Is it a Bad Tube or a Bad fuse (Fuse Fatigue)?

My D. Sachs Kootenay 120 started having distortion on the right channel. It sounded like static. So I switched out #1 (right side) with #4 (left side). I did the same with # 2 & 3. Then when I turned on the amp, the right channel was now gone. All tubes are lit up but no sound in right channel. The amp has fuses on each power tube. Sure enough, after checking the socket I find #1 fuse is blown. 

Here’s my dilemma. Which tube is bad? #4, now in #1 slot? Or # 1, now in #4 slot. Don says not to use that tube because it blew the fuse. But with the static in right channel prior to changing the tubes around, I’m skeptical and wonder what if the culprit was the fuse on #1 socket? So I look up the idea of a weak fuse. Do fuses become weakened over time and found that is the case. Its called fuse fatigue. Everything else wears out over time so why not a fuse

So this begs the question. Are the KT88 Gold Lion tubes still good and was the main culprit fuse fatigue?  These fuses are subminiature 250 Mv. The tubes are only 1 -2 yrs old and usually get weekend workouts. So I’m skeptical. But I don’t have a tester and don’t want to use my amp as a tube tester. And I’m told that a tester may not pick up the bad tube? So what are my options other than spend the $200 for a new pair? Thanks for the help


Fuses are designed to be good or to blow. an in-between situation that you call "fatigue" is very rare. My guess is the fuses are fine.

Pick one location and put the old tubes into that location one at a time until you identify the bad tube.

The others will be fine so long as your auto bias can bias them. I much prefer manual bias. Manual bias with a meter is a type of tube tester. I can see which tubes are degrading.

@carlsbad2 Thanks. Your take on fuses are the same as Don’s. They work or they blow. Thus he believes the tube is bad. But which one since the original tube had static in the same channel & same socket? I believe in fuse fatigue. But it may be rare as you say. A search on fuse fatigue brings up a lot of articles so its not just my belief. But I apparently need to read more about it. I'd like to know if this is rare or somewhat common. Of course that still doesn't solve the question

fuse fatigue - Search (

@jond Thanks. I have talked to Don via email. He doesn’t buy into fuse fatigue which is the crux of my thinking that its possible that it was a bad fuse & not a bad tube. I suppose it is safer to buy a pair of KT88’s than repair a power supply


Fuse fatigue as a reason for a fuse to blow may exist  but until it blows it won't make static.  that's my thoughts.

Best of luck,


@carlsbad2  Fuse fatigue as a reason for a fuse to blow may exist  but until it blows it won't make static

That makes sense. I originally thought that one of the tube pins was dirty, which is why I swapped the tube placement. My thinking was that If the other tube had static, then I'd know it was in the amp and not the tube. I was surprised when there was no signal. I didn't expect that. This aroused my curiosity. That tube had been playing fine in the other channel. Then the tube that had the static was now playing fine as well. This makes me think it was not a tube failure but possibly a weakened fuse or some other gremlin in the amp.