I have a couple of nice little 3/4" fuse holders so thought I'd put them to some good use in this JCM800 2204 I am currently building.
Do you have a preference of where you would locate fuses if you even used them there? I could fuse each end of the HV secondary on the way to the diodes, or fuse them right after the diodes or, just fuse the main B+ after the first filter cap. The latter doesn't seem to be the best because should problems arise with the filter cap or diodes, so, perhaps catching things early is the best? Right at the HV secondary coil legs?
Whats ya think.
Thank you!
Best,
Phil
Fusing the HV, B+, etc....
Moderators: pompeiisneaks, Colossal
Fusing the HV, B+, etc....
I’m only one person (most of the time)
Re: Fusing the HV, B+, etc....
Merlin makes a pretty convincing argument for doing it that way:
http://www.valvewizard.co.uk/fuses.html
Re: Fusing the HV, B+, etc....
Phil, fusing is a tool to use that could address a wide variety of future concerns. What are your concerns and why?
Re: Fusing the HV, B+, etc....
Thanks folks,
I'm not anticipating any terrible electrical situation from this amp, I'm going to buy good parts and components and try to overrate them a tad but, I figured why not. I have seen different sections of mass produced amps fuse various parts of their amps so I figure if it doesn't compromise anything electrically and it would only help in the unlikely but possible event of a bad filter cap or diode problem, then I don't see whynot. All tube amps are going to have some issue of some sort, someday so, there always some room for concern, whether superficial or critical. I'm not looking to be talked out of it unless there is a darn good reason but I just see fusing as cheap insurance. If nothing ever happens, great.
I think I did read Merlins article but, I'll go read it again.
When I raped my Bogner Alchemist to build a 20W plexi, I found around 5 fuses in various parts of the amp besides the main primary fuse. That surprised my a bit, but I was impressed at the willingness to protect and serve the amp, even it obsessively so. One of them did in fact blow, and I had to replace it.
Best and thanks!
Phil Donovan
I'm not anticipating any terrible electrical situation from this amp, I'm going to buy good parts and components and try to overrate them a tad but, I figured why not. I have seen different sections of mass produced amps fuse various parts of their amps so I figure if it doesn't compromise anything electrically and it would only help in the unlikely but possible event of a bad filter cap or diode problem, then I don't see whynot. All tube amps are going to have some issue of some sort, someday so, there always some room for concern, whether superficial or critical. I'm not looking to be talked out of it unless there is a darn good reason but I just see fusing as cheap insurance. If nothing ever happens, great.
I think I did read Merlins article but, I'll go read it again.
When I raped my Bogner Alchemist to build a 20W plexi, I found around 5 fuses in various parts of the amp besides the main primary fuse. That surprised my a bit, but I was impressed at the willingness to protect and serve the amp, even it obsessively so. One of them did in fact blow, and I had to replace it.
Best and thanks!
Phil Donovan
I’m only one person (most of the time)
Re: Fusing the HV, B+, etc....
Phil, imho it is always worth including extra protection than just a mains side fuse in a valve amp. The hassle for many is that where to fuse, and what fuse value to use and why, are quite technical queries to assess in detail - but detail is what is needed in order to have some reality about what to expect.
The following link is to a technical discussion on fuse related protection in valve amps - perhaps see if you can navigate that and come to your own awareness of what sections of your valve amp can be fused. If you aren't sure on some aspect then I suggest you post about that and hopefully we can assist you through the learning curve and to confirm details. Ciao, Tim
https://www.dalmura.com.au/static/Valve ... fusing.pdf
The following link is to a technical discussion on fuse related protection in valve amps - perhaps see if you can navigate that and come to your own awareness of what sections of your valve amp can be fused. If you aren't sure on some aspect then I suggest you post about that and hopefully we can assist you through the learning curve and to confirm details. Ciao, Tim
https://www.dalmura.com.au/static/Valve ... fusing.pdf
Re: Fusing the HV, B+, etc....
Dead right, Tim. Fusing can be a challenge to get right, even to a pro.
To start with, fuses are not rated for the current they'll blow at. They're rated for a current they will carry semi-forever without blowing. Their actual opening current is above their current rating by some amount.
How much the over-current is affects how much time it takes to clear. For small over-currents, the time to blow may be hours or days.
This is exploited in making "slow blow" or time-delay fuses. These fuses have elements that have a high-mass element that take time to heat up to melting, so they withstand s short-term current pulses without actually melting. This makes slow-blow fuses good for AC mains applications, where a power-on inrush current will not clear them as long as the current settles back now to a lower level. Tubes, transformers, and carbon composition resistors happen to be resistant to high pulse currents, so the time lag in blowing is a good match for things like guitar amps. Not so for most solid state electronics, where a current even marginally over the component's max current can kill it. Even fast-blow type fuses can't clear fast enough to protect, say, a power transistor. This caused a lot of misery to designers in the 1960s and 1970s where the designers' experience with tube equipment left them unprepared for sudden-death semiconductors.
The fuse makers finally got around to publishing charts of I-squared-T, the relationship of fuse opening current to the energy (current squared times time) absorbed by the fuse element. This helped narrow the actual fuse opening characteristic down a lot, and made designing with them simpler. You were still left with the manufacturing variation of the fuses, but this was a lot smaller tolerance band than current-squared-times-time. In the 1960s and 1970s, EEs also worked out a lot of ways to have semiconductors self-protect by sensing and limiting their own currents. This could be fast enough to protect them where a fuse would not.
As to where to put fuses, I like the idea that fuses protect what's before them in the current path, not what's after them. This is particularly true for AC mains fuses. These are used not to protect, for instance, the insides of a guitar amp. Instead, they're there to protect the AC mains wiring and prevent the amp from starting a fire. The presumption is that if the AC mains fuse blows, something is already dead after the fuse. Where possible, it can make more sense to use a current limiter/clamp or a sense-and-shutdown circuit. That can protect things before the protection circuit.
For fusing in an amp design, one could rank components in terms of how much they cost and how difficult they are to replace.
Tubes are simply going to die someday. That's why they're in sockets, not soldered in. They are also resistant to pulse-current overloads. Fusing tube current supplies (to me...) only makes sense where the tubes are so very expensive that even one tube dying is a disaster. Even then, I'd probably use some kind of current-voltage-power sense and shutdown. I have designed a circuit that monitors output tube currents and shuts things down before the tubes can die, as in the instance of lost bias voltage, etc. A B+ fuse might protect the tubes, but I feel much more capable of designing a circuit that can be tailored to the expected disaster. An active current limit or clamp can protect things upstream of it, where a fuse might, but also might not.
The power transformer and then the output transformer are the two most individually expensive and hard to replace parts in most amps. It's really worth protecting these parts. A B+ fuse is best thought of as protecting the OT. Using a current limit and/or shutdown circuit on the output tubes really is limiting the current into the OT. It also protects the filter caps and rectifiers from an output tube short. Protecting the power transformer is probably best done by fusing the secondaries. A fuse in each separate secondary winding section will keep that secondary's current below the current that damages the PT, and protect the PT against damaging loads.
To start with, fuses are not rated for the current they'll blow at. They're rated for a current they will carry semi-forever without blowing. Their actual opening current is above their current rating by some amount.
How much the over-current is affects how much time it takes to clear. For small over-currents, the time to blow may be hours or days.
This is exploited in making "slow blow" or time-delay fuses. These fuses have elements that have a high-mass element that take time to heat up to melting, so they withstand s short-term current pulses without actually melting. This makes slow-blow fuses good for AC mains applications, where a power-on inrush current will not clear them as long as the current settles back now to a lower level. Tubes, transformers, and carbon composition resistors happen to be resistant to high pulse currents, so the time lag in blowing is a good match for things like guitar amps. Not so for most solid state electronics, where a current even marginally over the component's max current can kill it. Even fast-blow type fuses can't clear fast enough to protect, say, a power transistor. This caused a lot of misery to designers in the 1960s and 1970s where the designers' experience with tube equipment left them unprepared for sudden-death semiconductors.
The fuse makers finally got around to publishing charts of I-squared-T, the relationship of fuse opening current to the energy (current squared times time) absorbed by the fuse element. This helped narrow the actual fuse opening characteristic down a lot, and made designing with them simpler. You were still left with the manufacturing variation of the fuses, but this was a lot smaller tolerance band than current-squared-times-time. In the 1960s and 1970s, EEs also worked out a lot of ways to have semiconductors self-protect by sensing and limiting their own currents. This could be fast enough to protect them where a fuse would not.
As to where to put fuses, I like the idea that fuses protect what's before them in the current path, not what's after them. This is particularly true for AC mains fuses. These are used not to protect, for instance, the insides of a guitar amp. Instead, they're there to protect the AC mains wiring and prevent the amp from starting a fire. The presumption is that if the AC mains fuse blows, something is already dead after the fuse. Where possible, it can make more sense to use a current limiter/clamp or a sense-and-shutdown circuit. That can protect things before the protection circuit.
For fusing in an amp design, one could rank components in terms of how much they cost and how difficult they are to replace.
Tubes are simply going to die someday. That's why they're in sockets, not soldered in. They are also resistant to pulse-current overloads. Fusing tube current supplies (to me...) only makes sense where the tubes are so very expensive that even one tube dying is a disaster. Even then, I'd probably use some kind of current-voltage-power sense and shutdown. I have designed a circuit that monitors output tube currents and shuts things down before the tubes can die, as in the instance of lost bias voltage, etc. A B+ fuse might protect the tubes, but I feel much more capable of designing a circuit that can be tailored to the expected disaster. An active current limit or clamp can protect things upstream of it, where a fuse might, but also might not.
The power transformer and then the output transformer are the two most individually expensive and hard to replace parts in most amps. It's really worth protecting these parts. A B+ fuse is best thought of as protecting the OT. Using a current limit and/or shutdown circuit on the output tubes really is limiting the current into the OT. It also protects the filter caps and rectifiers from an output tube short. Protecting the power transformer is probably best done by fusing the secondaries. A fuse in each separate secondary winding section will keep that secondary's current below the current that damages the PT, and protect the PT against damaging loads.
"It's not what we don't know that gets us in trouble. It's what we know for sure that just ain't so"
Mark Twain
Mark Twain
Re: Fusing the HV, B+, etc....
Thanks guys, I will check out the resources that you have mentioned.
My history in fusing (for amps) has been relegated to trying to determine a main primary fuse for my 2 amp builds. This may have come from one of your mentioned resources where I calculated the V/A for each PT secondary coil, added them up and divided that into the primary voltage. That would have given me the supposed maximum current that I may have expected through the primary winding. The article then said to double the current value of the fuse that had been calculated. So, for example, one of my amps calced out to a max of .9A for the primary coil current, so I used I believe a 2A fuse. (don't remember it if was a slow-blow or fast blow, probably used whatever was recommended.
It was a few years ago but, I worked as a test technician at Mersen fuses where we manned a lab with 2 HUGE generators that yielded thousands of volts with the capability of 10's of thousands of amps (if short duration!). We spent our days testing for breaking current, I2T, and a number of other specifications that I cant even remember. Our days were spent calculating time constants (for DC) and power factor (for AC) and even setting which angle of the sine wave was fist applied to the fuse under test (fuses that were designed to blow within a half cycle). So, where I"m no fuse expert, I'm also no stranger to some of the circuit and fuse specifications that apply. I just hope non of my amps ever blow with the ferocity that the Mersen fuses did - huge white hot balls of molten sand and filaments) That job really stressed me out and I was relieved to leave!
If I had just winged it, I may have done something similar to my amps, and simply chose a fuse that was 1.5 to 2.0 times higher that the maximum expected HV current expected and hoped for the best but, I'm getting the feeling here I should dig a little more deeply. Which I will do!
Thanks again guys, more to look into.
Best,
Phil D.
My history in fusing (for amps) has been relegated to trying to determine a main primary fuse for my 2 amp builds. This may have come from one of your mentioned resources where I calculated the V/A for each PT secondary coil, added them up and divided that into the primary voltage. That would have given me the supposed maximum current that I may have expected through the primary winding. The article then said to double the current value of the fuse that had been calculated. So, for example, one of my amps calced out to a max of .9A for the primary coil current, so I used I believe a 2A fuse. (don't remember it if was a slow-blow or fast blow, probably used whatever was recommended.
It was a few years ago but, I worked as a test technician at Mersen fuses where we manned a lab with 2 HUGE generators that yielded thousands of volts with the capability of 10's of thousands of amps (if short duration!). We spent our days testing for breaking current, I2T, and a number of other specifications that I cant even remember. Our days were spent calculating time constants (for DC) and power factor (for AC) and even setting which angle of the sine wave was fist applied to the fuse under test (fuses that were designed to blow within a half cycle). So, where I"m no fuse expert, I'm also no stranger to some of the circuit and fuse specifications that apply. I just hope non of my amps ever blow with the ferocity that the Mersen fuses did - huge white hot balls of molten sand and filaments) That job really stressed me out and I was relieved to leave!
If I had just winged it, I may have done something similar to my amps, and simply chose a fuse that was 1.5 to 2.0 times higher that the maximum expected HV current expected and hoped for the best but, I'm getting the feeling here I should dig a little more deeply. Which I will do!
Thanks again guys, more to look into.
Best,
Phil D.
I’m only one person (most of the time)