Tube drifting down as you play more and more

[redlir]

Hey guys,

Just wondering if anyone has ever seen this or can give me feedback.

I have a new set of Gold Lion KT66's that I'm using in my plexi. I biased them at 36ma just for now while I test them out. I notice that one tube stays at 36ma fine when I take measurements after playing. The other tube seems to drift down further and further the longer I play. To the point where it goes down to 10ma or so.

If I leave everything idle for 15 minutes, it slowly comes all the way back up to 36ma.

So, is it a bad tube? Do I just need to break it in longer or something?

Thanks!

[Ferrari-Dude]

I've had a similar experience with a pair of Tesla Rosnov EL34 NOS tubes but the deviation was not quite as severe as yours.

I fired up my Marshall (Jubilee 2553) and let the tubes warm for about 5 minutes. I biased them at 34.5mA left and 35.5mA right. Over the course of 1/2 hour one of the tubes climbed as high as 39mA and as low as 30.5mA. I'm guessing here but I suspect that as the tubes heat up completely, the internal properties change and possibly some of the side effects of the manufacturing are worked out.

Ultimately, my pair of EL34's sound great and are working fine.

There is probably a burn in period that you need to work through. I would consult some other tube experts though just to be sure.