What causes tubes to not hold the bias rating?

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As an example if you bias an amp to 28ma or whatever it may be but then after playing a while and rechecking the bias it now reads 23ma, is this an issue?

What could cause this? An amp issue or a bad tube?
 
Tubes can change bias current draw as they warm up. Make sure they have a chance to settle in to their final values before you call it done.

LIne voltage changes will also cause bias current variations, so you should measure your line voltage and make sure it is the same as when you set the bias.

I wouldn't worry about a few mA.
 
Tubes can change bias current draw as they warm up. Make sure they have a chance to settle in to their final values before you call it done.

LIne voltage changes will also cause bias current variations, so you should measure your line voltage and make sure it is the same as when you set the bias.

I wouldn't worry about a few mA.
Ok thanks but how long should it take to warm up? And I only worry because the amps are recommended at a certain ma. So if I set it there and it lowers, I have to bias every day and that just seems wrong to me. Just worried if there is an issue.
 
Just let it sit there for 5 or 10 minutes and watch the reading. It should stabilize at some point. If it is drifting wildly, you may have bad tubes. I've seen that with some EH fake Mullard brand EL34s that would drift all over the place and even go into redplating.

As I mentioned, wall voltage can change drastically throughout the day. If you are having to bias every day, you need to measure your wall voltage and make sure you note the bias at that particular voltage. I suspect it is fluctuating.

Also, tubes can change quite a bit during the initial break-in period, which is why many amp manufacturers do a 24-hour burn-in to let everything settle in. If your tubes are new, bias them up, play for a few days, and then re-check, again, making sure they wall voltage is the same as the first time (amp manufacturers use a variac to set the exact voltage every time).

However, you are probably worrying too much about it too much. It just isn't that critical to constantly be trying to hit one magic bias number and keep it there. If the bias current jumps way up and the tubes begin to redplate, you likely have a leaky coupling capacitor or a bad tube, but I doubt that in this case.
 
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Just let it sit there for 5 or 10 minutes and watch the reading. It should stabilize at some point. If it is drifting wildly, you may have bad tubes. I've seen that with some EH fake Mullard brand EL34s that would drift all over the place and even go into redplating.

As I mentioned, wall voltage can change drastically throughout the day. If you are having to bias every day, you need to measure your wall voltage and make sure you note the bias at that particular voltage. I suspect it is fluctuating.

Also, tubes can change quite a bit during the initial break-in period, which is why many amp manufacturers do a 24-hour burn-in to let everything settle in. If your tubes are new, bias them up, play for a few days, and then re-check, again, making sure they wall voltage is the same as the first time (amp manufacturers use a variac to set the exact voltage every time).

However, you are probably worrying too much about it too much. It just isn't that critical to constantly be trying to hit one magic bias number and keep it there. If the bias current jumps way up and the tubes begin to redplate, you likely have a leaky coupling capacitor or a bad tube, but I doubt that in this case.
Ok thanks, appreciate the info and I feel better.

I did what you said, let it sit for a while. They are stable and only 2ma lower than what I had originally set it for so I think I am ok.
 
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