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.
Understanding the schematic is key to maintaining, modding, or cloning this beast of a pedal. ⚡ The Core Architecture
The Blackstar HT-Dual is a modern classic in the world of tube-driven distortion. Unlike "starved plate" designs that run tubes at low voltages for show, the HT-Dual uses a high-voltage internal power supply to get genuine valve compression and harmonic richness.
The 12AX7 requires a specific filament voltage (usually 6.3V or 12.6V). If your pedal isn't lighting up or passing signal, this is the first place to check for a blown component. ⚠️ A Critical Warning The HT-Dual is not a beginner project for DIY repair. Lethal Voltages: The internal 300V rail can be fatal.
Both halves of the tube are utilized. One half typically handles the initial gain stage, while the second provides additional clipping and drive.
A high-impedance stage that preserves your guitar's high-end before the gain stages.
Blackstar’s proprietary "Infinite Shape Feature." On the schematic, this looks like a specialized tone stack that shifts the center frequency and response between "American" (tighter, scooped) and "British" (woodier, mid-forward) voicings. 🛠️ Key Schematic Sections to Study
The HT-Dual is essentially a two-channel tube preamp in a box. The schematic revolves around these critical stages:
