The pedal uses op-amps (often TL072 or similar) to boost the input signal before it hits the tube. This "pushes" the tube into saturation more effectively.
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
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 pedal uses op-amps (often TL072 or similar)