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Re: LL1660S Anomaly

Posted by KevinC (M) on May 1, 2006 at 05:07:08

In Reply to: LL1660S Anomaly posted by arnoldc on April 30, 2006 at 18:39:31:

The ringing is not an anomaly, per se. It is a result of one or more "resonances" that usually occur at frequencies greater than 20KHz and are the result of interaction between the tube's source impedance, the load impedance and the transformer's stray capacitance and leakage inductance. Resonances like these will occur with almost all interstage transformer/tube combinations. You could "tune" a combination by choosing a tube and operating conditions to suit a particular IT, but this is not a temporally inconsequential exercise.

These resonances can be reduced through one of several means. One of these requires adding a resistive load across the secondary to "swamp" the resonance. Typically this requires a pretty low value of resistance, reduces the gain significantly, and in my experience, "kills" the sound. Another involves empirically determining an RC network across the secondary that will damp the primary resonance somewhat. This can be sonically benign and partly effective. But it raises a point worth thinking about: if the cure is worse than the disease, sonically speaking, then why do it at all? Personally I find that these resonances don't disturb my listening pleasure at all so long as they occur above 20KHz or so.

In your particular case, I suspect that the resonances are made worse by the fact that you are using the transformer to step-up signal levels (effectively 1:1+1). For the best sound, as well as the resonance issue, I find it best to use the IT 2:1+1. Of course, that entails a loss of 6dB of potential gain, but I solve this by using a lower Rp and mu driving tube (6BX7) and direct coupling the 6SN7 to it. This gives more gain and better output tube control at the expense of a bit more complication. But to my ears, the complication is more than justified by the sonic improvement.

Kevin Carter
K&K Audio
www.kandkaudio.com

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