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RE: Why do my NOS power tubes take so much to bias compared w/ new production?

Posted by blazter (A) on April 29, 2007 at 15:16:19

I've been using a Cary amp for years and have a nice selection of NOS power tubes. I recently picked up some newer monoblocs that use EL-34 tubes & have one of those fancy bias meters built right into the amp. Well, when I stuck my EL-34 Amperex Double-D Getter tubes in there the amp ran out of juice before it would reach the reccommended bias level, I mean I had turned the bias knob all the way and still the Amperex didn't get as high as they should in one of the amps. I stuck the new production SED EL-34's in there and they biased at about HALF the turn required to bias the NOS Amperex tubes? I recall in my Cary amp I really had to crank that bias knob to get these to bias as opposed to my Tung-Sol 6550's but they always went up high enough eventually. All the Amperex test out just fine and I hve about 3 quads of these saved up over the years and all of them are the same as far as how you have to crank the bias knob to get them where they are supposed to be.

Any reason why this EL-34 Amperex model takes so much more juice to reach the same numbers on the bias meter as new production El-34 or even my Tung-Sol 6550?

Is there anything I can do to the new amp to get a little more juice out of the Bias knob so I can correctly bias the Amperex tubes?

I love these Double-D Getter tubes and they get "close" to the reccommended bias level but not quite there on one of the amps and just reach the level needed on the other monobloc?

HELP|!!!!!!!!!!!!

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