Blowing Fuses. Dennis Had Inspire 300B SET

You answered your own question.  Replace the rectifier.  

Power fuses are usually slow blow. 2A sounds reasonalble but 3A would not be unusual. 2A is 240 watts. Your amp will pull more current warming up and may actually pull more than 2A for a slort time, thus the slo blow.

I would try a 3A slo blow but first I would replace all tubes. the cause of fuse blows in tube amps is almost always a bad tube. The first suspect was the rectifier because you saw an anomaly there, but it could have been challenged because of a short in one of the tubes it powers. I always have spare tubes, maybe not a spare set of my premium high quality tubes but a spare set of cheap but known good tubes.

Sizing a fuse by the OEM is not generally the precision engineering that you would think. the signal path is carefully constructed with components chosen carefully to achieve the desired result. but the fuse sizing is often a bit of kentucky windage as the designer balances protection of the component with reliable power to the component. some OEMs will put more margin between operating current and fuse current than others.

Even if you try a 3A fuse and it works, I’d try to figure out what changed and fix it. that is harder with autobias. If, on the other hand, you have a short in a tube or similar fault, the 3A will blow just as fast as the 2A.

I like manual bias, partly because you can look at the tube current for each tube.

good luck

Jerry

@atmasphere Your opinion is noted and considered overly optiomistic at best.  You are a manufacturer and have an overly proud opinion of manufacturers IMHO.  (not that I have anything bad to say about you or Dennis Had). 

erry

@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

@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