What a coincidence. We are having a lively discussion of fuse values in the thread "Need help choosing mains fuse value" in the ChipAmp forum. Though I know very little about fuses, I do know that one purpose of the mains fuse is to prevent a person from getting electrocuted in the case of a short to the chassis. So you always always always need a mains fuse. Period. End of sentence. Now, if you want to know what should be the value of that fuse, I would check out the last few posts in the thread I mentioned above.
If the amp is properly grounded a short to the chassis should blow the mains breaker.
The innrush current limiiter only limits the start up current. It should drop in value to a near short as it heats up. It will NOT act like a fuse, in fact they will explode or burst into flames if exposed to a short. Trust me I have seen it happen, and it will stink up the house.
The main reason for a fuse is to save the transformers if a tube or capacitor shorts out. A shorted power supply cap, or a shorted rectifier tube should blow the mains fuse if it is properly sized. A dead short on a power transforrmer secondary without a fuse will fry the transformer. The process can cause a fire and WILL stink up the entire house!
A shorted output tube should blow a properly sized mains fuse. No fuse or too big a fuse will cause a dead OPT.
Several users have had their SSE's saved by the line fuse. The usual culprit was output tube runaway.
Been there, done that (when I was young and more stupid). Actually, I just overloaded the transformer, it overheated, and blew the fuse in the circuit panel of the house. It did indeed stink up the house. The transformer was still hot the next day!!
Always, always use a properly sized mains fuse. A good rule of thumb:
Fuse rating = 1.3 * TrafoVA/Vmains
Fuse rating in Ampere
TrafoVA = VA rating of trafo in Watt
Vmains = mains voltage in Volt
~Tom
I took it a step further. I stuck a screwdriver into a working SSE to see what it takes to blow the fuse.
A 1.6 amp slow blow fuse will instantly pop when the B+ is shorted. It will blow within a few seconds when the plate and cathode of one of the output tubes are shorted together. Unfortunately it will also blow at turn on sometimes if an inrush limiter is not used.
A 2 amp fuse will instantly pop when the B+ is shorted. It will blow within a minute or so when the plate and cathode of one of the output tubes are shorted together. During this minute the rectifier tube will be in cardiac arrest and may not survive but the transformers will be OK. This may not be true if an OPT with a high DCR is used (One Electron comes to mind).
You are referring to an NTC device. A PTC device can be (and usually is) used as a sort of self-resetting "fuse".
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