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05-02-2024, 12:37 AM
Hi Guys
Fuse calculations are given in TUT3 and elsewhere in the series, and also here on this forum:
Fusing for Bias Supply? Fusing for the OT?
post-2
In my amps, I fuse every supply so there is proper protection for each winding of the PT, and that includes the bias supply.
You can always look at every component as a potential breaking point which maybe cause a cascade of failures. On the other hand, you have to put your faith in something. So, say you fuse the bias supply and that fuse opens - it could be a nuisance failure or because of a legitimate fault, does not matter. Have you also fused the OT? If not, then the OT may be in danger. If you have fused the OT as well, then that fuse will open second and protect the $$ items.
You could devise all kinds of interlock approaches between the bias supply and the plate supply, but then you are relying on these extra components. Are they more reliable than the core circuit components? They shouldn't be.
The reliability rates for most components is pretty fantastic. I choose components for reliability and circuits for tone, therefore I do not follow a specific prototype circuit to attain a particular tone.
The bias winding in a PT is more likely to fail because it is of such light construction compared to the other higher-current windings than for any other reason. Vibration during normal operation can break the wire and then the bias winding no longer functions. I've had this happen on larger PTs and on subsequent runs spec'd the bias winding to be much heavier itself to have a margin against a repeat failure. Really, it was a fluke of manufacturing that lead to that failure.
The tensioning of the wire during manufacture is called "scaning". I had a small audio transformer fail because of a scaning issue - very tiny wire and many many turns. Talking to the manufacturer, I determined that I could use the rest of the batch by adding one resistor to the circuit which did not effect the basic performance but protected the transformer. Another manufacturing issue but also another transformer!
Since transformers are so expensive, i have no hesitation to add tons of fusing to protect them. You have to abandon the larger fuse sizes and go with 5x20mm or even smaller. A friend of mine uses pico fuses that solder to the PCB. I can't say i like that method but for what he builds he has never had a fuse failure and really nothing he builds is going up in smoke anyway
Have fun
Fuse calculations are given in TUT3 and elsewhere in the series, and also here on this forum:
Fusing for Bias Supply? Fusing for the OT?
post-2
In my amps, I fuse every supply so there is proper protection for each winding of the PT, and that includes the bias supply.
You can always look at every component as a potential breaking point which maybe cause a cascade of failures. On the other hand, you have to put your faith in something. So, say you fuse the bias supply and that fuse opens - it could be a nuisance failure or because of a legitimate fault, does not matter. Have you also fused the OT? If not, then the OT may be in danger. If you have fused the OT as well, then that fuse will open second and protect the $$ items.
You could devise all kinds of interlock approaches between the bias supply and the plate supply, but then you are relying on these extra components. Are they more reliable than the core circuit components? They shouldn't be.
The reliability rates for most components is pretty fantastic. I choose components for reliability and circuits for tone, therefore I do not follow a specific prototype circuit to attain a particular tone.
The bias winding in a PT is more likely to fail because it is of such light construction compared to the other higher-current windings than for any other reason. Vibration during normal operation can break the wire and then the bias winding no longer functions. I've had this happen on larger PTs and on subsequent runs spec'd the bias winding to be much heavier itself to have a margin against a repeat failure. Really, it was a fluke of manufacturing that lead to that failure.
The tensioning of the wire during manufacture is called "scaning". I had a small audio transformer fail because of a scaning issue - very tiny wire and many many turns. Talking to the manufacturer, I determined that I could use the rest of the batch by adding one resistor to the circuit which did not effect the basic performance but protected the transformer. Another manufacturing issue but also another transformer!
Since transformers are so expensive, i have no hesitation to add tons of fusing to protect them. You have to abandon the larger fuse sizes and go with 5x20mm or even smaller. A friend of mine uses pico fuses that solder to the PCB. I can't say i like that method but for what he builds he has never had a fuse failure and really nothing he builds is going up in smoke anyway
Have fun
