The reason tuning fuses are unsuitable for fusing tubes is quite simple. When a glass fuse blows in a DC circuit a plasma is formed in the glass cylinder. This plasma continues until the power supply has dumped all of its energy into that tiny fuse making it either explode or melt the holder. Because this dumping of energy goes through things like cathode resistors they can be destroyed. A proper 1/4 amp fuse blows around that value and its not at all critical. However when a fuse goes plasma the current can be many many amps. it is fact only limited by that 1-10 ohm cathode resistor, the output transformer primary resistance and the hot conducting tube.. Plasma conducts lots of current and the voltage drop is very small as the path is short. When the fuse blows there may be 500 volts in the power supply caps divided by even 50 ohms that's 10 amps, not 1/4. Since the output transformer is in this path we should fear for its life. If the primary wire is thinner than the cathode resistor wire it may be the thing to break the circuit and bye bye output transformer. In the RM-9 the cathode resistor opened up. That is the one and only cathode resistor that has every opened in a RM-9.
Standard fuses are rated to break circuits at 1500 amps and high breaking fuses 10,000 amps. Now you might say Roger is crazy talking about breaking 10.000 amps in a 1/4 fuse but you see from the previous paragraph plasma does not care that the fuse was 1/4 amp because now it has hot vaporized metal for a conductor. One other point, in a AC circuit things are quite different as AC goes through zero 120 times a second and the plasma dissipates rapidly.
How the make theses fuses is quite simple. They fill them with very fine sand.When the element vaporizes the sand fills the space and there is no place for the plasma to form. Gee isn't that simple. The may be made of glass or ceramic (ceramic is simply stronger). Just because a fuse is ceramic there is no guarantee it is filled with sand. So start reading the breaking ratings on Littlefuse and Buss data sheets.
When I called the distributor of Tuning fuses and asked if they were high breaking he did not know. I then said, I have ceramic fuses in my amplifier, he said these will be fine as they are ceramic too... oh boy
After I repaired the RM-9 that was damaged by tuning fuses i decided to do a little investigation into them. There were 8 of them so you do the math. They cost more than the tubes they are meant to protect and dont protect them at all.
The maker of these fuses does something that really demonstrates his lack of knowledge of fuses. There is no breaking rating on these fuses and I doubt they know what a breaking rating is. Its not in any of their tests. Out of simple curiosity I broke one open and found the fuse wire was slipped into a narrow Teflon tube. This is part of their "technology" and is not an easy thing to do. This Teflon tube obviously makes things worse as it further contains the plasma.
When people who dont know what they are doing start doing something bad people who know what they are doing need to speak up.
Roger A Modjeski
Music Reference
Ram Tubes...
Standard fuses are rated to break circuits at 1500 amps and high breaking fuses 10,000 amps. Now you might say Roger is crazy talking about breaking 10.000 amps in a 1/4 fuse but you see from the previous paragraph plasma does not care that the fuse was 1/4 amp because now it has hot vaporized metal for a conductor. One other point, in a AC circuit things are quite different as AC goes through zero 120 times a second and the plasma dissipates rapidly.
How the make theses fuses is quite simple. They fill them with very fine sand.When the element vaporizes the sand fills the space and there is no place for the plasma to form. Gee isn't that simple. The may be made of glass or ceramic (ceramic is simply stronger). Just because a fuse is ceramic there is no guarantee it is filled with sand. So start reading the breaking ratings on Littlefuse and Buss data sheets.
When I called the distributor of Tuning fuses and asked if they were high breaking he did not know. I then said, I have ceramic fuses in my amplifier, he said these will be fine as they are ceramic too... oh boy
After I repaired the RM-9 that was damaged by tuning fuses i decided to do a little investigation into them. There were 8 of them so you do the math. They cost more than the tubes they are meant to protect and dont protect them at all.
The maker of these fuses does something that really demonstrates his lack of knowledge of fuses. There is no breaking rating on these fuses and I doubt they know what a breaking rating is. Its not in any of their tests. Out of simple curiosity I broke one open and found the fuse wire was slipped into a narrow Teflon tube. This is part of their "technology" and is not an easy thing to do. This Teflon tube obviously makes things worse as it further contains the plasma.
When people who dont know what they are doing start doing something bad people who know what they are doing need to speak up.
Roger A Modjeski
Music Reference
Ram Tubes...