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.
Hi Jay:
It sure doesn't sound like the "normal range of behavior" to me. Sounds like stuff that should drive you CRAZY. I'm impressed you're so calm.

Sounds like you have a current problem. That would explain the fuse and the tube. You have a current problem perhaps because there isn't enough resistance or a short throwing to much current where it shouldn't be and it's right in the first stage or PS of the amp. Rather than listening to my rambling I suggest you go to someone who knows Dynas inside out. Joe Cursio. His site is here- Joe's but he posts at this forum all of the time
AsylumDIYtubeforum.If you put ST70 in the title of your post (maybe you have to do it 2-3 times but probably not) and ask for help Joe is likely to pipe in. He knows every wire in the thing on a first name basis and he is a nice guy.

Hi, Clue, thanks for your reply. No, the problem hasn't driven me crazy yet. I did, at the misguided advice of a local tech, unnecessarily purchase a new transformer, but the day I spent putting it in was very relaxing and got me more in touch with my amp, a good thing. I actually found a thread at the Asylum site on short-lived rectifiers in an ST70 that Mr. Curcio had weighed in on. It was inconclusive but a good point for me tag on to. I also downloaded an ST70 repair guide from Curcio's site that I hadn't seen before -- another good starting point. So, I think I'll be able to get to the bottom of this. Best regards.
You have one of three problems, or a combination thereof.

(assuming that the output tubes are properly biased)

Either the filter caps are bad, or the value of them is too high, causing excessive current to exist in the rectifier tube, especially during turn-on. Thus the tube wears out prematurely.

Another problem that has cropped up in this twilight of the tube era is the rectifier itself. There have been a number of Sovtek 5AR4s that have shown up in the last few years or so that really don't meet 5AR4 specs- they were really closer to 5Y3s. If you happen to have installed one of these, you can expect trouble.

The newer 5AR4s may have this problem corrected. If I were you I would compare whatever new 5AR4 you get to an old American one and see if they look similar. Its not gonna work if the wrong part is in the circuit!

Good Luck!
Ralph, thanks for addt'l input. I hope I get some free time soon to do some diagnosis.