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Lately I've been working on a Vacuum Tube Wien Bridge Oscillator, and I made it with a cathode follower as the output tube to provide a low output impedance. It worked and everything is fine, but every time i load it, the wave gets clipped a little and you need to readjust the negative feedback. So i was looking through the original Hewlett Packard Audio Oscillator schematic (The HP 200A) and i noticed it has an output transformer. This stunned me. From what i know audio transformers don't usually have a wide frequency response, sometimes even less than the audible range, and the 200A Oscillator was designed to produce frequencies from 30Hz to 35Khz. Even the best audio transformers sacrifice either low end frequency response for high end frequency response or vice versa. Then I noticed a very intricate feedback from the output to the cathode of the previous tube via another coil on the primary of the transformer. I decided to play around with it here in EC and this is what I got. A flat frequency response from 10Hz to 200Khz with a -3dB attenuation at the corner frequency. The way i achieved it is through the 5:20 transformer which supresses the peaking of higher frequencies, the 42pF capacitor, which acts as a bootstrap at higher frequencies, thus broadening the response a little, and the 17H coil in the negative feedback path, to boost the low end response (it passes only low frequency signals to the capacitor, which as we all know in this configuration increases the gain) . 17H may seem a lot, but in reality it probably can be somewhat miniaturized, because the current rating is less than 5mA, so you can get the thinnest wire you can find and it would still be a hundred times thicker then what is necessary. Moreover i think you can use a gyrator to emulate the choke. The waveform is ugly at frequencies lower than 30Hz because the current draw increases and the tube enters cut off, but is perfect at anything above.
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