Fixed Bias vs. Adjustable

M., although I'm sure there can be pro and con arguments made each way, I don't agree that simply implementing auto-bias directly results in 40% greater tube life. In fact, if the tubes aren't bad and you double-check the bias every once in a while and make an adjustment if needed, I doubt there's any longevity difference at all - why would there be, as long as the bias never goes outside the acceptable range?

Zaike,
It's a trade-off...

By embedding an electronic bias circuit you get rid of noisy potentiometers by nature and introducing Zenner diodes i.e. SS "intruders". Well anywhay VTL and many more tube amps are using SS power supplies so why not get an electronic controll over bias...? It's just simply a matter of effort and certainly money. They started implementing a bias B+ fuse that will prevent the grid voltage go above -40V but fuses and potentiometers are relatively equal sources of imperfection and noise as SS devices for electronic bias. IN addition you get a trouble-free and protected operation of the amplifier and and and tube life by nearly 40%.

Marakanetz, I am not an electronics expert to defend this position, but it is my understanding that many tube amp makers (presumably including VTL, but see below) prefer fixed bias because they believe it results in better sound. My supposition would be that simple auto-bias circuits, though convenient, may be susceptible to modulation by the audio signal, and that a fixed-adjustable bias supply with good regulation will avoid this problem, but I cannot say for sure if this is the issue. Also, I have never found it to be a significant problem to keep power tubes within their designated operating range over time unless one is going bad anyway (in either my VTL or C-J amps), and would dispute the contention that minute drifts in the precise bias-point settings are of much importance sonically, or for durability. Getting back to VTL for a sec, I notice that they are advertising their new 12-tube Siegfried mono as having automatic bias control, but it is apparently not a conventional type of auto-bias circuit; according to their website, the process is logic-controlled by a microprocessor, and the adjustment only occurs during the silences in between music selections, implying that bias is fixed during play. They obviously didn't go to these lengths to cut corners or save dollars, so I must assume they feel this is important to obtaining best sound from this premium product.

Most of the the power amplifiers especially tubed ones ought to be with adjustable bias. For the tube amps working in SET operation biasing is still on demand since tubes do have a large parameter tolerance between each other hence it's important to matche the tube to the amplifier rather than to themselves when dealing with SET. Tube amps need to be re-biased periodically and SS amps are biased by the manufacturer once or when you're replacing an output transistors or blocks. High quality powerfull transistors nowdays are manufactured with very minimal parameter tolerance and need only be slightly adjusted with potentiometers or electronically depending on the class of operation and certainly bias level. If SS power amplifier works on low-bias operation than it's possible to have a fixed bias even for the output devices. Motorolla, Fairchild or Sanken transistors are perfect devices that work distortionless on the small bias operation.
Bias is adjustable in power amplifiers different ways either with potentiometers or by semiconductors electronically keeping the bias voltage at neccessary bias margin.
After realizing the huge benefits to tubes and to the tube amp reliability I've repeated Quicksilver's electronic bias controll and mounted it to my VTL amplifiers. Now I don't have to bias-re-bias them at any time and if tube goes bad only the tube goes bad thx god.
The driving stages and preamplifiers may have a fixed bias since small-signal devices have correspondedly substatially smaller parameter tolerance hence in these domains bias may set to be fixed.