So in my insatiable curiosity about electron tubes(I've been struggling to find a comprehensive textbook that discusses them-any title suggestions???)I came across the Barkhausen-Kurz Oscillator. So my questions is as follows: 1. What is the equation(based on applied voltage and current) for the oscillator output frequency? 2. I presume that this is a sinusoidal oscillator. Are there any electron tube relaxation oscillators that can be driven by a DC input? If not, how would a single component relaxation oscillator tube be constructed?