Measuring DC Resistance
Calculating speaker impedances is a good starting point (see also links from the Cabinet Wiring Diagrams page for mathematical equations). But to really know what OT tap to use on your tube amp, especially if your speaker or cabinet contains no explicit impedance information, measuring DC resistance (called Re) is important to determine the best OT tap to use.
The OT taps on Sound City amps represent the three typical nominal impedance values: 4, 8, and 16 ohms. You will need the following items:
- For a single speaker, all you need is a digital volt meter (DVM) or digital multi-meter (DMM).
- For a speaker cabinet, all you need is a bare 1/4-inch phone plug (w/ its barrel removed) and a DVM/DMM.
- For multiple speaker cabinets, all you need is a bare 1/4-inch phone plug (w/ its barrel removed), a DVM/DMM, and enough speaker cables to connect the cabinets together.
To measure Re of...
- ...a speaker, just touch the DVM/DMM leads to the terminals on the speaker, or
- ...a single speaker cabinet, insert the bare phone plug into one of the cabinet's input jacks, set the DVM/DMM for ohms, and touch each DVM lead to each of the two terminals on the 1/4-inch phone plug, or
- ...multiple speaker cabinets, connect the cabinets together using speaker cables then insert the bare phone plug into one of the cabinets' unused input jacks, set the DVM/DMM for ohms, and touch each DVM lead to each of the two terminals on the 1/4-inch phone plug.
An Re measurement of...
- ...more than 2 but less than 4 (usually around 3.3) = 4 ohms nominal; therefore, set your OT tap for 4 ohms.
- ...more than 4 but less than 8 (usually around 5 to 6) = 8 ohms nominal; therefore, set your OT tap for 8 ohms.
- ...approximately 13 to 14 = 16 ohms nominal; therefore, set your OT tap for 16 ohms.
Rules of Thumb
- Always set the OT tap to the next highest nominal impedance value; e.g., if you measure Re of 3.3 ohms, set the OT tap to 4 ohms, etc.
- Avoid connecting two or more cabinets with different nominal impedances to the same tube amp, which typically provides OT taps for 4, 8, and 16 ohms.
I hope this helps.