The HT-Dual is essentially a two-channel tube preamp in a box. The schematic revolves around these critical stages:
The pedal takes 16V AC and boosts it internally to roughly 300V DC. This "B+" voltage is what allows the 12AX7 (ECC83) tube to operate in its linear region, providing real headroom. Blackstar Ht Dual Pedal Schematic
A high-impedance stage that preserves your guitar's high-end before the gain stages. The HT-Dual is essentially a two-channel tube preamp
A dedicated filtering circuit at the end of the chain that rolls off high frequencies to mimic a 4x12 cabinet. Blackstar Ht Dual Pedal Schematic