Tube amp troubleshooting question

Hi, ‘Goners,

I am using an AVA Super 70i, Van Alstine’s makeover of the Dynaco ST70. It was factory-rebuilt in January 2001, and I bought it used (or used-used, as it were). I have had the 5AR4 rectifier tube go bad on me twice, both times with only 7-10 months of moderate use. The first time, the fuse started blowing at startup. Replacing the rectifier with a new Sovtek 5AR4 fixed that. Seven months later, the power supply transformer developed a healthy and annoying buzz, and the output tubes seemed to be sagging, requiring significant rebiasing. I replaced the transformer and tried out fresh output tubes, both without effect, before thinking of the rectifier tube. Once again, putting in a new rectifier returned the amp to normal.

I’m wondering if I should suspect any deeper problem that might be causing the rectifier tube failure, or if this sounds to be within the normal range of 5ar4 behavior, perhaps mixed with a little bad luck. The tube is inexpensive, so the inconvenience and cost are not significant to me. But I’m curious as to whether I should be wise to any other likely problems.

I don’t know much about electronics, but I can recognize what part is what, use a multimeter, solder competently, and stay out of trouble. I have a schematic and other diagrams. As I mentioned, the audio circuitry is quite young; also, I checked the connections I made when installing the new transformer, and they look good. Thanks for any tips.