How to determine fuse rating

[Randall]

My question seems to have stalled as an aside to another post about another subject, so I shall present it again. I installed a fuse in a 1959 Hammond M3 amp that did not previously have one. The amp idles at 0.93A. How do I determine what amperage fuse to use in this case? It has 5U4, two 6V6, and 8 preamp tubes of various types.

[nevetslab]

My question seems to have stalled as an aside to another post about another subject, so I shall present it again. I installed a fuse in a 1959 Hammond M3 amp that did not previously have one. The amp idles at 0.93A. How do I determine what amperage fuse to use in this case? It has 5U4, two 6V6, and 8 preamp tubes of various types.

If you have an AC Mains Power Analyzer, or an accurate AC RMS Ammeter, you'd start with probably a 3A S/B fuse installed. Run the amp up to full power at High Line (127VAC, assuming you have a variac). See what the current draw is with the amp into good visible clip. Your Mains fuse value will want to be above this value, to prevent nuisance blowing. Let's say it pulls 2.0A. Drop the AC mains back down to nominal line voltage (120VAC), and the amp in clip now draws 1.7A. Select a fuse value 50% above that, which would be roughly 2.5A. You will want that fuse to blow under component failure, but not under typical overload conditions that would occur in use, such as loading the amp with half the rated load impedance.

Now, do you use a Slow Blow fuse/Time Delay Fuse, or a FAST Blow fuse? I tend to select the Slo Blo variety on AC Mains fuses. On most fuse data sheets, at 200% fuse rating, they call for a 5 Second Max hold before blowing. When you look at the curves on the Fuse Value (Horizontal) vs Time (Vertical), the Slow Blow fuse can handle much higher short-term current overloads than the Fast Blow variety. Such as LIttelfuse 312 (FAST) vs 313 (Slow).

I'm sure R.G. has a more exacting procedure in the selection process.

[trobbins]

Some additional details on fuses, and a process to select them using PSUD2, is provided in link - it is a bit more technical, but may assist.
https://www.dalmura.com.au/static/Va...p%20fusing.pdf

[Old Tele man]

Nice article!

[Jazz P Bass]

Don't forget the voltage rating.

That is the voltage level that the fuse can withstand without internally arcing over in the event of a failure.

[nevetslab]

Some additional details on fuses, and a process to select them using PSUD2, is provided in link - it is a bit more technical, but may assist.
https://www.dalmura.com.au/static/Va...p%20fusing.pdf

Excellent document, now added to my database for further study. Thanks for providing that!

[Randall]

While that certainly is an informative in depth article, I don't see where it helps me to choose which A value for the primary of my amp. I usually use 250v Timed delay. Since this amp has a tube rectifier, turn on surge shouldn't be an issue, so someone convince me a T 2A 250v fuse isn't adequate?

[nevetslab]

Don't forget the voltage rating.

That is the voltage level that the fuse can withstand without internally arcing over in the event of a failure.

While always using industry standard AC Mains fuses carrying 250VAC ratings from any of the normal sources (Bel, Bussman, Littelfuse, Schruter, etc), it still never fails to surprise me finding a failed 32VAC Automotive Fuse stuffed into a fuse holder. Look at the fuse data sheet on Littelfuse type 312 or 313 3AG fuses. The voltage rating on those above 10A is 32V. You have to select a different type, such as their type 314 3AB ceramic sleeve Fast, or type 326 3AB ceramic sleeve Slo Blos. The other mfgrs have similar ratings and alternate types for the higher current ratings.

Littelfuse Cartridge Fuse pages.pdf

[Dave H]

I usually use 250v Timed delay. Since this amp has a tube rectifier, turn on surge shouldn't be an issue

Even with a tube rectifier there's still the turn on surge of the transformer and heaters.

[eschertron]

Is there a way to guesstimate, based on the VA rating of the PT, what may be safe to prevent the PT's thermal fuse from blowing? I'm thinking something like
fuse rating = VA/nominal line voltage * 120% (or 150%, or...)
and in absence of the VA rating, a guess at the full power load. (Of course this is beginning to trace the steps of Nevetslab's instruction)

This would not be a technical answer, as the answers above are, but should prevent the amp bursting into flame while simultaneously minimizing nuisance faults. Erring on the side of more nuisance faults, and less bursting

[nevetslab]

Is there a way to guesstimate, based on the VA rating of the PT, what may be safe to prevent the PT's thermal fuse from blowing? I'm thinking something like
fuse rating = VA/nominal line voltage * 120% (or 150%, or...)
and in absence of the VA rating, a guess at the full power load. (Of course this is beginning to trace the steps of Nevetslab's instruction)

This would not be a technical answer, as the answers above are, but should prevent the amp bursting into flame while simultaneously minimizing nuisance faults. Erring on the side of more nuisance faults, and less bursting

Are you referring to the internal thermal fuse buried inside the windings? Class B insulation is 130 deg C, so you'd be safe using an OOR Thermal Fuse in the 115 deg C to 130 deg C, which would have to be fully insulated, as the metallic case types have the case common to one of the leads. Unless you've found where the thermal fuse is, and lucky enough to for the build to have installed a small cardboard tube for the thermal fuse, so it IS in the direct thermal environment of the windings. If not, and all you're left with is laying one in on the inside of the copper Flux Band (if there is one), then the temp rating selection is lab work & educated guess work.



This is the thermal fuse I found inside a Fender Deluxe Reverb PT, and is actually a 142 deg C 10A rated part. Interesting the choice of 142 deg C, that being above the thermal class rating of the windings.

To get at it requires surgery....removing the core bolts without breaking them, removing the end bells, unsoldering the copper Flux strap around the core, cutting open the insulation paper wrapped around the termination layer between lead wires and the magnet wires, then digging to find where that thermal fuse is. Some are just laid in on one of the winding layers with appropriate insulation. This one had a cardboard tube cavity, so replacement is feasible, once you're beyond the surgery. Broken core bolts can stop the entire process with the resulting frustration, so beware.

[eschertron]

Are you referring to the internal thermal fuse buried inside the windings?

Right! I guess my question was an awkward way of asking how to NOT worry about that thermal fuse. I know the line fuse is to protect the venue, not the equipment; like a lot of us, I'd like to think it can be used to protect the equipment too. Last year, I burnt up a PT because I had undersized it in my spec. The 2A (or whatever size I used, I'll have to look it up) time-delay fuse didn't even break a sweat. I'd like to not make that mistake again.

[trobbins]

Although that article focuses on how to gauge a fuse for the secondary side, I added some guidance towards the end on AC side fusing and how to estimate how much the inrush may get to, and how to relate that to fusing.

Anything more than a broad-brush estimate sort of requires a detailed technical assessment to narrow down the expected operating conditions, and even then it will be based on a few estimates.

Another approach to take is a bit more statistical - for example, use the lowest value fuse that could be just acceptable, and make a note of how often the amp is used and the fuse blows. When the fuse has blown say 7-10 times, then increase the fuse rating by one current increment, and repeat. After a year or two you may be using a fuse rating that hasn't blown for quite some time, and that may give you confidence that you are using an appropriate fuse rating.

Another approach is a bit more risky - for example use a fuse rating that should do the job. If it doesn't blow after many uses, and a long period of time, then it may be the best value to use - but it may not.

Just my 2 cents worth

[Old Tele man]

Also, always *inspect* the blown fuse link (clear glass body assumed): if it's *splattered* that indicates a sudden high current load, but if the link is just *sagged* and pulled-apart in the middle that indicates a slowly increasing (temp?) load, ie: a "slow gradual blow" rather than a "sudden explosive blow."

[R.G.]

Good call, OTM!! I've seen fuses where the inside was mottled-silvered from what I think was the filament vaporizing.

@ eschertron
I can't think of a good way to prevent the internal thermal fuse from blowing with an external fuse. Current sensitive fuses are designed so that the self-heating of the filament makes the filament melt at some value of current/time/energy and limit (over)currents, so there's a lot of work put into making their self heating matter and be predictable. Thermal fuses are designed to sense temperature and open, not be internally self heated. There's a maximum current for thermal fuses, but the idea in a thermal it to make it relatively insensitive to current, instead sensing the temperature of its surroundings.

True, over currents do cause heating, but they may not open with so-called "soft-short" failures. These can let temps creep up without necessarily blowing a current fuse. Thermal fuses were a solution to the safety-regulation requirement that the worst possible combination of conditions should not cause dangerous things to happen. Having catastrophic currents limited by current fuses, the next possible issue would be (relatively) slow overheating. Thermal fuses catch a lot of these kinds of conditions. The safety regs don't say "put in a thermal fuse". Instead, engineers found that thermal fuses in the transformers were a good way to pass safety tests.

Just speculating, I'd say that one way to avoid tripping thermal fuses is to sense transformer temperature somehow and shut things down electronically before the thermal fuse can trip. Something like a resettable snap-action temperature sensor on the laminations might do it, although I'd naturally tend to over-engineer a solution with active temperature sensors, a microcontroller, solid state relays, bluetooth reporting and so on.