If you are saying that you cannot get the bias reading high enough, even with the pot turned up all the way, then something is wrong.
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Has it always been this low? If you used to be able to get 700mV and it has dropped down to 450mV then the tubes may be fading away.. It could be something else but the tubes would be the number one suspect. The easy test is to try a fresh set of tubes but that is a bit expensive.
If it has always been 450mV maximum then something else is going on. Are these the tubes that came with the amp? If not, since all 300B tubes are not the same, these might not be a good match for your amp.
SWAMPWALKER - Even when the pots are turned to maximum, bias will not reach recommended 700. I know it's the tubes because I tried different positions and it followed with the same results.
HERMAN - Three out of the four can bias to 700 no problem. The one at 450 is the same brand and was purchased new.
MARAKANETZ - Problem is just with the one tube. I know it's not the amps.
How will I know when a 300B tube is on it's last legs?
Enen though the music is still there (little degradation sonically), should I continue using this tube or replace it immediately?
"HERMAN - Three out of the four can bias to 700 no problem. The one at 450 is the same brand and was purchased new."
Ah Ha, that would have been nice to know up front. You knew one out of the four was different but didn't share that with us.
"How will I know when a 300B tube is on it's last legs?"
You now know, you have a weak tube. It has low emmission. You have 2 choices, replace the one weak tube or the whole set. If they have relatively low hours I would replace just the one, but if they have a lot of hours I would replace the whole set.
NO return on a defective tube makes them a non-viable supplier, IMO. Sounds like they are using their customers as QA/QC. For 20 bonus points, what large software supplier uses the same business model? Seriously, unless your vendor is 30% lower in price than the competition, then their policy in unsupportable.