Blowing Fuses. Dennis Had Inspire 300B SET

@atmasphere I've made a pretty good career mysefl, unscrewing the things that OEMs have screwed up. This spring I spent 2 weeks in rural North Carolina helping a vendor figure out how they screwed up a multimillion dollar order.   If I hadn't been there to show them their mistake, they would have sent me $1M worth of identically screwed up product.  

It just makes me shake my head when people post the opinion that the OEM not only knows everything but is the only person that can know about their product.  It is impossible that a lowly private citizen can understand anything about the equipment.

In this case, my observation was that the fuse is at the low end of what is normally used and it is possible that the 2A is blowing because it is on the edge.  If it is indeed a fault, then the 3A will blow just the same.  I did suggest other things to try first.   3A is only 360 watts, nothing is going to blow up.  

You might say that I am increasing the chances of blowing an "expensive" component like an output transformer.  That transformer is probably $200 and I'd much rather replace it than ship the amp back and forth across the country twice.  I would have been reluctant to make this suggstion on an amy using hand wound Japanese transformers.

Absolute statements like "never use a higher rated fuse that is in there" always ring false to a thinking man.  Now "generally that won't solve the problem and could cause something to break" would have been less adversarial.

Finally, I told him that that was what I would do, I let him make his own decision.

We haven't heard badk from the OP.  replacing tubes may have solved the problem.  I would expect it to.  And that's what I suggested first.

Jerry

@carlsbad2 I probably do see this from a different perspective. I’ve worked on so many different pieces over the years and I’ve seen attempts to install higher rated fuses. About 1/2 the time it ended in tears.

If you put a 3A fuse were a 2A fuse was indicated, there will be 50% more current allowed. If all that current is feeding a short in a rectifier, you may not have enough time to shut things down before that power transformer gets fried. Some transformers are not nearly as durable as some others!

Now you might argue that a transformer so easily damaged is substandard or the like and I won’t disagree. All I’m saying is apparently this amp has been made for a while so there are multiple examples out there that seem to run fine on a 2Amp fuse, just like this one did until whatever came along and knocked it out.

So, as a thinking man, the idea of putting in a higher rated fuse rings false to me. Actually, it rings scary. I’ve seen poorly engineered power transformers catch fire; its not pretty and they were able to do that with a properly rated fuse. I don’t know how well engineered this power transformer is, but why go where angels fear to tread?

@atmasphere I don’t think we’re going to agree here. I’ll just say that your thought that the fuse is always properly sized to prevent a "poorly engineered power transformer" from blowing is an example of what I meant by overly optomistic. You think they screwed up the power transformer but properly sized the fuse?

I’ve reviewed calculations for setting $100,000 breakers and the number of judgement calls to program a very sophisticated piece of equipment would probably shock you. Of cours good engineers make good judgement calls. So I’ll stick with my assessment that here is Kentucky windage in choosing a fuse. I have a much smaller amp on my stand right now that uses a 3 amp fuse. It’s total power when running is 37 watts.

You seem so passionate about this that I would guess your fuses are carefully sized and you know what components are limiting. At least much more than most. But have you looked at every failure mode? I put a wrong tube in an amp that had two pins shorted (that was the only difference between this tube and the correct tube). No fuses blew but a resistor did. I guess a resistor is a fuse of sorts. and it only took 10 minutes to replace. I’ve seen amps blow, fortunately usually my own, but a couple of times they were mine, especially when I was young. Never have I seen a fuse protect the amp. I’m sure it happens sometimes.

I blew the Class D amp in a subwoofer that I forgot was hooked up when a friend wanted to turn up my system and try to hurt his ears (I guess that must have been his goal?) The fuse blew but so many components were smoked on the motherboard that I just replaced the entire thing. Fuse didn’t help at all.

Edit to add, I guess I seem pretty cynical here.  I'm not really.  I think  the failure rate is very low, but the cause of the few failures we see is what I a cynical about. 

Jerry

@atmasphere I'm headed to the shop or a while.   Hope I didn't say anything you thought was offensive and thanks for the discussion.

Jerry

I’ll just say that your thought that the fuse is always properly sized to prevent a "poorly engineered power transformer" from blowing is an example of what I meant by overly optomistic. You think they screwed up the power transformer but properly sized the fuse?

@carlsbad2 No, this isn’t what I think nor did I suggest that.

What I did say is that somehow this and other amps like it seem to work with a 2A fuse. That suggests that since this one did at one time, it should be able to do it again. But I don’t know what its like (although the photo looks OK) and since I don’t want to be on hook for a part that might be underrated or the like, I won’t put in a fuse that is beyond the suggested rating. That isn’t the same thing as saying there are parts that are underrated or poorly engineered.

You’re right that we’re conservative about our fuse ratings. They have served us well in the last 45 years. When a fuse blows in our stuff there’s always a good reason.