6. Audio Signal Output transformers Output transformers for tubes: These are step down transformers, because a tube is a device with medium impedance and medium gain, but we want a low voltage output signal with low impedance. Note, you step down the impedance with the square of the step down ratio. Such a transformer beautifully gets out of tube technology what is inside. The output impedance really gets very low, when you use a tube with good gain, and medium impedance. Such medium impedance tubes have often very good linearity. As an example, use for instance the NOS 7N7 it is low cost, and easy to use. It even has a metal shielded base, which you can ground to the amplifier chassis. If you parallel the two systems in one tube, gain will stay the same. DC current will double from 9 to 18mA , and output impedance goes down from 7770 to 3850 Ohms. Drive a 9:1 step down with this tube, will give an effective gain of 2x and an output impedance of 20x20 lower, so you get an amazingly low 9.6 Ohms at the output. Such a pre amp can drive any amplifier, virtually hum free, be it an 600 Ohms or a high impedance (47k) type. Interconnection cables can be simply twisted, like with silver wire, self made. Many other examples are possible of course, and you will see, there is a Lundahl for almost every tube. Select the tube for good linearity, because you will have some reasonable output signal, peak to peak. Such tubes have medium gain, like from 10...40. Do not tale more gain than you need, because lower gain tubes have lower plate impedance, and that is always helpful to get the best out of a transformer. When you found one, you will soon see, many tube can be used beautifully. Indeed this is an ideal application for signal transformers.
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