[TKonrad.NOLA]

Bought a Cary SLI-100 last year, checked the bias for the first time today and it was super high.

[SET Man]

Hey!

   Wait a minute... you've just checked the bias of your amp since you bought the amp last year? You are lucky that those tubes are good and your amp is robust. It is  a good good idea to check the bias when you turn on the amp for the first time.

   After the first check you should check it every 3-6 months. Turn it on let it warm up for 10-15 mins before checking the bias and then check again an 30 mins to an hour later to confirm they are correct and don't drift too much after the first check, readjust them as needed.

   Tubes bias drift overtime as it age. But shouldn't be a wild swing, that could indicate a bad tube or something have gone wrong with the amp. Well, maybe more knowledgeable people will fill in here. Anyway, my current amps for the past 21 years are auto bias, still I opted for bias meter so every time I turn them on I would take a quick look just to be sure.

Buddy

[TKonrad.NOLA]

Hey!

   Wait a minute... you've just checked the bias of your amp since you bought the amp last year? You are lucky that those tubes are good and your amp is robust. It is  a good good idea to check the bias when you turn on the amp for the first time.

   After the first check you should check it every 3-6 months. Turn it on let it warm up for 10-15 mins before checking the bias and then check again an 30 mins to an hour later to confirm they are correct and don't drift too much after the first check, readjust them as needed.

   Tubes bias drift overtime as it age. But shouldn't be a wild swing, that could indicate a bad tube or something have gone wrong with the amp. Well, maybe more knowledgeable people will fill in here. Anyway, my current amps for the past 21 years are auto bias, still I opted for bias meter so every time I turn them on I would take a quick look just to be sure.

Buddy

Yeah they said it comes from the factory with the bias set and I just went with that.  I was meaning to buy a meter and finally got around to it.

[Tyson]

First question - is there a bias pot that lets you set the bias?  If so, can you use that pot to re-set the bias for this tube?  Sometimes tubes drift out of bias over time & you just need to adjust the bias pot to get them back in line.

If that doesn't work, then swap your tubes around & see if the odd readings follow the tube.  If so, then it's a bad tube.

[SET Man]

Yeah they said it comes from the factory with the bias set and I just went with that.  I was meaning to buy a meter and finally got around to it.

Hey!

    They all do but it is worth to recheck it after shipping and etc. Also, another factor is your line voltage level can have effect on the bias also, depending on how well the amp was designed.Take a reading of your AC line voltage before measure, make a note of it and check it again the next time you bias your amp. I used to live in Manhattan NYC in a big apartment building and I remembered the line would fluctuate a bit sometime peak at 127V!

    Anyway, this are things that we have to deal with using tubes. Can't believe that the whole world used to run on tubes for decades! I got into tube for the first time back in 1997/8 and now with all tubed system, well except the DAC.

     By the way, just checked out your amp on Cary's website. Man! 100wpc of tube power! Love the look of those KT150 tubes, reminded me of the Gherkin building in London.

Buddy

     

[jjss49]

power tubes need more bias current as they age, as the tubes become less efficient in their amplification task - if a tube amp is biased correctly with new tubes installed and adjusted for the first 50-100 hours, one usually needs to gently and slowly increase bias current in extended use...

[TKonrad.NOLA]

First question - is there a bias pot that lets you set the bias?  If so, can you use that pot to re-set the bias for this tube?  Sometimes tubes drift out of bias over time & you just need to adjust the bias pot to get them back in line.

If that doesn't work, then swap your tubes around & see if the odd readings follow the tube.  If so, then it's a bad tube.

Yes there are bias pots.  I adjusted both back to factory recommendations.  Was just curious as to why it had drifted so high.

[FullRangeMan]

Yes there are bias pots.  I adjusted both back to factory recommendations.  Was just curious as to why it had drifted so high.

All valves vary bias some less others more, one of that least varies bias is the wonderful GM70, it is said it run an full year with rock solid bias, one of that most varies is the 6C33 made in the 1980s and 90s due it has two cathodes, however the 6C33 1960s and 1970s are stable, mainly the Svetlana.

With use the tube heats and cools and the resulting fluctuation of voltage and current during use makes the bias vary or even fluctuate mainly when the valve is put into use cold, without heating and also when the amp has no soft start and no voltage delay.

Here there is some stuff :
https://www.thegearpage.net/board/index.php?threads/bias-drift-question.2124914/

https://www.diyaudio.com/forums/tubes-valves/154436-bias-drift.html

https://www.diyaudio.com/forums/tubes-valves/345502-tubes-bias-change-age.html