Discussion in 'Tube Audio' started by GordonW, Jan 16, 2009.
Gents - All this talk about using ten ohm resistors as fuses leads me to a better idea. How about putting REAL fuses in there! I always fuse the B+in my tube amps. Once the amp is running properly, turn it off, discharge it, pull one B+ lead and insert a DC current meter in series. Then run the amp to near clipping and read the current. Say it's 250 ma. I double it and use a 500ma, Fast-Blo fuse. Don't depend on a resistor to burn out, it's too slow a process. And don't depend on the mains fuse to protect anything either. It's there simply to prevent a fire from starting. The mains fuse has to be chosen to pass the high voltage currents and the heater currents as well. So it may not blow just because one tube red-plates. Repeat the process for the other amp channel if it's stereo.
A fuse doesn't allow you to sample the cathode current. You'd need a fuse AND a resistor, for EACH output tube.
Given that, a 1/4 watt 10 ohm resistor works better as a fuse, than nothing. It's saved transformers for me, in several cases. It will usually blow faster than even a global B+ fuse- because it's small enough to only handle the ONE tube it's connected to- in contrast to the B+ fuse, which has to handle ALL the output tubes. U
There's at least one Pioneer solid state receiver that uses a 1/8 watt resistor for the same purpose. The component number is FRn instead of just Rn for "fusible resistor".
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