Class A vs. Class AB

This describes how a tube pair shares the workload. In Class A, each tube handles the full signal alone, so it needs enough idle headroom to never cut off — safe idle percentages run hot (roughly 70–90% of max dissipation). In Class AB, the pair trades off: as one tube's current rises on a signal peak, the other's falls, so the pair's combined heating is naturally lower than either tube running Class A alone — safe idle percentages run cooler (roughly 50–70%).

Cathode bias is a different axis entirely

It's about how the bias voltage gets set, not how the pair shares load. A cathode resistor is self-regulating: as current rises, the voltage it develops rises too, gently pulling the tube back down. That negative feedback is why cathode-biased amps tolerate running much closer to a tube's full rated dissipation (into the 90s%) without the runaway risk a fixed-bias amp would have at the same percentage — which is why this calculator gives Cathode Bias its own gauge-zone option above rather than folding it into Class A or AB.

Fixed bias vs. cathode bias, circuit-wise

Fixed bias uses a separate negative supply run to the grid, with the cathode near ground. Nothing self-corrects — if it drifts hot, it stays hot until someone checks it, which is why fixed-bias amps need periodic verification and usually have a trim pot. Cathode bias sets itself via the cathode resistor; there's normally no trim pot, and the cathode-resistor mode above is diagnostic — confirming what the resistor's already set, not something you dial in.

70% isn't sacred

These percentages are a convention, not a datasheet limit — the real safe point depends on your actual measured voltage, the tube's true dissipation rating (which varies by maker and drifts with age), and chassis ventilation. Red-plating — the anode visibly glowing — means you're already well past safe, not a line to creep up to and back off from. And once you're in a sane range, the "right" bias is partly a tone call, not just a number — it's worth listening to the amp as you adjust, not just chasing a target percentage.

Measurement methods, safest to riskiest

• Existing cathode resistor — safest and simplest, but only applies if the amp actually has one.
• The output transformer's own winding resistance — non-invasive, no need to open anything up, but relies on knowing that winding's exact DC resistance, which isn't usually published precisely and varies between individual transformers.
• One half of a center-tapped push-pull primary — more accurate than a full-primary or center-tap reading for push-pull amps specifically, since each half carries only one tube's DC current on its own. Still relies on knowing that half's own resistance — measure it, don't guess it.
• A dedicated shunt or an inline octal probe — more accurate than relying on the OT's own winding, since a purpose-built shunt or probe has a known, precise value, but installing one usually means opening the chassis.
• Breaking a live connection to insert a meter directly in series — the most direct reading, and the one to avoid unless you're experienced: it puts a meter probe into the middle of a live high-voltage path.

Read the full breakdown, with worked examples and a measurement-method comparison table →

The step-by-step procedure and calculator above get you a number. This section is about what that number actually means, and why the "safe" target moves depending on the amp in front of you.

Class A vs. Class AB — a worked example

Class A vs. Class AB describes how a tube pair shares the workload, not how the bias voltage gets set. In Class A, a tube handles the entire signal cycle by itself, so it needs enough idle current in reserve to swing through a full cycle without ever cutting off — that headroom requirement is why Class A safe-bias targets run hot, typically 70–90% of the tube's max rated dissipation. In Class AB, the pair splits the work: as one tube's current rises on a signal peak, the other's falls, so the pair's combined heating stays lower than either tube running Class A alone would produce. That's why Class AB tolerates a cooler idle point, typically 50–70%, without introducing audible crossover distortion — the headroom is coming from the other half of the pair, not from the idle bias itself.

As a worked example: take a tube rated for 25W max plate dissipation, run at 400V plate-to-cathode. A 70% target works out to about 43.75mA (25W × 0.70 ÷ 400V). That same 70% figure lands in very different places depending on which class it's read against: for Class A it's still comfortably cool, nowhere near the 90%+ range where things get hot. For Class AB it's already sitting right at the edge of the safe range — the same percentage number, the same milliamp reading, but a different amount of headroom left depending on which class the amp is actually running.

Cathode bias vs. fixed bias

This is a separate question from Class A/AB — it's about how the negative grid bias voltage is produced, not how the pair shares load.

Fixed bias runs a dedicated negative supply to the grid, with the cathode sitting near ground. The bias voltage is set once (via a trim pot) and stays put — nothing in the circuit corrects it if it drifts. That's exactly why fixed-bias amps need periodic rechecking: tubes age, the bias supply can drift, and nothing will warn you except rising heat.

Cathode bias generates its own bias voltage from the tube's own current: current flows through a cathode resistor, the voltage it develops raises the cathode above ground, and that offset is the bias voltage. It's a self-correcting loop — if current starts to climb, the cathode voltage climbs with it, pulling the tube back down. There's usually no trim pot because there's nothing to trim; the resistor already set it. That self-regulation is also why cathode-biased amps tolerate running much closer to a tube's full rated dissipation than a fixed-bias amp safely could at the same percentage.

Measurement methods compared

Five ways to get a current reading, in order of safety and invasiveness:

Red-plating and why 70% isn't a hard ceiling

Common bias percentages (like 70% for Class AB fixed bias) are a convention, not a datasheet limit stamped on the tube. The actual safe point for a given tube on a given day depends on several things that convention can't account for: the true measured plate voltage, the tube's real dissipation rating (which varies between manufacturers and drifts as the tube ages), and how well the chassis ventilates that heat away. Red-plating — the anode visibly glowing, usually a dull orange-red in a dark room — is a sign the tube is already well past a safe operating point, not a boundary to creep up to and then back away from. By the time color is visible, damage is already underway. Once you're in a reasonable range, the last adjustment is often more about tone than a target number — it's worth playing through the amp as you dial it in rather than treating any single percentage as gospel.

Dual triodes and other edge cases

Dual-triode preamp tubes like the 12AX7 family are in this calculator's tube list too, already rated per section rather than for the whole envelope — a 12AX7 shows around 1.0–1.2W max dissipation per triode, matching current manufacturer datasheets. A triode has no screen grid, so there's no screen-current split to worry about; the plate current is simply the current measured. The two triodes in one envelope are independent — each has its own plate and cathode pins, so bias and measure them one at a time rather than trying to average or combine a reading across both.

Open this section on its own page →