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Pros and cons ....... >
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Biasing is this magical, mythical black art that only the inhuman can understand and comprehend. Your mortal brain cannot fathom the decimation of lives and spawn you will destroy by simply " biasing @ 80% "...
All kidding aside, you must think of bias as a dictionary would prescribe it. Bias is simply setting the " idle " running point of the tube to some arbitrary point between off ( not making sound at all ), and melting down. Idle simply means that when the tube is not amplifying an input signal, but is on and able to conduct, its bias is set to X % of max dissipation.
What NEEDS to be understood is how an amp operates and why each bias topology has a pro and a con. I will be brief, but the % of max dissipation is not meant to be an operational target as much as a basic guideline for knowing where you are at.
Cathode Bias: Cathode bias is done by elevating the cathode above ground and referencing its grid to ground as well. The disparity in their relative difference to ground creates the bias. I.E. The grid is further from ground potential than the cathode, making it appear more negative than the cathode. Since there is no absolute zero to ground for either part of the tube, it will self-regulate to a bias point. Because ground potential never changes for either of those parts, this means that bias can ONLY shift downwards under heavy load ( high output volumes ). This is because the HT power supply will sag down ( lower voltage ) which makes the tubes bias colder as well. This is why cathode-biased amps are biased very close to class A at idle. The bias will only cool down more as the amp's volume is turned up.
Fixed Bias: This is not what you think it means. It is fixed bias because the bias voltage sent to the grid of the tube is set/fixed by a separate, dedicated, bias circuit. That voltage is fixed or set, to be a certain voltage, and at idle will be fixed at the voltage that you set it for. The cathode in a fixed bias circuit is connected directly to ground. This means that the cathode will ALWAYS have a relative operating point between its plate and the voltage sent to the grid. The reason for this is that the bias circuit that supplies the negative bias voltage fluctuates up and down just as the HT power supply voltage does. As the power supply voltage sags down, the bias voltage shifts up. I.E. the positive HT voltage and the negative bias voltage collapse together closer to ground potential. If the bias voltage shifts up ( less negative ), this makes the tubes bias hotter, which is a problem at high volumes where the HT supply will sag down, making the tube bias even hotter still.
So what are the pro's and con's of each topology?
Cathode bias:
Pro's:
1. Operates and sounds like a class A amp at lower volumes
2. Has more dynamic range than Fixed bias amps.
3. Has sag and bloom to the notes as you play the amp. Feel more chewy under the fingers, than most fixed bias amps.
4. Breaks up sooner due to the typical output topology.
Con's:
1. Bias often shifts down into Class B (bad-sounding crossover distortion) at high volumes.
2. While having better dynamic range than fixed bias amps, it has less headroom and will distort more easily.
3. Setting bias can ONLY be done by changing a large wattage resistor and typical values may not be ideal.
4. There is substantial compression and note bloom at higher volumes that is not ideal for certain musical genre's.
Fixed Bias:
Pro's:
1. Less note bloom and less compressed feel, make notes jump out more and are less chewy feeling under the fingers.
2. Easy to set bias by simply adjusting a bias trim pot.
3. More efficient topology allows for higher wattage output power.
4. Bias can be finely tuned and set for the desired tone at operating volume if desired.
Con's:
1. Bias shifts up at high volumes making the tubes susceptible to meltdown if ran hard for prolonged periods of time.
2. More complicated and expensive circuitry to create the bias voltage supply.
3. Less dynamic range than cathode-biased, or class A amps. Simply means that the quietest quiet and the loudest loud are closer together than other bias topologies. Most fixed bias amps utilize negative feedback in their output reducing distortion and keeping the stage cleaner longer, with a sudden onset of distortion. Take the feedback loop out and it will work more like cathode-biased amps. Meaning it will distort sooner, but because the amp must be biased colder at idle, the quietest quiet, is louder than a class a, or cathode-biased amp which are biased nearer full output power at idle. This is why there is more dynamic range and sustain in cathode-biased amps. The amp biases hotter as the volume decreases, increasing the dynamic range. You could say that cathode biased and class A amps act like an expander circuit.
4. Less ability to balance bias between tubes. There is only one bias supply, so it must set a bias for all output tubes in the amp. Cathode bias can have independent bias resistors for each tube much more easily.
Some of these Pro's and Con's are subjective, as are some of the generalized ideas of how each topology " sounds " and " reacts " to playing. If you build two amps tit for tat with the only difference being the bias topology, they will sound and react more alike than different. The operational differences may be more apparent when ran to the different extremes, but under normal operation, they would be hard to tell apart.
So when it comes to bias, it is not about the percentage as much as the operational goal. 80% of max dissipation is only 80% of theoretical max dissipation. We are not 100% sure what the actual max dissipation of any particular set of tubes will be, so we can only assume. This is why tech's tend to use scopes to see what the output is doing. They can know that at 75% of theoretical max dissipation, there is no crossover distortion, and the tubes don't red plate when played at full tilt boogie, so all is good. Others can listen, measure the voltages and tell that the bias is fine, especially if there is no red plating. Some people like their tubes biased cold, some hot. Some like the sound of tubes on the verge of melting down, while others prefer their tubes to last significantly longer. I prefer my fixed bias amps to be biased on the hotter side. I shoot for 70% and if I can go higher without melting down, I will, IF AND ONLY IF, the sound improves when going above 70%. If there is no improvement in sound quality once breaching 70%, I turn it ( the bias ) down until the sound becomes bad, then back up to the point where it sounds good again and note the bias percentage. If it is near 70%, I will call it good. If it is below 60% I start to wonder a little and will bust out the scope to see what's going on. I want as hot a bias as I can get to the point where sound quality no longer improves. If I have to bias above 80% to get the sound quality I want, I start to get worried. You are in the danger zone here with fixed bias amps. Much beyond 75% in fixed bias amps, you start to get into the money zone. This is where you burn up tubes quickly and risk taking out transformers if you do go into meltdown.
With cathode-biased amps, I shoot for 90% - 95% of max dissipation at idle. If it doesn't melt down at idle, it's good. If I am sitting at 98% and still no red plating, it's good, but it's also got me worried. If I am at 85%, and I can go higher I will. If I can't get the bias to be below 100% and the next standard value resistor only gets me to 90% ish, I will call it good. This is usually not an issue though in cathode-biased amps. You can usually get about 5% accuracy with standard-value resistors. Generally though, if you are at 93% with value A, value B will be too hot for comfort even if you end up close to 98%. So you will often have to live with the colder 93% number. You have to play the hand you are dealt sometimes.
To close this long post, I will address the bias question more directly. The only real consideration for bias is the operational goal of the amp at operating volume. You want the amp to be biased so that at idle, when it is easy to acquire the numbers you need to do all the math with, the amp will be in the sweet spot when at operating volumes. If you don't play very loud, you can often bias fixed bias amps rather hot quite safely. Conversely, if you play quiet with a cathode-biased amp, you may be better off biasing on the cold side. If you play loud with a fixed bias amp, you will want to consider carefully how much money you want to spend on tubes. If you bias hotter, you will be replacing them more often. Whereas with a cathode-bias amp that you play loudly with, you can generally bias fairly hot, and not have issues, as the bias will cool down more as you play louder and louder. You are not looking for a number per se, as that number will be different for any given set of tubes. The bias percentage is just a number that gives you a good idea of where to shoot for off the bat.
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