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Does an amps bias "drift"?

kselbee

Silver Supporting Member
Messages
2,710
Can bias change over time or do you really only need to bias when changing tubes?
 

macmax77

Silver Supporting Member
Messages
13,285
Can bias change over time or do you really only need to bias when changing tubes?
Yes, with tube wear as long as I know, but do not quote me on this since I am not even sure, just think so.
 

Geetarpicker

Member
Messages
3,054
The output tubes themselves can surely drift. I’ve had plenty pairs of Russian and Chinese power tubes that started off nicely matched but over time were way off from each other making proper bias iffy at best. If you are lucky a set of tubes will drift but stay reasonably matched so you can rebias and be good to go. I check bias every few months on my amps that see several gigs a week
 

support

Senior Member
Messages
849
The bias voltage doesn’t normally drift, but things do change over time. As a tube ages, its conductance decreases slightly. That effectively increases the negative bias voltage, which itself didn’t change. The question is, how much time are we talking about?
 

HotBluePlates

Member
Messages
12,208
Can bias change over time or do you really only need to bias when changing tubes?
The amp applies a bias-voltage to the tube to control the tube-current.

In a fixed-bias amp, the applied bias-voltage doesn't drift much or at all.

The tube-current that happens can & does drift, sometimes a lot. Early in a tube's life, it will drift a lot and in a short amount of time.

For those new tubes above where tube-current drifts a lot, some vendors will do a burn-in to hopefully get beyond the point where the tube's behavior settles down.​
No, you don't really need to re-check the bias unless you want to. Forums massively over-emphasize the importance of tube biasing. Really, we mostly want to be sure the tubes don't overheat & melt.

_____________________________

Cathode-bias amps will exhibit less apparent tube-current drift. The current drift causes an immediate reaction that tends to oppose the drift.

Cathode bias is self-adjusting; it minimizes the apparent bias & characteristic differences among a groups of different tubes. As long as the tubes aren't overheating when first installed, we generally assume the tubes will remain good until we decide the tubes should be replaced. This assumption is very-valid with cathode bias, to a greater degree than with fixed-bias.
 

kselbee

Silver Supporting Member
Messages
2,710
The bias voltage doesn’t normally drift, but things do change over time. As a tube ages, its conductance decreases slightly. That effectively increases the negative bias voltage, which itself didn’t change. The question is, how much time are we talking about?
I have a Victory V30 and it seemed to start sounding "fuzzy". I changed the power tubes but it didn't really change much so wondering if it needs biased. Or maybe it's the preamp tubes? It just doesn't seem to sound as good as it once did, but I also have noticed that my ears hear things differently over time so maybe my tastes are just changing? Hard to know.
 




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