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K&K Audio / Lundahl Transformers Welcome! Need support, you got it. Or share youe ideas and experiences. |
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K&K Audio / Lundahl Transformers Welcome! Need support, you got it. Or share youe ideas and experiences. |
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In Reply to: Driver Impedence and Interstage Questions posted by Horn-a-copia on August 3, 2003 at 21:58:29:
Lots of good questions... Let me see if I can do justice to them!Basically, the interstage transformer (IT) transforms the voltage as the turns ratio and the impedance as the square of the turns ratio. Let's set up an example. Let's imagine a SE driver with an Rp of 1.2K driving a triode output tube through an IT with a turns ratio of 1:2. At the simple level, for every volt of signal developed at the plate of the driver, 2 volts appear at the grid of the output tube. However, the grid of the output tube "sees" a source impedance of 1.2K x (2 squared) or 4.8K. The result for a step down connection is just the reverse: 1 volt becomes 0.5 volt and the reflected impedance is 300 ohms. Big difference!
With transmitting tubes like the 845 there are two challenges. The first is an adequately low Rp driver to ensure that the grid capacitance can be driven up to an adequately high frequency, and the second is that the driver can provide enough voltage to the grid to get the full power out of an 845, which requires a pretty massive grid swing. Unfortunately, while a step down IT solves the first problem, it aggravates the second. Your idea of a P-P driver, while heresy to the SE devotee, is not such a dumb idea, because it largely cancels even order harmonics in the driver stage, while potentially providing a lower frequency cut-off than a paralleled SE driver stage with the same tube. However, it doesn't buy you lower drive impedance than a paralleled driver.
My advice is to choose a low Rp tube with a high power supply voltage maximum, so that you can swing the required grid voltage and have a low driving impedance, which takes us right back to the short list of tubes in your second paragraph.
Kevin Carter
K&K Audio
www.kandkaudio.com
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Follow Ups
- Re: Driver Impedence and Interstage Questions - KevinC 07:01:21 08/04/03 (2)
- Re: Driver Impedence and Interstage Questions - Horn-a-copia 07:40:33 08/04/03 (1)
- Re: Driver Impedence and Interstage Questions - KevinC 07:33:55 08/05/03 (0)
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