For the DIY builders and the spec-heads, the 2x56 setup usually follows a specific architectural philosophy: 2.5 Volts (AC or DC). Amplification Factor (µ): Roughly 13.8. Plate Voltage: Typically run between 180V and 250V.

High-quality, matched pairs of vintage 56 mesh plate tubes are increasingly rare and expensive.

It features a distinct "shoulder" glass shape (S-12 or ST-12).

These tubes are notoriously "microphonic." If you tap the chassis of the amplifier—or even walk too heavily across a hardwood floor—you might hear a "ping" through your speakers.