Bias Setting.............

[MX117.MC7150]

Have two (2) quads of Tung Sol 7581a tubes, which I find superior to EL34s, KT77s & KT88s in my amp. All sound great and differences are not major.
The first quad of my 7581a has about 1200hr so I ordered another quad to put on the shelf. When I received new tubes I always burn them in for about 25-30 hours "just to make sure"..........
The bias I normally use for the quad is 160mA measured with my Fluke 77. When I rolled in the new set I was very surprised that the bias was about 210mA, so reset to the wanted 160mA. After burn-in I removed the new set and put on the shelf and rolled the old ones back in. And, bias was about 115mA, so had to crank back up to 160mA.
Does this tell me anything? Should I worry? All sounds good. Well, not really. Sounds great. Am very pleased with my system.

 

[GordonW]

Not really abnormal. 6L6 class tubes, such as 7581As, have an acceptable variance range of transconductance and bias current. That's why we have bias adjustments!

It's not abnormal for a new set of tubes to be "hotter" than an old pair- as the cathode wears, the current must be turned up, to keep the same operating point.

Regards,
Gordon.

 

[gadget73]

I've also had tubes shift a fair bit from new out of the box to 20-30 hours of use. They generally stay fairly stable from that point until they start to get really tired.

 

[MX117.MC7150]

The quad with ~1200 hours had needed no adjustment after the initial 25-30 hours. Was surprised by the difference between the two quads.
Have a couple of sets of KT88s that I bias at 200mA and the difference between the two sets is <10mA.

 

[JohnT123]

There is a lot of variation in the production process due to materials and assembly variation. Each will have a different idle current within a range. Retailers buy thousands and group them based on their critical characteristics. Idle current at a fixed voltage is a very common way to sort them. More so than transconductance. Some will sort them further by matching multiple characteristics like idle current AND transconductance. The bathtub curve is in effect. Failure or change happens early in use, rapidly tapers off and then stabilizes. As they reach end of service life failures and changes in performance rise sharply..