Tube DIY Asylum

Posted by MaTJ (A) on March 10, 2003 at 16:31:52

In Reply to: Distortion question --- Need expertise !!! posted by Eric Chan on March 10, 2003 at 06:58:27:

Hi Eric,

Check the 300B Western electric datasheets at the WE site. They have nice distortion curves and distortion specified for different operating points.
The transformer distortion is somewhat more difficult. More airgap is good but you'll also need the inductance to match the reflected impedance or else the loadline will become elliptical resulting in more distortion. For pretty straight loadlines I'd recommend a reactance eaqual to the reflected impedance at the desired -3db frequency. So for a 3.5K opt -3db at 20Hz: 3.5k/(20*2pi)=28H. With enough core area to keep the flux levels in the linear area; Basically just get an opt designed for the job...
But this is not the whole story as a reactive speakerload will present a reactive load on the tube anyway. So you also need to consider your amp/speaker combination. And a reasonable amount of H2 in your amp can be used to cancel the H2 of your speakers suggesting the choise of a lower reflected impedance to achieve lower total thd.

Regards, Marijn