Acoustic Vacuum

Woo Audio WA33 Gen 2 · Standard tube rolling reference

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What fits

Nine sockets, three families. The socket decides; a tube that is not on this sheet does not go in the amp.

Stock tube brands and the Gen 2 changes are being confirmed from Woo Audio's own pages by the research run; until then this sheet shows the family facts only.

Combinations

Every 2A3 against every rectifier. Each cell is the blended profile; warm cells lean lush, cool cells lean lean. Click a cell to load it into the roll panel.

Bars, left to right: warmth · bass · air · detail · speed · dynamics · stage · background. Hover or focus a cell to read it.

Hover a cell.

Profiles are expectations blended from construction physics and tallied forum and review descriptors, weighted power 55 %, rectifier 30 %, driver 15 %. They are not measurements. Evidence counts rise as the research run verifies claims.

Bench

The engineering behind the lore. Drag the values; the curves are computed from datasheet models, not drawn.

Signal path

One loop, source to headphones. Amber dots are the hot phase, steel dots the cold phase; inside each tube, electrons leave the filament and land on the plate. Watch the two 2A3 plate currents: both stay above zero for the whole cycle. That is class A. The transformer takes their difference, which is what push-pull means. Hover, tap or tab to a stage to read what it does. Representative topology, not Woo's schematic.

Swipe the figure sideways to follow the path.

AC
wavy wire, dots surge with the signal
pulsating DC
after the rectifier, before the filter
DC
flat wire, steady dots
electrons
pale dots, filament → plate, gated by the grid

Plate curves and load line

2A3 Koren model (μ 4.2). The hyperbola is the 15 W plate limit; the line is the load each tube sees. Push the bias or the load and watch the swing.

Driver stage

The 6C45 in front of each 2A3. A 2A3 has a μ of only 4.2, so it needs about ±45 V on its grid for full output; a high-transconductance driver makes that swing from about a volt at its own grid, and its low plate resistance keeps the drive stiff. Move the supply, the bias and the plate resistor, and watch the gain, the swing and the plate limit.

Model: Child–Langmuir three-halves law fitted to the KB's μ and gm for the 6C45.

Heaters and warm-up

Nine tubes, two ways to heat a cathode. The 2A3s and the rectifier are directly heated: the filament is the cathode, so it emits within seconds. The 6C45s are indirectly heated: a heater warms a separate cathode sleeve, which takes tens of seconds. B+ arrives with the rectifier, before the drivers are awake.

    Datasheet: every filament and heater rating (KB), the amp's total draw (Woo's spec), the EML 5U4G-Mesh slow start (EML, via the KB) and the 5 to 10 minutes to operating temperature (Woo's quick-start guide). Typical, not measured: the other warm-up times and the shape of the B+ curve, including its small rise while the drivers are still cold.

    Rectifier sag

    A rectifier is a resistor that changes with current. Each curve's drop at rated current comes from the KB; below that it follows the three-halves law, and past the rating it turns dotted. More drop, more sag, softer dynamics. The EML 5U4G-Mesh is one current 5U4G; the Takatsuki TA-274B, which Woo lists for the WA33, is one 274B. Volts are at the filter input, before the choke.

      Output transformer

      Woo publishes no transformer data: no turns ratio, no primary impedance, no inductances. Every value on this card is an assumption you can move. Turns ratio squared sets the reflected load; primary inductance sets the bass corner; leakage inductance sets the treble corner. The one related figure Woo gives is the whole amp's response, −2 dB at 10 Hz and 60 kHz, marked ◇ on the plot. Change the headphones and the load line on the 2A3 curves above moves.

      Inside the bottle

      Hover the parts. Switch the plate to see what "mesh" actually means.

      Electrons boil off the heated filament, pass the grid, and land on the plate. The grid voltage gates the flow; that is the whole amplifier.

      Market

      Reputable dealers that ship to the USA. Prices are as fetched on the date shown, in the dealer's currency with a USD estimate at ECB reference rates.

      Order

      Library

      The reference entry for each family: datasheet numbers, makers, NOS, warnings, and what owners say. The full entries live on their own page so this one stays a scroll, not a book.

      Open the full library — every datasheet number, maker, NOS variant, warning, and verified owner claim, with sources.