A question about biasing new tubes

[johnnytheg]

Hi all, I just replaced seven year old Svetlana EL34 tubes (one of the tubes shorted) with Mullard re-issues. When I hooked up the new tubes and let the amp warm up (twenty minutes) I hooked up my multi-meter. The previous older tubes had the bias set to 1.56 (dynaco specifications) but when I took the readings off the new tubes the bias read 1.30 on both the left and right side. I would imagine the bias would be much higher in the new tubes as they would require less bias than the older tubes. Any idea why the new tubes would read so low? I adjusted the bias to be at 1.56 with the new tubes, but I am just curious why this might be.

 

[peterh]

Hi all, I just replaced seven year old Svetlana EL34 tubes (one of the tubes shorted) with Mullard re-issues. When I hooked up the new tubes and let the amp warm up (twenty minutes) I hooked up my multi-meter. The previous older tubes had the bias set to 1.56 (dynaco specifications) but when I took the readings off the new tubes the bias read 1.30 on both the left and right side. I would imagine the bias would be much higher in the new tubes as they would require less bias than the older tubes. Any idea why the new tubes would read so low? I adjusted the bias to be at 1.56 with the new tubes, but I am just curious why this might be.

Tubes have a large spread in characteristics, +-25% is not unusual. That's why bias needs adjustment !

 

[johnnytheg]

Ok good, thanks for that. I just wanted to make sure everything was normal.