I found these two versions of this rectifier mod for ST70's ising a GZ34 or equivalents on DIY threads and thought it would be of interest to those who have been having problems with tube rectifiers. Looks pretty simple and and easy. Both appear to be the same mod.
Van Alstine version:
For your information, we recently posted a small upgrade to our Ultimate 70, Ultravalve, and even stock Dyna St-70 amplifiers. All that is necessary is to install two 1N4007 diodes, one in series with each high voltage power transformer lead to the 5AR4 rectifier tube. The diodes will mount on unused 5AR4 socket pins. The purpose is to prolong the life of the rectifier by reducing the impact of cold hard turn-on AC voltage to the tube.We recommend this for any vacuum tube amp using a 5AR4 rectifier tube.[u]