SO, what happens if I don't have the bias adjusted?

[stevel]

I hear about people biasing amps "hot" and things like this, so I imagine there's a little lee-way in biasing, but what happens if I don't re-bias at a power tube change?

In fact, really, what is biasing?

Biasing "hot" sounds like it would eat up the tube(s) faster.

And if your amp has had the bias set for a set of tubes, and you replace them with some other tubes (let's say anywhere from the exact same brand to some different brand) what's going to happen? How far off can it be?

Also, does bias change over the life of the tubes? For example, can you have the bias set when the tubes are installed, and then after 6 months of playing the tubes "go out of bias" or anything like that?

TIA,
Steve

[hamfist]

Biasing basically is setting the idle current drawn by the tube.

Some sets of tubes can be WAAAAY off compared to some others in terms of bias needs. So if you don't re-bias you could end up with a new set that simply draws so much current that it red-plates and fries your tubes (hopefully not taking any other components out on the way !). I think you'd be fairly unlucky to have this happen straight away, although you could easily have a new set that just ran a fair bit hotter, and didn't last very long.

Conversely, you could get a new set that ran much colder than your previous set. Some amps sound harsh, ice-picky and generally unpleasant if run with the bias too low, so obviously your tone could seriously suffer in that respect.

Tubes can drift a bit in their bias needs as they age. This mainly seems to happen in the first 48 hours of their lives. So, good tube dealers pre-burn in their power tubes for 24 or 48 hours and only then do they match them up into pairs and quads.
As for further significant drift later in life I, personally, haven't experienced that. Although I would suspect that if it happened it would indicate that the "drifting" tube was on the way out.

[TweeDLX]

While I always recommend biasing after retubing, Groove Tubes claims their (I think it's 1-10) numbering system of tube matching means you just bias for the first set, and buy sets with the same number when you need new ones. I have no experience with them, so I'm not 100% sure about it.

Mike

[Trout]

The OP needs to post what amp actually,

If its a Cathode biased amp in most cases a tube swap(of same type) is simply plug and play by design.

If the amp is fixed bias, then there could be reason for concern.