Not entirely accurate....waldo041 wrote:the power output tubes are either a fixed or adjustable bias. fixed bias a tech needs to physically change out a couple resistors to lower or raise the bias to the tubes. adjustable bias uses a potentoimeter to bias the tubes. the hotter a tube runs the faster it breaks up ie.. less headroom. the colder a tube is biased the later it breaks up ie.. more headroom.bucketorain wrote:could you explain that a bit further please?Jon S. wrote:As a fan of really cheap fixes that cost little and sound good, an overlooked option for folks who want a bit more of the Jerry tone from what you already own is to actually lower the power tube bias a bit from its optimal level. Try it sometime, see what you think of its impact on your amp's edge/smoothness and dynamic range - you might be surprised at what you hear.
Technically all output sections with tubes are either fixed bias or self(cathode) biased. All amps are adjustable in one way or another. It's just a matter of how labor intensive it is. The play on words came with the introduction of variable resistors, pots, trimmers, etc, on the bias circuit. It's a mistake to refer to an amp with a trimpot as "Adjustable Bias" but it has become so commonplace that it's become the acceptable term. It's still a "Fixed Bias" circuit. There are many amps that are cathode biased that have a trimmer in circuit as well and these amps are referred to as self biasing even though you can adjust the range in which the cathode swings.
As for how hot or cold you push a power tube, this does not follow any hard fast rule such as a hot tube equals less headroom, or a cold tube equals more headroom. It has more to do with the type and quality of the tube construction and type of bias scenario, fixed or cathode. Example being a GE 6550. If you were to run this tube at 45-55 percent of it's plate dissipation, it would be quieter and exhibit a harsh clipping. Pushing it up into the 60-70 range raises the perceived headroom and the tube operates much more efficiently.