6.Experiments

12AX7 dual triode tube

Filament 1

(view from bottom)

You will notice on the ampli¯er schematic that both triode elements inside the 12AX7's glass envelope are being used, in parallel: plate connected to plate, grid connected to grid, and cathode connected to cathode. This is done to maximize power output from the tube, but it is not necessary for demonstrating basic operation. You may use just one of the triodes, for simplicity, if you wish.

The 0.1 ¹F capacitor shown on the schematic "couples" the audio signal source (radio, musical keyboard, etc.) to the tube's grid(s), allowing AC to pass but blocking DC. The 100 k- resistor ensures that the average DC voltage between grid and cathode is zero, and cannot "°oat" to some high level. Typically, bias circuits are used to keep the grid slightly negative with respect to ground, but for this purpose a bias circuit would introduce more complexity than it's worth.

When I tested my ampli¯er circuit, I used the output of a radio receiver, and later the output of a compact disk (CD) player, as the audio signal source. Using a "mono"-to-"phono" connector extension cord plugged into the headphone jack of the receiver/CD player, and alligator clip jumper wires connecting the "mono" tip of the cord to the input terminals of the tube ampli¯er, I was able to easily send the ampli¯er audio signals of varying amplitude to test its performance over a wide range of conditions:

Amplifier circuit

Radio

. . .

Audio

input

. . .

phones

"Phono" plug

"Mono" headphone plug

A transformer is essential at the output of the ampli¯er circuit for "matching" the impedances of vacuum tube and speaker. Since the vacuum tube is a high-voltage, low-current device, and most speakers are low-voltage, high-current devices, the mismatch between them would result in very audio low power output if they were directly connected. To successfully match the high-voltage, low-current source to the low-voltage, high current load, we must use a step-down transformer.

Since the vacuum tube circuit's Thevenin resistance ranges in the tens of thousands of ohms, and the speaker only has about 8 ohms impedance, we will need a transformer with an impedance ratio of about 10,000:1. Since the impedance ratio of a transformer is the square of its turns ratio (or voltage ratio), we're looking for a transformer with a turns ratio of about 100:1. A typical automotive ignition coil has approximately this turns ratio, and it is also rated for extremely high voltage on the high-voltage winding, making it well suited for this application.

The only bad aspect of using an ignition coil is that it provides no electrical isolation between primary and secondary windings, since the device is actually an autotransformer, with each winding sharing a common terminal at one end. This means that the speaker wires will be at a high DC voltage with respect to circuit ground. So long as we know this, and avoid touching those wires during operation, there will be no problem. Ideally, though, the transformer would provide complete isolation as well as impedance matching, and the speaker wires would be perfectly safe to touch during use.

Remember, make all connections in the circuit with the power turned o®! After checking connections visually and with an ohmmeter to ensure that the circuit is built as per the schematic diagram, apply power to the ¯laments of the tube and wait about 30 seconds for it to reach operating temperature. The both ¯laments should emit a soft, orange glow, visible from both the top and bottom views of the tube.

Turn the volume control of your radio/CD player/musical keyboard signal source to minimum, then turn on the plate supply switch. The voltmeter you have connected between the power supply's B+ output terminal and "ground" should register full voltage (about 170 volts). Now, increase the

value.

You may be pleasantly surprised at the quality and depth of tone from this little ampli¯er circuit, especially given its low power output: less than 1 watt of audio power. Of course, the circuit is quite crude and sacri¯ces quality for simplicity and parts availability, but it serves to demonstrate the basic principle of vacuum tube ampli¯cation. Advanced hobbyists and students may wish to experiment with biasing networks, negative feedback, di®erent output transformers, di®erent power supply voltages, and even di®erent tubes, to obtain more power and/or better sound quality.

Here is a photo of a very similar ampli¯er circuit, built by the husband-and-wife team of Terry and Cheryl Goetz, illustrating what can be done when care and craftsmanship are applied to a project like this.

6.2Voltage comparator

PARTS AND MATERIALS

²Operational ampli¯er, model 1458 or 353 recommended (Radio Shack catalog # 276-038 and 900-6298, respectively)

²Three 6 volt batteries

²Two 10 k- potentiometers, linear taper (Radio Shack catalog # 271-1715)

²One light-emitting diode (Radio Shack catalog # 276-026 or equivalent)

²One 330 - resistor

²One 470 - resistor

This experiment only requires a single operational ampli¯er. The model 1458 and 353 are both "dual" op-amp units, with two complete ampli¯er circuits housed in the same 8-pin DIP package. I recommend that you purchase and use "dual" op-amps over "single" op-amps even if a project only requires one, because they are more versatile (the same op-amp unit can function in projects requiring only one ampli¯er as well as in projects requiring two). In the interest of purchasing and stocking the least number of components for your home laboratory, this makes sense.

CROSS-REFERENCES

Lessons In Electric Circuits, Volume 3, chapter 8: "Operational Ampli¯ers"

LEARNING OBJECTIVES

² How to use an op-amp as a comparator

SCHEMATIC DIAGRAM

ILLUSTRATION

-

+

INSTRUCTIONS

A comparator circuit compares two voltage signals and determines which one is greater. The result of this comparison is indicated by the output voltage: if the op-amp's output is saturated in the positive direction, the noninverting input (+) is a greater, or more positive, voltage than the inverting input (-), all voltages measured with respect to ground. If the op-amp's voltage is near the negative supply voltage (in this case, 0 volts, or ground potential), it means the inverting input (-) has a greater voltage applied to it than the noninverting input (+).

This behavior is much easier understood by experimenting with a comparator circuit than it is by reading someone's verbal description of it. In this experiment, two potentiometers supply variable voltages to be compared by the op-amp. The output status of the op-amp is indicated visually by the LED. By adjusting the two potentiometers and observing the LED, one can easily comprehend the function of a comparator circuit.

For greater insight into this circuit's operation, you might want to connect a pair of voltmeters to the op-amp input terminals (both voltmeters referenced to ground) so that both input voltages may be numerically compared with each other, these meter indications compared to the LED status:

-

+

Comparator circuits are widely used to compare physical measurements, provided those physical variables can be translated into voltage signals. For instance, if a small generator were attached to an anemometer wheel to produce a voltage proportional to wind speed, that wind speed signal could be compared with a "set-point" voltage and compared by an op-amp to drive a high wind speed alarm:

6 V

LED lights up when wind speed exceeds "set-point" limit established by the potentiometer position.

6.3Precision voltage follower

PARTS AND MATERIALS

²Operational ampli¯er, model 1458 or 353 recommended (Radio Shack catalog # 276-038 and 900-6298, respectively)

²Three 6 volt batteries

²One 10 k- potentiometer, linear taper (Radio Shack catalog # 271-1715)

CROSS-REFERENCES

Lessons In Electric Circuits, Volume 3, chapter 8: "Operational Ampli¯ers"

LEARNING OBJECTIVES

²How to use an op-amp as a voltage follower

²Purpose of negative feedback

²Troubleshooting strategy

SCHEMATIC DIAGRAM

ILLUSTRATION

-

+

INSTRUCTIONS

In the previous op-amp experiment, the ampli¯er was used in "open-loop" mode; that is, without any feedback from output to input. As such, the full voltage gain of the operational ampli¯er was available, resulting in the output voltage saturating for virtually any amount of di®erential voltage applied between the two input terminals. This is good if we desire comparator operation, but if we want the op-amp to behave as a true ampli¯er, we need it to exhibit a manageable voltage gain.

Since we do not have the luxury of disassembling the integrated circuitry of the op-amp and changing resistor values to give a lesser voltage gain, we are limited to external connections and componentry. Actually, this is not a disadvantage as one might think, because the combination of extremely high open-loop voltage gain coupled with feedback allows us to use the op-amp for a much wider variety of purposes, much easier than if we were to exercise the option of modifying its internal circuitry.

If we connect the output of an op-amp to its inverting (-) input, the output voltage will seek whatever level is necessary to balance the inverting input's voltage with that applied to the noninverting (+) input. If this feedback connection is direct, as in a straight piece of wire, the output voltage will precisely "follow" the noninverting input's voltage. Unlike the voltage follower circuit made from a single transistor (see chapter 5: Discrete Semiconductor Circuits), which approximated the input voltage to within several tenths of a volt, this voltage follower circuit will output a voltage accurate to within mere microvolts of the input voltage!

Measure the input voltage of this circuit with a voltmeter connected between the op-amp's noninverting (+) input terminal and circuit ground (the negative side of the power supply), and the output voltage between the op-amp's output terminal and circuit ground. Watch the op-amp's output voltage follow the input voltage as you adjust the potentiometer through its range.

You may directly measure the di®erence, or error, between output and input voltages by connecting the voltmeter between the op-amp's two input terminals. Throughout most of the poten-