Choose how many speakers you have and how they are wired; the diagram redraws to match, and the total impedance the amp sees updates with it. Series adds, parallel divides — and matched speakers always split the power evenly, however you wire them.
Hover, tap, or focus a speaker to read its share.
Two rules cover every guitar- or bass-amp cabinet. They come straight from the way current and voltage divide, and they are worth committing to memory.
Ztotal = Z1 + Z2 + …
One path through all the speakers, so the same current flows through each and the impedances stack. Two 8 Ω speakers in series read 16 Ω; four read 32 Ω.
1 / Ztotal = 1 / Z1 + 1 / Z2 + …
Every speaker sees the full drive voltage on its own path. For matched speakers that is just Z ÷ n — two 8 Ω speakers give 4 Ω, four give 2 Ω.
Two series pairs, wired in parallel
Add within each pair, then combine the pairs in parallel. Four 8 Ω speakers become two 16 Ω strings in parallel — back to 8 Ω, the everyday way to wire a 4×12.
| Wiring | Speakers | Total (each = Z) | With 8 Ω speakers | Power each |
|---|---|---|---|---|
| Single speaker | 1 | Z | 8 Ω | 100% |
| Two, in series | 2 | 2 × Z | 16 Ω | 50% |
| Two, in parallel | 2 | Z ÷ 2 | 4 Ω | 50% |
| Four, in series | 4 | 4 × Z | 32 Ω | 25% |
| Four, in parallel | 4 | Z ÷ 4 | 2 Ω | 25% |
| Four, series-parallel | 4 | Z | 8 Ω | 25% |
A common worry is that wiring speakers in series "starves" them while parallel "drives them harder". With identical speakers, neither is true. In series the same current passes through every driver, so each dissipates the same I²Z. In parallel every driver sees the same voltage, so each dissipates the same V²/Z. Either way, and in series-parallel too, matched speakers split the amplifier's output in equal fractions — half each for two, a quarter each for four. What the wiring changes is the total impedance, and therefore whether that impedance matches the amp; it does not change how the power is shared.
The equal-share rule only holds while the speakers actually match. Mix a 4 Ω and an 8 Ω driver and the current or voltage divides unevenly, one runs hotter than the other, and the tidy fractions break down. In a guitar or bass cabinet the speakers are normally identical for exactly this reason, and every scheme in the tool above assumes matched drivers.
A tube amplifier reaches the speaker through an output transformer, and that transformer only reflects the intended load up to the tubes when the speaker impedance matches the tap it is wired to. Present the wrong impedance and the whole load line shifts — a lighter load than the tap raises the voltage the tubes swing and stresses them; a heavier load lowers the available power. The single-ended tweed Champ-style circuit in this archive winds its output transformer for roughly 5 kΩ : 8 Ω, expecting one 8 Ω speaker; the push-pull tweed Bassman-style circuit uses a transformer fixed at a 2 Ω secondary to feed its four 8 Ω speakers wired in parallel. Both are just the wiring rules above, chosen to land on the transformer the builder specified.
Speaker figures for the two presets are read from the parts lists of the tweed Champ-style and tweed Bassman-style circuits in this archive. For the transformer theory behind an impedance match, see Reuben Lee, Electronic Transformers and Circuits (1955), and this site's transformer reference.