Vox’s recent AV series is an example of hybrid amps that pair 12AX7 preamp tubes with a solid-state power amp.
Hybrid amps combine tube and solid-state technology. After selling Fender to CBS, Leo Fender co-founded Music Man, which made well-regarded amps with solid-state (i.e., tube-less) tone-shaping sections (preamps) and vacuum-tube-driven power sections. Since then, many manufacturers have also experimented with this approach, as well as combining tube-driven preamps with solid-state power amps.
Both for the sake of your tone and the safety of your gear, it’s important to make sure the impedance load of the speakers connected to your amp matches the ohm rating specified on the jack.
Impedance: The resistance (measured in ohms) in an electronic circuit. It plays an important role in every aspect of your tone: Your guitar has an output impedance, and your effects and amps have input, output, and internal impedances. But while you can probably live happily without worrying about the impedance of any of these, there’s one area where it’s essential to be aware of impedances—when connecting your amplifier to speakers. Your amp’s speaker-output impedance should match the impedance of the speaker(s) it’s connected to. Worst-case scenario, an impedance mismatch can cause serious damage to your amp. Here are a few things to keep in mind:
Your amp’s output-transformer impedance needs to match the total impedance of all the speakers connected to it—not simply the ohm rating listed on one of the speaker labels. Anytime there’s more than one speaker, you need to do a few calculations.
To keep things simple, let’s say you have two 8-ohm speakers:
If you connect them in series, the impedances are simply added together. (In a series connection, the amp feeds the positive lead of one speaker, that speaker’s negative lead then feeds the positive lead of the second speaker, and the second speaker’s negative lead feeds the amp). Your total: 16 ohms.
Three common speaker-wiring scenarios for guitarists and bassists: Two speakers in series, four speakers in parallel, and four speakers in series-parallel.
If you connect them in parallel—both speakers getting the amp’s positive and negative leads—the math gets more complex, but it’s easy enough when the speakers have the same impedance: Simply divide the single-speaker impedance rating by the number of speakers. In this example, each of our speakers is 8 ohms, and we’ve got two speakers, so 8 divided by 2 results in a total load of 4 ohms.
What happens if you have four 8-ohm speakers? All four in parallel would be 8/4 = 2 ohms. That’s uncommon but not unheard of. All four in series would be 32 ohms—which is pretty much unheard of. Most 4x12 cabs have their speakers wire in series-parallel. In other words, the four speakers are wired as two sets of two speakers, and within each pair, the two speakers are wired in series—and then each set of speakers is wired in parallel. Let’s break down the math: When each of the 8-ohm pairs is connected in series, you end up with two 16-ohm loads (remember, we add series-wired impedances). Then, when we connect the two 16-ohm pairs in parallel we count each pair as if it were a single speaker, and therefore divide by two: 16/2 = 8 ohms.