Blowing Fuses. Dennis Had Inspire 300B SET

You answered your own question.  Replace the rectifier.  

Yes. Thanks. I did, and it toasted another one, stopping the others I guess from warming up. It is self-biasing amp. 
 

to be honest, I have two tube amps, and it’s the first time I’ve ever been listening in the middle of a session and for the first time with this Dennis had amplifier, that the amplifier failed or any amplifier turned off so I guess that’s the way it works when that happens. 

I am not sure about the 2amp fuses? Is it right to have a SLOW BLOW fuse as the main fuse? It looks like that is what was present since I bought it used?

If the fuse keeps blowing the problem is not with the fuse. It’s elsewhere.

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

If the rectifier is what initially failed look for a capacitor linked to the rectifier and then check its condition.

DeKay

My friend went thru this, he tried several fuses, tube changes, a repair person, finally back to Maker, came back fixed.

I have a Cary amp from back in the Dennis Had days, and also from back in those days I had a DIY "Power Bank" (a kit from CAD) which consisted of four more power supply capacitors connected to the amp with an umbilical cord.  The amp started blowing the 3A SB AC power fuse on start up (not every start up, but frequently enough that I kept a lot of spares on hand).  I disconnected the Power Bank and now I literally cannot remember the last time I blew that fuse.

With all that typed, I realize that you didn't say that you had your amp connected to a power bank, but, as @dekay  mentioned, your issue could be related to a bad cap, and specifically one of the large power supply caps.

I had a similar issue many years ago with a Music Reference amp. Turned out to be one of the output transformers needed replacing. Not an easy or cheap repair.

I would try a 3A slo blow

@moose89 Please ignore the above advise- never, ever put a fuse higher than the rated fuse in any product unless specifically told by the manufacturer that its OK.

Solid state amps usually have fast blow fuses. Tube amps almost always have slow blow fuses. All the 'zigzag' fuses I've seen are fast blow. I would get a Littlefuse or Bussman slow blow fuse as a replacement so as to know that the fuse you're installing will actually do the job.

If it continues to blow you have other problems; the amp should be checked out by a qualified technician.

@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

@moose89 , I’ll relate another problem I once encountered with my Cary amp and how I fixed it:

I’d turn the amp on and on one side the bias would run away high regardless of where the bias pot was set (not a self-biasing amp). I emailed Cary about it (this was the new Cary, not the Dennis Had Cary), and when they did get back to me, which was none too quick, they basically quoted me their shop rate and advised me to send them the amp. It’s an 80 lb. amp and I could not find the original box so I wasn’t crazy about that idea.

On another audio site I posted a thread (similar to what you have done) and a very patient DIY member who was quite generous with his time got back with me and together we troubleshot my amp and I fixed it.

So what he suggested I do, and what I believe you could do, is put it on a work table and remove the bottom panel (assuming that is where you would gain access). Then with a multimeter, using various settings (ohms for resistors and the capacitor setting for caps) check everything left and right and see if the readings are the same or if they are wildly different.

In my application, one of the signal caps measured way different than the other 3 using the capacitor setting of the mm. The member who was "talking" me through this suggested I buy a cap checker on Amazon that I could verify my buggy reading with the cap in the circuit. I did, and the reading continued to be bad so I bought all four new signal caps and that fixed my amp.

Since a capacitor related to the rectifier tube was previously mentioned, and I was once blowing 3A SB AC power fuses left and righton start up  that I am about 99.9% sure was related to one or more power capacitors, you could start by checking those against each other.

However, if you do decide to take this approach, either make or buy a shunt so you can be sure that all your caps are discharged before you get started.

@moose89 -- I would contact Dennis directly with your issue. You can contact Mr. Had at w4usr@att.net or radiom_dr2856nho@members.ebay.com. I have a Dennis Had Inspire 45 Amp and LP3.1 Preamp, and Mr. Had has answered numerous questions for me, usually responding within 24 to 48 hours. He is a very nice guy and is responsive to fans of his gear. My gear was made in 2018 (he signs and dates each piece of gear on the bottom with usually a gold or silver sharpie. I live in Winston-Salem, NC, which is about 80 miles from where Mr. Had lives.

 

A friend of mine has one of Mr. Had's 300B amps paired with a pair of Klipsch Cornwalls.  One of the sweetest sounding combinations I've ever heard.  I am looking at adding one to my collection, but my next purchase is going to be a tube Phono Preamp that Mr. Had builds every now and then.  I have Klipsch Forte's matched with my Inspire 45 (that's another story -- like an idiot I had the 45 hooked up to Polk Audio Monitor 10's before I moved my Forte's; 2 watts a channel into 89 efficiency speakers was NOT SMART!).  

Turn on the amp without the tubes installed. If the fuse blows send the amp in for repair. If not, then there's a 99% chance it's a bad tube.

 

A Dim Bulb Tester can save you some fuses.

I am in New Jersey if you need assistance.

 

I found a gut-shot of a recent Inspire 300B amp (Steve Huff).

The rectifier is lower/left.

 

 

DeKay

@carlsbad2 I'm not certain how you arrived at this:

Your opinion is noted and considered overly optiomistic at best.  You are a manufacturer and have an overly proud opinion of manufacturers IMHO. 

My prior comment is based on experience. I started my career in 1974 servicing consumer electronics and that is how I put myself through engineering school.

You don't put a fuse of a higher rating in any kind of equipment, especially if that equipment is consistently blowing fuses.

Bad Things tend to happen.

I have no idea how you might have arrived at the 'overly proud opinion' thing. To me the comment makes no sense, so I suspect its not accurate 😉

 

@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.

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

Keep us updated, @moose89  , and let us know how it all works out.

By all accounts that I am familiar with, Dennis is a good dude and backs his products.

Plugged in. Turned on without tubes. Fuse is stable. Waiting on tubes now. 

After getting the rectifier tube, the problem still remains as a blown fuse continues and the replacement of the blown fuses continue. The only tube that was able to get warm was the rectifier tube and my Western Electric 300 bees showed signs of life before the fuse did its job. I have contacted Dennis Had and hopefully he will help me. 

@moose89 Does the amp blow its fuse if the power tubes are not installed but the rectifier is?

@atmasphere 

After reading this thread on this day after Christmas, I am reminded of what a class act you are.  Thanks for your contributions and the best to you and yours.

Fed Ex has it now, (I think).
 

Shipping with them is 3rd World it seems, at times. 

it didn’t blow fuses with NO Tubes, turned on… with tubes, only the rectifier got warm, as it blew a fuse. 
 

Dennis awaits the shipment. Thanks for your interests!

To Dennis I gave my apologies. I blew up the electrolytic capacitor. I inserted a Cary Bias meter into the jack inlet on board the amp, purely no knowledge on the consequences of THAT action! 
 

Dennis took it back, and repaired it, scratching his head on how it could have happened, until we spoke on the phone. 
 

Thanks for the followup.  I remember that @dekay did mention a cap linked to the rectifier, and I did allude to my amp blowing the 2A SB fuses frequently when I had it connected to the extra cap power bank.

I skimmed back through the thread because I thought I remembered you saying it was a self biasing amp, but now I cannot finnd that?  If it is not a self biasing amp, what jack did you plug the meter into?  I take it that it was NOT the bias jack.  If that amp is NOT  a self biasing amp, and does need to be manually biased, how/where are you supposed to plug the meter in?

 

My lack of knowledge. I plugged a Cary Bias Meter into the jack hole on top. It measures plate, not bias. My bad… bias meter went arrow to the right… yet the amp continued to run for weeks thereafter. 

Here’s Dennis:

INSPIRE 300B AMPLIFIER

I have not written up a formal manual for the new Inspire 330B amplifiers. Please read the following information below:

Make sure you insert the WE 300B tubes with the Western Electric logos facing forward. Large pins forward.

The 5U4 or 5AR4 rectifier tube is located to the far right up front of the power transformer. The 6SL7 driver vacuum tube is centered between the 300B output tubes

When first powering up from a cold start you may hear a slight hum in the loudspeakers as the

300B tubes auto bias up. This is normal.

You may notice the amps runs warm to the touch on the power transformer side. The heat generated is from a non-inductive power resistor dissipating the heat through the aluminum chassis. This resistor is part of the DC filament power supply. If you remove the bottom panel of the amplifier you will see the large aluminum resistor mounted to the side of the chassis near the power transformer.

The 1/4 inch jack located on the top of the amplifier behind the left facing 300B tube is for doing a simple, 300B cathode voltage check/reference. Your amplifier test voltage is:

10

vdc. Date: / 119 23

To calculate the plate current of the 300B output tube:

Simply plug in the 1/4" two circuit jack and hook the black lead to negative and the red to the positive of your DVM meter and measure the DC voltage of the output 300B tubes. Simply divide the DC voltage shown on the meter by 490 (auto bias resistance value) and this is the

plate current for both tubes operating. Example divided the indicated de voltage by 490 = Plate

current of both tubes then divide this total by 2. Now you will know the platte current to each

tube. 68 volts de divided by 490 = .138 divide by 2 = 69 ma. per 300B

To operate as a basic amplifier simply open up the front panel volume control to maximum.

These are the basics. Enjoy your new Inspire 300B amplifier and be sure to fill out the Western Electric 5 year warranty cards for the 300B tubes. List Inspire by Dennis Had as the distributor on the WE form.

Thank you and have a wonderful day

Dennis J. Had

78

490

=. 143=2 = 7/ ma each