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


Why not put new fuses in and try it again? you don't have a tester so unless your going to change them all you have to try it with new fuses. If it blows the fuse again you know its a tube and will have a good idea which tube. just make sure you use the correct fuse rating/type for the application.  Also look for red plating that could be a good indication other things in the amp are the issue. is the amp self biased? could be a bias resistor failing in that #1 spot. I'm not sure what bias scheme they use. 

good luck. 

Glennewdick, Sorry I didn't make it clear, but I have replaced the fuse and have another set of tubes in place which plays fine. My question is whether the fuse caused the problem in and of itself which means the tubes are good. Or did the tube blow and take out the fuse?

Glennewdick, There was no red plating. Also, The amp is auto biasing. I hate to throw away $200 worth of tubes, especially when they don't have that much time on them. 

Artemus are you able to get the tubes tested? Otherwise that's quite the quandary perhaps sending a note to Don Sachs might give you some clarity?

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.

That’s a pretty nice amplifier to just use a guess what caused a fuse to blow.

These fuses are subminiature 250 Mv.

Is that a typo mistake? Is that suppose to be 250V rated fuse? What amperage rating? How tight is the fit of the fuse in the fuse holder clips?


I would get the tubes tested before ever using them again. Using a Google search this comes up. Any of them close to you?

Kingsport, TN, tube amplifier repair shops

Call around. First check for review ratings of their repair services.


@jea48 Yes, typo. here's the fuse 250V

0034.6008 SCHURTER Inc. | Circuit Protection | DigiKey

I would get the tubes tested before ever using them again. 

I agree. I don't want to use my amp for a tube tester. I was just fishing to see if anyone had had a similar circumstance w/ bad fuse rather than tube. The way this happened is a curious event to me. The fuses fit fairy tight. They just slide in and the amp isn't that old.

Thanks much for the link. I know the hifi shops don't have testers. But I can't believe I never thought about music shops having testers and techs. Duh. And I am a musician!!! I feel kinda stupid. I do have a friend with a TV7 tester. But he is really busy with work but said I could borrow it. I was getting ready to look up the instructions on how to use the tester but would feel better with someone experienced with a tester. I'll check the links those out


Post removed