How to Use an Oscilloscope With Tube Amps

Each square represents one unit of measurement.

Tube amplifiers produce a high-power AC electrical signal. To analyze the output waveform, frequency, and voltage characteristics at the tube amplifier output, a special device called an oscilloscope is needed.

An oscilloscope displays a graphical representation of an AC electrical signal. From this representation, voltage can be calculated by measuring the waveform amplitude. Frequency can be calculated by measuring the waveform period, and dividing by the time measurement scale selected on the oscilloscope.

Things You'll Need

• Tube amplifier
• Amplifier output cable
• Amplifier input cable
• Amplifier input source (such as a CD player or radio)
• Oscilloscope with clamp probes

Instructions

1. Plug one end of the input cable into the input source. Plug the other end into the amplifier input receptacle. Plug one end of the output cable into the amplifier output receptacle. Turn the amplifier volume to its minimum setting.

2. Turn on the oscilloscope. Place the clamp probe on the grounding ring below the oscilloscope display. Set the horizontal control until the video line is aligned with the oscilloscope display horizon. Remove the probe from the grounding ring, and attach the probe to the amplifier output cable tip.

3. Turn on the amplifier, and allow the amplifier to warm up for a few seconds. Turn on the input device, and slowly turn up the amplifier volume to approximately 10% volume.

4. Adjust the oscilloscope voltage (or "Volts/Division") scale until you can see the entire voltage waveform. Adjust the oscilloscope period (or "Time/Division") scale until you can isolate one individual wave cycle.

Tips & Warnings

• Count how many squares between the lowest and highest points in the waveform; this is the "peak-to-peak" voltage value. Count how many squares between where the wave crosses the horizon going up and where it crosses going down, and multiply by two; this is the "period" value. Divide 1 by the period value to get the frequency in Hertz.

References

• Class 'A' Exposed and Explained; Randall Smith; Mesa Engineering; 2005
• Ohio State University: How to Use an Oscilloscope