Workbench / Output Stage

Output Power Estimator

Max clean watts from B+, load and topology.

Output Stage

Topology

Plate supply B+

Saturation voltage

Pentode knee ≈ 30–70 V, triode higher

Primary Raa (plate-to-plate)

Transformer efficiency

Push-pull output stage

Plate V1Plate V2

Output power

80.5 W

Primary power

89.5 W

limited by voltage swing

Peak plate current

459 mA

per valve

Plate efficiency

68 %

theory max 78.5%

Engineering Notes

How it works

The maximum clean output power of a valve stage is set by how far the plates can swing before the valves saturate (or cut off), and by the load the transformer reflects.

Push-pull class AB

Each valve drives half the primary, which presents Raa/4. At full power each plate can swing from B+ down to the saturation voltage, so the peak swing is Vpk = B+ − Vsat, and the full primary sees twice that:

P = (2·Vpk)² / (2·Raa) = 2·(B+ − Vsat)² / Raa Ipk = 4·Vpk / Raa

Single-ended class A

A single valve must never cut off, so the swing is limited both by voltage (B+ − Vsat) and by current: the plate current can only swing down to zero from Iq, limiting the peak voltage to Iq·Ra:

Vpk = min(B+ − Vsat, Iq·Ra) P = Vpk² / (2·Ra)

Finally, the output transformer loses a few percent in its winding resistance and core. These figures are first-order estimates; real amps lose more to B+ sag and screen-voltage limits.

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Miller & Grid Stopper

Output Transformer Match