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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.
jayboard

Showing 1 response by atmasphere

atmasphere's avatar
atmasphere
11,637 posts
 
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!
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