# Old Tubes and Bias



## JimiGuy7 (Jan 10, 2008)

Okay, so I was cleaning the pots and inputs on my JCM 900 and I decided that maybe I should check the bias on the tubes. They look to be the stock Marshall tubes that came with it. I check the bias and it was at like 8ma and I was like WHOA!!!!! What the hell???!!!! So I turned up the bias pot and I normally bias these around 36-40, but I couldn't even get above 26ma. So, could this just be a case of the tubes are just done and they won't reach there optimal bias anymore? Or, does something need to be fixed or changed? I ordered a new set today and should have them in a couple of days, but don't want to put in a brand new set and hurt the tubes or anything else. Should I expect the bias to go up properly with a set of tubes that are not worn out? I know the tubes are done, but is something else? If you guys could let me know what your opinions are on this that would be great. Thanks in advance.


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## WCGill (Mar 27, 2009)

Tubes do drift with use, usually when they are newer and first installed. This is pretty extreme, perhaps they are worn. 36-40 means nothing without knowing the plate voltage.


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## dtsaudio (Apr 15, 2009)

It does sound like the old tubes are done. 
dial back the bias before you install the new ones. The new tubes could run hotter than the old and may draw too much current when installed.


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## JimiGuy7 (Jan 10, 2008)

I should have mentioned that I do believe the tubes to be at least 20 years old. I ordered new tubes today and will report the results. Hope it works.


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## Church-Audio (Sep 27, 2014)

JimiGuy7 said:


> I should have mentioned that I do believe the tubes to be at least 20 years old. I ordered new tubes today and will report the results. Hope it works.


Age means nothing when we are talking about tubes it's hours of use we need to look at. You should invest in a bias meter like the ted weber bias rite that measures plate current and plate voltage. This will tell you if you use the weber bias calculator where your tubes are in the scale of things. If tthe amp sounds good ??? Leave them until it does not. Or get them tested. My favourite tube right now is the tungsol el34b sounds great in marshall amps. I guess the other question is how are you checking bias exactly?


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