to fuse or not to fuse

[spuffock]

I find that a trend in modern valve amplifiers (for guitar) is the inclusion of fuses in the heater supplies. Without exception the fault, failure of all valves to heat, is caused by the fuse being open circuit. I have never found a shorted heater to be the cause of the problem. Further, in 40 years I have never found a burned transformer caused by a shorted heater. When I find these fuses they get replaced with a heavy wire link. Comments, anyone?

 

[Nigel Goodwin]

I can't say I've ever seen a fuse in the heater circuits?, and I certainly wouldn't include one in anything I designed.

 

[Optikon]

I find that a trend in modern valve amplifiers (for guitar) is the inclusion of fuses in the heater supplies. Without exception the fault, failure of all valves to heat, is caused by the fuse being open circuit. I have never found a shorted heater to be the cause of the problem. Further, in 40 years I have never found a burned transformer caused by a shorted heater. When I find these fuses they get replaced with a heavy wire link. Comments, anyone?

Fusing is... important. I would keep fusing and figure out the right size & type (clearly the examples you speak of were undersized)

 

[Bob Scott]

With the demise of tube circuitry and multiple tapped power transformers, is it possible that separate power transformers are being used used for heater supplies, and therefore need separate fusing?

Bob

 

[RODALCO]

I would consider it good practise to put a fuse in the heater supply.
If say 2 Ampères is drawn for the heaters, fuse it with a 3 Amp slow blow 6 x 30 type fuse.
Allowe for 1 amp extra and use a slow blow, often the fast blow fuses are used because they are cheaper.
A multivoltage transformer for a valve amplifier is very expensive, for the sake of a couple of dollars it is cheap safety.

 

[Nigel Goodwin]

I would consider it good practise to put a fuse in the heater supply.
If say 2 Ampères is drawn for the heaters, fuse it with a 3 Amp slow blow 6 x 30 type fuse.
Allowe for 1 amp extra and use a slow blow, often the fast blow fuses are used because they are cheaper.
A multivoltage transformer for a valve amplifier is very expensive, for the sake of a couple of dollars it is cheap safety.

Perhaps you would care to suggest what paricular failure such a fuse might be protecting against?.

 

[RODALCO]

Perhaps you would care to suggest what paricular failure such a fuse might be protecting against?.

Protecting the supply transformer 6.3 volts ac winding against overload or a short circuit.

A multivoltage transformer for a valve amplifier is very expensive, for the sake of a couple of dollars it is cheap safety.

If a separate 240 / 6.3 volts filament TX is used you can leave it out, but large currents can flow for a period of time. They could also start a fire.
I look at it as a safety issue.
You can of course fuse it the primary side, but still need a slow blow fuse to allowe for the TX inrush current.

Raymond

 

[Nigel Goodwin]

Protecting the supply transformer 6.3 volts ac winding against overload or a short circuit.

That didn't actually answer the question

Where (and how?) would this ficticious overload or short circuit take place?.

 

[Hero999]

If you're going to protect the secondary side then I would use a PTC resistor rather than a fuse as it won't need replacing.

 

[spuffock]

No,go on, answer Nigel's question. Go into some detail about this mythical fault that the fuse is protecting against. The transformer can go short, but this is before the fuse. Think of the heater. A bunch of thin , insulated wire , the ends of which are crimped into a heavy wire to go to the valve base. So, the bulk of the heater CAN go short, but what of the thin, high resistance wire going to the valve pin? It'll beat the fuse to it!
I've had open heaters, but never in conjunction with an open fuse. Usually, the contact resistance of the fuse clips causes the fuse to heat up, the spring pressure drops, the fuse heats up some more, the whole thing runs away and the fuse falls to bits. Replace the fuse and it packs up in the middle of the next gig with the same fault. (Reputation gone). Replace the fuse clip and it's back next year. Solder a bit of copper wire in, never get the fault again.

 

[RODALCO]

Fuse on 6.3 volt supply

spuffock : Go into some detail about this mythical fault that the fuse is protecting against.

Basically for a fault in the wiring to the heaters.
These heater wires are often twisted to reduce hum.
If insulation fails between the two wires.
Valve amps do run hot and insulation can soften.

ok, you can argue that 6.3 volts is a low voltage and you need a good contact area to make a large current flow.

Frayed wires because of sharp edges of the aluminium chassis, short between pin 4 and 5 on the valve socket.

 

[mneary]

The reason for the fuse on the heater line is probably that on the occasion of a secondary short (however unlikely), the current in the primary won't be enough to blow the main fuse. This would happen if the rated power for the heater winding is a small percentage of the total power. The predictable outcome is a smoking transformer, or fire.

If the transformer were robust enough to draw sufficient current in the primary, then the secondary fuse wouldn't be necessary. This would be accomplished by overdesigning the heater winding.

A PTC or self-resetting fuse is an option.

Safety agencies don't like even the remote possibility of fire (that's their job). As accidents decrease it's an opportunity to dig deeper into the less likely hazards. Your ladders have a notice not to stand on the paint shelf...

[edit] "When a bureaucracy's reason for existence is threatened, it typically generates new missions." Desperately Seeking Mission: Why the State Department's Gone Green --- Peter VanDoren [\edit]

 

[spuffock]

Hold it a minute! I don't recall ever seeing a fuse with a proper chassis holder, only the silly little clips on printed circuit boards, where to actually short the wiring would require some interesting acrobatics on behalf of inanimate objects.
In the event of a well designed (expensive) fuseholder, the heating and subsequent failure of the fuse would never occur.
My conclusion is, politicking by half informed health and safety fanatics, with spin from accountants (bean counters). SHORT 'EM OUT!

 

[Optikon]

The reason for the fuse on the heater line is probably that on the occasion of a secondary short (however unlikely), the current in the primary won't be enough to blow the main fuse. This would happen if the rated power for the heater winding is a small percentage of the total power. The predictable outcome is a smoking transformer, or fire.

If the transformer were robust enough to draw sufficient current in the primary, then the secondary fuse wouldn't be necessary. This would be accomplished by overdesigning the heater winding.

A PTC or self-resetting fuse is an option.

Safety agencies don't like even the remote possibility of fire (that's their job). As accidents decrease it's an opportunity to dig deeper into the less likely hazards. Your ladders have a notice not to stand on the paint shelf...

[edit] "When a bureaucracy's reason for existence is threatened, it typically generates new missions." Desperately Seeking Mission: Why the State Department's Gone Green --- Peter VanDoren [\edit]

From a safety standpoint, you are spot on. I don't understand all the resistance (pun intended) for using a fuse!

If it helps protect against high voltage faults or fire in case something *unintended* happens why not? Seems very foolish to say Oh, such and such a fault will never happen. Ahem, never say never.

 

[Nigel Goodwin]

From a safety standpoint, you are spot on. I don't understand all the resistance (pun intended) for using a fuse!

If it helps protect against high voltage faults or fire in case something *unintended* happens why not? Seems very foolish to say Oh, such and such a fault will never happen. Ahem, never say never.

Simply because they fail for no reason, making for a very quiet gig!

There also seems no reason for a fuse in the heater circuit, it's not something which is going to cause a problem - assuming it's designed and built with even a very tiny bit of care!. I've been mending valve amps for many decades, I've never seen any such fault, and never heard of anyone who has!.

Interesting mention of 'high voltage faults' when we're discussing a 6.3V heater supply

BTW, as someone from the valve era, I wouldn't consider using a valve amp!.

 

[Optikon]

Simply because they fail for no reason, making for a very quiet gig!

There also seems no reason for a fuse in the heater circuit, it's not something which is going to cause a problem - assuming it's designed and built with even a very tiny bit of care!. I've been mending valve amps for many decades, I've never seen any such fault, and never heard of anyone who has!.

Interesting mention of 'high voltage faults' when we're discussing a 6.3V heater supply

BTW, as someone from the valve era, I wouldn't consider using a valve amp!.

Well where does the 6.3 come from? I've lost track of what the OP is talking about now.. But generally, if one has mains connected equipment via tranformer for doing low voltage heating or cooking your breakfast, it had better be fused. That's all I was saying.

The fact that they fail for no reason is a misunderstanding by the designer. Not a valid reason for getting rid of them!
Wouldnt everybody feel better if somebody investigated the problem of why they fail. If a fuse is already fitted, why can't the OP just put in a larger rating?

 

[spuffock]

They DON'T fail for no reason. They fail because the crappy little fuse clips heat up at the poor contacts and the fuse falls to bits.
Put a higher rated fuse in and THE CRAPPY LITTLE FUSE CLIPS HEAT UP AT THE POOR CONTACTS AND THE FUSE FALLS TO BITS.
The only useful thing to do is solder a bit of heavy copper wire in, otherwise
( buggrit how do you get a bigger font in here?)

 

[Optikon]

They DON'T fail for no reason. They fail because the crappy little fuse clips heat up at the poor contacts and the fuse falls to bits.
Put a higher rated fuse in and THE CRAPPY LITTLE FUSE CLIPS HEAT UP AT THE POOR CONTACTS AND THE FUSE FALLS TO BITS.
The only useful thing to do is solder a bit of heavy copper wire in, otherwise
( buggrit how do you get a bigger font in here?)

OK.. now maybe you are on to something.. If the fuse rating is ok now the clips are the problem, then solve the clip problem.

If the fuse blows so rarely, why use clips at all? Wouldn't soldering the fuse work better? Or how about finding better clips?

These kinds of problems seem trivial to solve.

 

[badaxxe]

I have a 1977 Marshall Club and Country 2 -12 Combo and it has heater fuses on the rectifier board.


Quote: BTW, as someone from the valve era, I wouldn't consider using a valve amp!.
What would you use? I have Boogies, Marshalls, Voxes, Sound Cities, Fenders, Gibsons, Supros, name it I have it or have had it. I have Line 6 gear including their flagship Vetta II, Fenders' Cybertwins and sorry dude TUBES RULE. I have been playing guitar live since I was 16 in bars and I am 48 now and gig everyweek. Every great player from Vai, Johnson, Beck, Clapton, Van Halen bla bla bla uses tube amps. Sure everywhere else (I love my McIntosh tube Stereo gear) tubes are history. Digital stuff sounds sterile.

 

[Nigel Goodwin]

Every great player from Vai, Johnson, Beck, Clapton, Van Halen bla bla bla uses tube amps.

Some great players use valve amps some of the time, many other great players use mainly transistor amps - and even ones who use valve amps often use transistor ones for recording.