Thanks PGPG, h21e is something i had not heard of. As for my original post i have taken another look at the schematic and i had assumed Q18 was simply the forth transistor of the output stage, but of course its not its part of the drive for Q19-Q21, silly me. That changes things as the Darlington tip122 should be able to drive the bases of the 6 x tip35c sufficiently as i would want each of them to supply a maximum of 3amp.