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Tube Oscillation Circuit Viewed On an Oscilloscope

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Published on Oct 25, 2009

The scope is connected with each channel reading separate ends of the phase inverter tube just past the coupling caps. One cathode (or emitter/source from the perspective of transistors) of the oscillator tube is connected to the cathode of the phase inverter at the junction of the PI's bias resistor and bypass capacitor. I say one cathode because with this particular circuit it takes an entire "twin triode" (two complete tubes in one valve) to accomplish the entire 360 degrees of phase inversion necessary for oscillation. The resistive/capacitive network provides 180 degrees and the tube itself provided the other 180 degrees. Eventually the output of the entire oscillator circuit is coupled to the anode (collector/drain) of the same PI. So in this sense the oscillator acts as a feedback loop, but not necessarily to the PI tube. "Feedback" is a technique used in amplification to either create signal gain or attenuation, and is accomplished by taking a portion of the output signal from an amplifier and rejoining with the input. It could easily be seen how feeding the output back into the input could create signal gain, but creating a signal loss isn't so apparent. Generally tubes and transistors amplify signals which when after amplification are compared to the originals end up being 180 degrees out of phase. Meaning what once were positive voltages are now negative and vise-versa (audible signal voltages consist of AC current which alternates from positive to negative polarity at various frequencies). Any frequency will cancel out that same frequency if it is of an opposing polarity. This is termed "negative feedback". Coupling two frequencies of the same polarity is termed positive feedback. In order for a tube to amplify voltage you need to have a positively charged anode, a neutral cathode and a negatively charged grid. The cathode is heated internally to a very high temperature and because it is coated with a material that holds a very high potential to conduct it then emits a large amount of electrons. These electrons form a cloud around the cathode and when agitated by the alternations of current in the grid when a signal is applied are then pulled towards the anode. It is important to note that the grid is at a negative voltage in it's quiescent state so any voltage applied to it causes it to swing either more negative or less negative, but still negative. They are attracted to the anode as the anode is at a positive polarity. Seeing as the total sum of electrons being attracted to the plate from the grid and the cathode is much greater than the voltage applied to the grid, amplification (or gain) takes place.

Now, the oscillator's own signal is fed to it's grid (the grid of a tube serves the same function as the base of a transistor or the gate of a JFET, though more like a JEFT as they are almost identical to a tube in how they work), and so long as the voltage gain of the tube is of a sufficient magnitude the signal being fed back to the grid increases and continues to increase with every cycle forcing the grid to swing closer and closer to positive voltage. Eventually this happens and is called "forward grid current". Once this happens, electrons are more attracted to the cathode which is more or less at 0 voltage as it is connected to ground, and the ability of the tube to amplify voltage gain plummets. Since forward grid current is brought about by positive feedback and positive feedback is a product of the gain of the tube eventually grid current stops and the gain of the tube begins to rise again: Oscillation. The frequency of oscillation is determined by the makeup of the feedback circuit and the magnitude of oscillation is determined by whatever portion of the phase inversion network of the oscillator tube isn't fed to ground.

The signal from the output of the oscillator is coupled with the signal exiting the phase inverter tube as it travels to the main output tubes.

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