Superconducting tapes with high-purity substituted Tl(1223) powders (> 95 wt.%, Tc ∼120 K) were produced by a new method based on an in-situ reaction under high pressure of helium (25 bar He/0.5 bar O2). A Ag(20 wt.% Au) alloy showed to be a good sheath material for an in-tape reaction up to 990°C, even when involving partial melting of the oxide which favors the grain growth. Due to an interaction between Pd and the oxide, a Ag(6 wt.% Pd) alloy is much less suitable for tape preparation. Reproducible transport critical current densities of 9 kA/cm2 (77 K, 0 T) were obtained for Ag(Au) sheathed Tl(1223) tapes with in-tape-reacted powder, the highest value being 10 kA/cm2. The magnetic-field dependence of jc of such tapes was improved with respect to Ag sheathed tapes with pre-reacted Tl(1223) powders. Locally textured samples with up to 50 μm large plate-like grains were obtained after partial melting in Ag(Au) sheathed tapes.