Heater supply fuse question - (A tad OT)

[MikeH]

My friend has a JCM600 that stopped working; I popped it open and found the fuse on the heater supply blown.  It says "T6.3L 250V".  Couldn't find it at the hardware store, but I think I found it at Radio Shack- would this be the same fuse (assuming it's the same size)?

http://www.radioshack.com/product/index.jsp?productId=2102764

I realize they're probably 10 times what I'd pay online, but I'm trying to fix this thing ASAP.

And of course there's also the question of "Is the blown fuse really the problem, or just a symptom"?  I guess I'll see. 

For extra credit: anyone know why there's a fuse on the heater supply anyway?  Schematic for reference:  http://www.drtube.com/schematics/marshall/jcm6-61-02.pdf (F101)

[defaced]

For extra credit: anyone know why there's a fuse on the heater supply anyway?

Because they put one there 
Actually, it's just good practice.  I know everyone has their reasons why and why not, but since it's not a typical pure AC heater supply, it makes sense to include one... see the next line as to why. 
If the fuse blew, you might have a shorted rectifier diode in that bridge rectifier if the new fuse blows.  Or for that matter, anything in that string could be shorted.  I'd pull the tubes to isolate possible issues with them before you install the new fuse. 

[MikeH]

Yeah, I had those thoughts in the back of my mind too... the tubes are suspect; they look pretty old.

[R.G.]

As defaced noted, it's because of the possibility of a shorted rectifier, capacitor, or heater element. I've done the math on this on another forum, and what can happen is that a short on a heater winding will not increase the AC power line current enough to pop the AC line fuse, but it will be enough to burn out the heater winding and possibly other things in the power transformer. The power transformer is the single most expensive part in the amp. A fuse in each secondary winding can protect the power transformer from load faults that would kill the transformer without popping the AC power line fuse.

The AC power line fuse is NOT to protect the transformer. It's to prevent fires caused by internal failures. If it happens to protect something inside, that's nice, but it's not why it's there. You need secondary fuses to protect the transformer.

[merlinb]

And of course there's also the question of "Is the blown fuse really the problem, or just a symptom"?  I guess I'll see.  

For extra credit: anyone know why there's a fuse on the heater supply anyway?  

Because it uses a rectifier. Simple AC heaters rarely suffer any problem so fuses aren't normally used. Rectifiers, on the other hand, do go bad, so fuses become obligatory. Consequently you should be suspicious of that part of the circuit.

[R.G.]

I used to think that the blown fuse was always a symptom, not a cause. I have had to correct that.

When a fuse element gets hot, it expands in length like most materials. So there is some flexing of the fuse element any time it gets near the fusing current. This flexing is not a problem with the middle of the filament which is hot, but stresses the connections of the element to the colder contact ends, and can, over time, break the element at the ends.

So a fuse which is poorly designed to be too close to opening, even on input surges, can simply wear out.

However, this is not a common situation. Suspect other things first, but keep this in mind in case all the other suspects' alibis check out. 

[PRR]

I suspect THIS fuse is working very close to rating. The RMS current into a cap-input rectifier is far higher than the DC current. There's also a prolonged start-up surge with all those huge caps.

It may just "wear out".

For today, put in another 6.3A fuse. (Have spares... it IS still very possible you have a simple short downstream.)

If it comes back with the same problem, consider a 7.5A fuse.

Personally, I don't think single-filtered "DC" is that much better than good raw AC. I don't know the beast, but on general philosophy, I would explore getting a good 6V 1.5A DC power supply to fit inside the box. Pull the rectifier, feed the small-tube heaters with pure DC instead of ripple.

[MikeH]

When a fuse element gets hot, it expands in length like most materials. So there is some flexing of the fuse element any time it gets near the fusing current. This flexing is not a problem with the middle of the filament which is hot, but stresses the connections of the element to the colder contact ends, and can, over time, break the element at the ends.

I got a good look at exactly what you're saying here when I replaced the fuse.  I figured I would start with a new fuse and see if it blew right away, so I had my magnifying glass over the fuse when I turned it on again so i could see just how quickly the fuse blew if t blew again.  When I flipped it on I could actually see the element in the fuse jump up and bend around for a couple seconds.

Anyway... seems to be rolling along fine now. Thanks for the info.

[defaced]

Low cycle fatigue at its finest. 

[Ronsonic]

I used to think that the blown fuse was always a symptom, not a cause. I have had to correct that.

When a fuse element gets hot, it expands in length like most materials. So there is some flexing of the fuse element any time it gets near the fusing current. This flexing is not a problem with the middle of the filament which is hot, but stresses the connections of the element to the colder contact ends, and can, over time, break the element at the ends.

So a fuse which is poorly designed to be too close to opening, even on input surges, can simply wear out.

However, this is not a common situation. Suspect other things first, but keep this in mind in case all the other suspects' alibis check out. 

Nuisance fuse blows do exist, but I always pretend otherwise until I've checked thoroughly.

The heater fuse seems to be the most prone to needless failure. It does get cycled pretty hard.

My question is why so many amps have a fuse in the bias circuit.

[R.G.]

My question is why so many amps have a fuse in the bias circuit.

Lack of clear thinking is the best guess I can make.

Anything which is protected from faults by the internal resistances doesn't need a fuse, and opening the bias supply is nearly always the wrong thing to do.

Of course, clear thinking is not evenly distributed. I remember that there are some Marshalls in which the standby switch not only disables the high voltage supply, it turns off the bias supply too. There is an interesting surge when the standby is turned back to the play position and the hot filaments let the tubes surge all the high voltage current they want until the bias supply ramps back to hold them off a bit. Wow. What a strange thing to put into production!