extremely efficient, as tubes idle comparatively cool when compared to cathode biased machines, simply because they can be biased such that they draw very low current unless driven by audio program content to do some real work. They require no heat producing power resistors under the chassis, so under chassis temperatures remain lower and they can harness the full plate power supply voltage, so they develop more audio power output than an equivalent cathode biased amp. Tube life is extended because the tubes don’t waste most of their useful hours idling at higher currents, even when producing no sound.
Power supplies still have to be quite beefy to provide the headroom to easily handle full output currents when all output tubes are driven into high current conditions while producing high output power. But generally, power transformers and other components never get as warm or waste as much energy as equivalent cathode biased systems, as most of the time the fixed biased output tubes do not draw appreciable current while idling or reproducing sound at low listening levels.
Sound quality of course is very subjective. Where fixed bias fans will claim that their amps have more “presence”, and up front ability, cathode bias fans will say that fixed bias amps are “shrill” and hard sounding.
Each method has its merits and demerits, but careful design, and very exhaustive listening tests, can produce a winner with either bias system. Both styles and have many “lovable” amps from both camps.