Fixed Bias vs. Adjustable

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.

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%.

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?