Choosing a Power Supply for a CB Linear Amplifier: Size the Amps, Not Just the Volts

Ask ten guys what makes a good power supply for a CB linear amplifier and nine of them will tell you the voltage. "It's a 13.8-volt amp, so I grab a 13.8-volt supply." True — but that's the easy half. The half that melts ring terminals, drops your output the second you key up, and makes a clean amp sound dirty on the air is current. Amps, not volts, are where most solid-state linear builds live or die.

This is the part people skip because the amp gets all the glory. But the supply is not an accessory you bolt on afterward — it is a component of the build, every bit as much as the transistors and the transformer. Treat it that way from the start and half your "why won't this thing hit like it should" problems never happen.

Why the power supply for a CB linear amplifier is part of the build

A solid-state RF stage is a current-hungry animal, and it wants that current in short, hard bursts every time you key down. The supply's job is to deliver those bursts without the voltage caving in. If it can't, the whole downstream chain suffers: lower output, more distortion, and a hot supply that shortens its own life.

Think of it like a bench project with three legs — the RF board, the cooling, and the supply. Pull any leg and the stool falls over. A brilliant amp starved by a marginal supply performs like a marginal amp. So we size the supply to the load the amp actually pulls, not to the sticker on the amp's case.

Current headroom matters more than people think

Here's the trap. A supply rated "40 amps" might mean 40 amps continuous, or it might mean 40 amps peak/surge with a much lower continuous number. Those are very different animals. SSB and CW key-down is closer to a continuous draw than most folks assume, and it's exactly when you need the supply at its best.

So the honest question isn't "does the supply's number match my amp's number?" It's "can this supply hold rated voltage at my amp's real draw, continuously, without sagging or shutting down?" That's why headroom is your friend. Sizing the current capability comfortably above your worst-case draw is the single most useful decision you'll make about your power supply for a CB linear amplifier.

Estimating the draw from your target output

You don't have to guess. You can put a design-target number on it with a little math, then verify on the bench. The chain is simple:

1. Start with your target RF output. Use your build's rated figure — don't borrow a number off someone else's amp.
2. Divide by efficiency to get DC input power. A solid-state RF stage commonly lands somewhere in the rough range of 45–65% efficiency as a design ballpark, but real efficiency depends on your class of operation, drive, tuning, and devices. Measure yours when you can, and treat any single number as a starting estimate, not gospel.
3. Divide DC input power by your supply voltage to get current. At a nominal 13.8V, that current figure is what your supply has to deliver.

Worked as an illustration only: suppose a build targets in the neighborhood of 300W PEP and you assume roughly 55% efficiency as a design target. DC input works out to about 300 ÷ 0.55 ≈ 545W. At 13.8V that's about 545 ÷ 13.8 ≈ 40A. That's an estimate to size around — your real draw depends on your supply voltage, drive level, and how the amp is tuned, so always confirm against your build's spec and your own meter. Now add headroom on top of that estimate rather than buying a supply that just barely meets it.

Regulation and sag: what happens the instant you key down

A supply's rated voltage means nothing if it can't hold that voltage under load. This is regulation, and it's where cheap or undersized supplies embarrass themselves. Watch the voltage at the amp's input terminals while you key up. If 13.8V collapses to 12-something the moment you transmit, the amp is being cheated out of the input power it was designed around, and output drops with it.

Some sag is physics — there are no perfect supplies and no zero-resistance wires. The goal is to keep it small and stable. A stiff supply with real current headroom, short and fat DC leads, and solid connections will hold voltage far better than a marginal supply on thin wire. Sag isn't only the supply's fault; it's the whole delivery path, which is why wiring gets its own section below.

Wiring and fusing — the delivery path is half the job

You can buy the perfect supply and still choke it with the wiring. Forty-plus amps is a lot of current, and it punishes undersized wire, corroded lugs, and long runs with voltage drop and heat.

1. Keep the DC run short and use heavy conductor. Voltage drop scales with current, length, and wire resistance. For a high-current amp, err heavier than you think you need and keep the supply close to the amp.
2. Make clean, tight, low-resistance connections. Proper crimped or soldered lugs on clean metal. A loose or corroded terminal is a hot spot and a voltage-drop source hiding in plain sight.
3. Fuse the DC line close to the source. A fuse protects the wiring from a short becoming a fire. Size it to protect the wire above your real operating current, and never delete it to "get more headroom."

Bring it back to the build

The supply isn't the boring box in the corner — it's what lets the amp actually be the amp. Size it for the current your build really pulls, give it headroom, watch the voltage hold under key-down, and treat the wiring and fuse as part of the design instead of an afterthought. Do that and the amp finally hits the way the build sheet says it should.

If you're planning a build from scratch, our build volumes walk the whole chain, and the free calculators take the guesswork out of the numbers. Get the supply right first, and everything downstream gets easier.

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.

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