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The story on output level and saturation in a SE transformer

Posted by KevinC (M) on November 18, 2004 at 06:20:00

In Reply to: hm... posted by kasra on November 18, 2004 at 03:23:06:

This is a question that is often asked: I want to use an LL1660 transformer that is gapped for 18mA with a tube that I am running at 25mA, so can I do it? The answer is as you might expect: That depends! It depends on the output voltage that you ask to get from the transformer. In line stage use where, normally, you are only asking for 0.5-3vrms, then the answer is "Yes!" This is because the 18mA bias spec puts the flux in the transformer at about one-half of saturation flux and at that point this transformer is capable of up to about 50vrms output. You have offset the core to a ~40% higher flux point at 25mA, but you are only asking the transformer for about 5% of its capability at 18mA, so this offset doesn't force the transformer to the edge of saturation with your signal needs. If, however, you were insisting on the whole 50vrms, you would be out of luck at 25mA, because the added DC current in the transformer ate up a lot of that audio signal capability. It's a tradeoff between accommodating DC current flow and AC signal flow in the transformer.

Kevin Carter
K&K Audio
www.kandkaudio.com

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