| 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.
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