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In the Forum: Audio Discussions
In the Thread: SET amplifiers
Post Subject: It means exactly what it means.Posted by Romy the Cat on: 12/17/2014
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 noviygera wrote:
 What does this mean? Let's say the OT is 5K:8 where is this loading coefficient coming from? And how can it be adjusted? If the R(e) of a driver is let's say 12ohm, does this mean the secondary of the OT must be 12ohms for optimal loading to the given driver? 
 
I do not undesired what you ask. It means exactly what it means. If you have OT wish has a certain transformer ratio that you can change the transformer ratio and it will affect loading. A tube that you use has more or less constraint known output impedance. Well, it wary with the way how you power the tube plate but let presume that the amp is finalized and the operation point is fixed. The very small fluctuation of the output impedance with plate voltage is negligible at this point.  The impedance of you driver is knows, would it be 4, 8 16 R or whatever. Let say your tube is 150R and you decided to use 10X transformer. So, your driver of let say 16R will "see" tube of 150R x 10 = 1500R. If you change the impedance of your driver voice coil or ration of your transformer then the "loading" of your tube will be change as well. Do not forget that as much as driver "see" the tube via the prism of transformer in the same way the tube "see" the driver via the transformer.  What is the optimal loading of a given driver and a given tube? It is hard to say, you can measure distortion but it would not be too illustrating in some cases. Generally you use tube that everyone know and look what other people use. Thai is good place to start only and you will move from there. Some companies do easy re-mapable transformers section and you can do with them anything you want. 

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