If you have ever wondered why one amplifier sounds glass-clean on sideband and another turns your voice into a wall of hash, the answer usually comes down to bias class. The whole AM vs SSB amplifier class C vs class AB question is really one question in disguise: does your mode carry its information in a constant carrier, or in a varying envelope? Match the bias class to the mode and the amp behaves. Get it wrong and you are spraying trash across the channels either side of you — and everyone on frequency knows it.
This is bench-level stuff every linear builder should have straight before winding a single toroid. Let's walk it the way you'd explain it to the guy next to you.
What bias class actually means
Bias class is just a fancy way of saying "how much of each RF cycle does the device conduct?" That figure is the conduction angle, and it sets the whole personality of the stage.
- Class A — conducts the full 360 degrees. Most linear, worst efficiency. You rarely see it in a big RF final because it runs hot for the power out.
- Class B — conducts roughly 180 degrees, one half of the cycle. Efficient, but crossover distortion right at the handoff makes pure Class B rare in practice.
- Class AB — sits between B and A. The device idles with a small quiescent current so it is already turned on when drive arrives, which smooths out that crossover region. This is the linear-amplifier workhorse.
- Class C — conducts less than 180 degrees, often much less. The device is biased below cutoff and only kicks on at the peaks of drive. Brutally efficient, and completely non-linear.
There is the trade in one paragraph: the smaller the conduction angle, the higher the efficiency, and the less faithfully the stage reproduces the input. Efficiency and linearity pull against each other, and every mode you run has an opinion about which one it needs.
Why SSB has to have Class AB
Single sideband is a linear mode, full stop. When you talk on SSB, all the intelligence — every bit of your voice — lives in a constantly varying envelope. There is no steady carrier holding station; the RF amplitude rises and falls with your speech. An amplifier feeding that signal has one job: make the envelope bigger without changing its shape.
That is exactly what Class AB is built to do. Because the finals idle with a set quiescent current, they stay in their linear region as drive swings up and down, so the amplified envelope is a faithful, scaled-up copy of what went in. Run SSB through a properly biased AB linear and it comes out the far end clean.
Where Class C fits: the carrier
Class C shines when the signal amplitude is constant, because then non-linearity does not cost you any information. A keyed carrier — think CW, dits and dahs of full-amplitude RF — or an FM signal never changes its envelope, so you can bias the finals hard below cutoff, let them slam on only at the peaks, and pocket that fat efficiency number. Nothing about the mode cares that the device is distorting, because there is no envelope to distort.
This is why builders reach for Class C on carrier-dominant work. It is the efficient choice when you are amplifying a steady tone rather than a voice envelope. The catch is the one every honest builder has to say out loud: voice AM is not a pure constant carrier. AM does have a fixed carrier, but modulating it puts a varying envelope on top — which is precisely why plenty of clean AM operators bias for linearity too, and why the "just run it in Class C" reputation of cheap CB amps is also the reputation for splatter.
AM vs SSB, Class C vs Class AB: the builder's read
So the honest summary of the AM vs SSB amplifier class C vs class AB decision looks like this. Pure keyed carrier or FM? Class C's efficiency is fair game. Anything with a voice envelope you care about — SSB for certain, and the modulation on AM — wants the linearity of Class AB. When in doubt, linear is the safe default; you give up some efficiency and gain a signal that stays inside your channel.
What happens when you run the wrong class
Feed a linear-mode signal into a non-linear amplifier and the physics bites back. A stage that cannot follow the envelope clips and reshapes it, and that distortion shows up as intermodulation products — new frequencies the amp manufactured that were never in your signal. On the air that is splatter: wideband hash smeared across the adjacent channels, a signal that sounds gritty and "wide," and neighbors keying up to tell you about it.
It is not a volume problem you can turn down. It is a linearity problem baked into the bias. The fix is not less drive alone — it is running the mode in a class that can actually reproduce it.
How selectable-bias amps solve it
Here is the clever part, and it is why a lot of serious builds have a mode switch on the front panel. You do not have to pick one class forever. A selectable-bias design changes the operating point of the same finals depending on what you are running.
- Flip to the linear position and the bias circuit feeds the finals their set idle current, parking them in Class AB for clean SSB and modulated AM.
- Flip to the efficient position and the bias drops the devices toward cutoff, moving them into Class-C-style operation for a keyed carrier where efficiency is the priority.
Mechanically it is usually a relay or switch steering the bias network — sometimes a small bias regulator that gets rerouted or disabled per mode. One amplifier, two personalities, matched to the signal in front of it. That is the whole appeal: you stop compromising and let each mode have the class it wants.
How to think about it as a builder
Start from the mode, not the schematic. Ask what the signal's envelope is doing, and the bias class picks itself: constant amplitude tolerates Class C and rewards you with efficiency; a varying voice envelope demands Class AB or it splatters. Devices follow the same logic — a common CB/ham RF power BJT like the Toshiba 2SC2879, rated in the neighborhood of 100W per device, will be perfectly linear or perfectly filthy depending entirely on how you bias it, not on the part number.
That is the mindset behind the AM-and-SSB builds we document: understand why the class matters, then wire the amp so it does the right thing on every mode you actually operate. Browse the full lineup on our volumes page, dig into the product library, or keep reading the rest of the bench blog for more of the same practical wiring theory.
Build it right — with the full documentation
Want the bias network, mode switching, and layout worked out end to end for both modes? The Gatekeeper Tribute Vol.12 — AM & SSB builds walks the whole thing at the bench, from setting idle current for clean Class AB sideband to running the carrier efficiently. It is the build guide that turns everything above into copper you can actually key up.
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Educational reference only. Always follow your specific device datasheet and build documentation, and verify all values independently. Working on high-voltage and RF equipment carries real risk — if you're unsure, get help from an experienced builder.