**Lab 03** Oscillator and Maybe a Flip Flop # Overview For this lab we'll try to build an early form of vacuum tube oscillator. This will be a short lab since getting the bistable form of the multivibrator to work with tubes at this low of a voltage has proven to be a very difficult task. I will add some explanation about why this is when I get some additional time, but for now, we just have this single circuit to build. We're still using the same 12AT7 as last time, so its datasheet is included below: * The [12AT7 High-Mu Double Triode](./resources/12AT7.pdf) Lecture 6 discusses a bit of the history and very loose theory of oscillators, but check out the links on the front page for more details and more legitimate discussion of multivibrators. All types of multivibrators are essentially two identical circuits simultaneously driving and being driven by each other...sort of a snake eating its own tail kind of a thing. Depending on the nonlinearity of your active devices as well as the gain and built-in "delay" between the two stages you can get either an astable format, a bistable format or a monostable format. In this lab we'll try to get the astable format. # The Abraham-Bloch Astable Multivibrator The circuit we'll build came about first in history. It is the Abraham-Bloch Astable Multivibrator. Two Triodes are placed in an inherently unstable form (due to the capacitors), and when adjusted just right, will result in a periodic oscillation. The circuit is not that complicated and can be built with one 12AT7 (same as last lab) and a handful of other components. I added potentiometers for all resistors so we have a lot of stuff to mess with. In general, when tuning the pots in this lab, remember this circuit should be *about* symmetric. In other words, the plate bias pots should be about the same value...same with the grid potentiometers. Deviations can of course be introduced, but the goal is to get this thing oscillating and then tweak pots one at a time to see how the signal amplitude and frequency evolve. ![Figure [Oscillator]: An Abraham-Bloch Multivibrator](./resources/abraham_bloch.png) The parts of the circuit are as follows (because of its symmetry, we can double-up our discussions for most parts!): * **The 12AT7 Dual Triode** is our amplification device with a touch of non-linearity. This allows us to drive one side high while the other side is low and vice-versa. * **Resistors $R_1$ and $R_3$** are the plate bias resistors, just like in the simple audio amplifier in lab 02, these establish the load lines of the two tubes. These should be roughly the same in value/orientation/tuning. * **Resistors $R_2$ and $R_4$** are potentiometers set up in a voltage-divider format with the tap going to the grid of each tube. They perform hybrid duty of tuning the grid leak to each tube's grid as well as modulate the overlal grid bias and (dis)charge time of the two capacitors. These should likely be kept close together in value for the system to oscillate. * **Capacitors $C_1$ and $C_2$** These two components add the "temporal disconnect" between the output of one stage and its impact on the other (and vice versa). Because of the feedback process, this therefore allows the circuit to shift between two states, neither of which it is stable in. * **Resistor $R_5$** is a shared cathode resistor. If you want (optional) you can put a 10 microFarad capacitor in parallel, though for this circuit it likely isn't needed. The value of this can be pretty low (though feel free to experiment). I believe it can even be trimmed to zero and that wlll work largely fine. Build the circuit. Use the oscilloscope to monitor the two plate voltages on either side. With some twiddling you should be able to get some oscillation. Once you've done that, massage the other pots to see what your range of oscillation frequency and amplitude is. When you're happy with that, consider hooking the device's output (taken from either of the plates to the input of your solid state amplifier in lab 01. Now you have a beautiful instrument. The five potentiometers you have here actually provide a huge space of variation for different waveform shapes. None of them sound "good", but they certainly can sound different as you vary things. # The Eccles-Jordan Bistable Multivibrator I really wanted us to build a flip flop using a very similar topology shown below (in fact the only real change is the removal of the two capacitors), but whereas oscillation can be obtained with relatively little non-linearity thanks to the explicit capacitors we added into the circuit, the ability for this circuit to sit in one of two mutually interchangeable states needs a real solid non-linearity to be experienced. In essence we need the output of one stage to effectively turn off the other tube, but when operating at such low voltages, the tubes just don't have the nice linear kinks in them that we could get if we were running at higher voltages. ![Figure [flipflop]: An Jordan-Eccles Multivibrator](./resources/jordan_eccles.png)