Re-Biasing frequency?

[mercury128]

Hi All

I completed my Mainline build near the end of October and have been enjoying it ever since. Over the past few weeks I started to notice that it was not sounding as good as it did when I first completed it, mainly in terms of soundstage.

I remeasured the voltages today and found that the voltages associated with biasing had dropped to 127 and 129. I was able to readjust them back up to 145 using the trim potentiometers and it is sounding great again.

My question is how often should I be expecting to check and re-bias? Is there anything that causes this to change over time?

Thanks
- Alex

[Paul Joppa]

Most tubes will drift at first, until the cathode is fully formed. After some tens (or occasionally hundreds) of hours, it's said to be adequate to check the bias once a year. Even used tubes need some initial break-in time, if they have been unused for many years.

The variation in time to fully stabilize is large; I've heard from 10 to 200 hours. It seems to be correlated with the longevity of the cathode. As I recall, trace amounts of silicon in the "melt" of the cathode allow faster stabilization but shorten life. Higher purity cathode alloys will allow longer life, but the material is costly and the initial forming of the cathode at the factory takes much longer, so they are more expensive.

[mercury128]

Hi Paul

Thanks for the info. I'll remeasure the bias from time to time to see if it continues to change.

- Alex