Bias is the single setting that decides whether your linear runs clean and lives a long life, or distorts, splatters across the band, and cooks its finals on the first hard key-up. It's also the setting most often skipped or guessed at. This is the calm, repeatable way to do it — whether you're running RF MOSFETs, bipolar power transistors, or glass.

The goal of Class-AB bias is simple: set just enough idle ("quiescent") current so each device is already slightly turned on before any RF drive arrives. That eliminates the crossover distortion you'd get from a cold Class-B stage, without wasting the power and heat of a true Class-A design. Get it right and your SSB sounds clean and your IMD stays low. Get it wrong in either direction and you've got a problem.

Safety first. Tube amplifiers hold lethal plate voltages (hundreds to thousands of volts) on the filter caps even after power-off — bleed and verify with a meter before touching anything. Solid-state amps won't shock you, but a mis-set bias pot can dump tens of amps and destroy a $40 transistor in under a second. Read your build's safety section before you start.

What "bias" actually controls

Every amplifying device needs a small standing current to operate in its linear region. In a transistor that means setting the gate (MOSFET) or base (BJT) voltage so a defined drain/collector current flows with no signal. In a tube it means setting the grid bias so a defined plate (or cathode) current flows at idle. That standing current is your quiescent current, or Iq.

  • Too little Iq (cold): the device shuts fully off between RF half-cycles → crossover distortion, raspy audio, and splatter onto adjacent channels.
  • Too much Iq (hot): wasted dissipation, excess heat, and on bipolar devices the real risk of thermal runaway — current rises with temperature, which raises temperature, which raises current.

What you need on the bench

  • A current meter in the main DC supply line (or the amp's built-in meter) reading the total idle draw.
  • A good DMM for bias voltage.
  • A dummy load on the output — never set bias into an antenna.
  • A way to remove RF drive (key with no audio / no carrier) so you're measuring idle only.
  • The device datasheet, and your build's recommended Iq target.

Setting bias on a solid-state amp (MOSFET / BJT)

  1. Start cold. Turn the bias pot fully to minimum (lowest Iq) before powering up. This is the step people skip and regret.
  2. Power up with no drive. Apply normal supply voltage, no RF in. Watch the idle current — it should read near zero at minimum bias.
  3. Bring up Iq slowly. Advance the bias pot until the idle current reaches the target for your design. As a general rule of thumb, RF power devices in an SSB linear are biased to a modest standing current per device — tens to a few hundred milliamps depending on the part — but always use the figure your build or the device datasheet specifies. Sneak up on it; don't spin the pot.
  4. Let it stabilize. Iq will drift up as the heatsink warms. Wait a few minutes and re-trim back to target at operating temperature. This is the difference between a stable amp and one that creeps hot.
  5. Verify under drive, then re-check idle. Run a short test transmission into the dummy load, then return to idle and confirm Iq held its setting.
Push-pull pairs: if your amp uses matched devices in push-pull (common on 2SC2879 and MRF-class CB/ham linears), the two halves should idle at similar current. A big imbalance points to a mismatched device or a bias network fault — fix that before chasing power.

Setting bias on a tube amp

The idea is identical — set idle plate current — but the method depends on the bias scheme:

  • Fixed bias: a negative grid voltage sets idle plate current; adjust the bias supply pot to the tube's recommended resting plate current.
  • Cathode bias: a cathode resistor self-sets the operating point; you change it by changing the resistor value, not a pot.

Always set tube idle current to the manufacturer's published figure for your plate voltage. Over-biasing a sweep tube or a 3-500Z to chase a few extra watts just shortens its life and dirties your signal.

Thermal runaway watch: on bipolar transistors, if idle current climbs on its own after you set it, shut down. The bias network needs proper temperature compensation (a tracking diode or thermistor on the heatsink). A MOSFET is more forgiving but still wants a stable reference.

Quick sanity checklist

  • Bias pot started at minimum? ✓
  • Idle current set to the spec'd Iq at operating temperature? ✓
  • Push-pull halves balanced? ✓
  • Set into a dummy load, not an antenna? ✓
  • Re-checked after a warm-up and a test transmission? ✓

Do those five things and your linear will run clean, stay legal-sounding on the band, and last. Skip them and you're rolling dice with your finals.

Run the numbers first. Use the free Ohm's Law & Power calculator to size your bias network resistors and check device dissipation before you power anything up.

Want the full bias setup for real builds?

Every Hood's Finds Linear Amp Build volume includes the exact bias procedure, target Iq, and a dedicated bias tab for that specific design — plus full SVG schematics and interactive calculators. 90 complete builds in the Master Bundle.

Get the free Builder's Quick Reference

Bias cheatsheet, transistor substitution tables, and the 10 mistakes that kill amplifiers. Free — no spam.

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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