New PI throws bias way off

[ClassicRock88]

Got some new Svetlanas for a 100 watt. Biased them at 32.5 ma. Burned them in for 30 min and rebiased at 32.5. Still hearing volume warbles and intermittent crackling so I replace the phase inverter. Play amp for 45 minutes and it sounds GREAT. Bring it back to idle (channel volumes and MV on 0) and check bias again. 55 ma. Adjust down. I now have it as low as I can set te bias at 38 ma.

Was the sudden current draw jump from further burn in or the new PI tube? Is the amp safe running at 38? I'm amazed that the Svetlanas lasted at 55 at all. Good job, Russians.

[Marshall Mann]

Hey Classic Rock,

I'm sorry, I'm not familiar with what am it is you are working on?

What is your plate voltage reading?

As a general answer, you can run tubes outside of there norm for short periods of time without doing any long term damage (ie at 55ma). Although I don't recommend it, I have a set of Sovetec 6L6's in my Madison that I have been running at near 100% dissipation for more than a year.

[ClassicRock88]

Marshall Mann,

It's my Super Bass clone. She runs at roughly 480VDC.

[Marshall Mann]

Marshall Mann,

It's my Super Bass clone. She runs at roughly 480VDC.

Hey classic Rock,

With 480 vdc on the plates I show 38ma at 70% dissipation (480vdc divided by 23 watts max dissipation, at 70% is 33.5 ma + 5 ma for the screens = 38.5ma) and 70% (38ma) is a very safe bias point.

[ClassicRock88]

Mann,

Thank ya sir! Much appreciated! May I ask where you get the 5 ma for the screens? That's a new one on me. I have always calculated it MPD/VDC * 70% is max safe bias.