Blowing Fuses. Dennis Had Inspire 300B SET

@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.

Even on billion $ hardware a adequate FEMA… sometimes isn’t….

Where i always ran into trouble was when really smart people had to much ego to admit they didn’t know everything….

I saw that enough, i made sure i had some non advocate reviews….and a personal coach to whack me when i needed it….

But let’s not kid ourselves, this particular amp ( 99.9% ) are built to a price point…

  I am happy to report that Dennis Had got back to me and told me after asking what type of rectifier tubes I had, to buy from Tube Depot, and to purchase a JAN Phillips Military rectifier…. also that I needed a slow blow 2 amp fuse, and if this does not work that he would take a look at it for me.

I want to say that you are all very helpful and I want to thank you for all chiming in on this one! Merry Christmas