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Thread: Immortal Mods: additional fuses, where to put the fuse holders?

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    Immortal Mods: additional fuses, where to put the fuse holders?

    Hi All,
    I have a build that has slowed considerably due to grappling with where to put fuse holders for Immortal Mods extra fusing. he recommends fusing each leg of the HT off the power transformer, and each leg of the heater wiring.

    This JTM45 type thing also has 5v for a tube rect.

    After some trial and error, I found I can fit two screw down fuse clips on the side of the inside of the chassis, right over the PT. The HT wiring runs right around there anyway so at least I can't see a problem with this.

    That leaves the 6.3v heater wiring fuses. The only place I could find in this chassis, and not much room in there, is between the power transformer and main board. There is just *barely* enough room mount the clips. The problem is running the wiring. I would need to run the heater wiring around the power transformer to one side of the clips near where the presence control is located (not so close but near there). This will make the heater wiring much longer, and also, running from the "output" side of the clips over to the first power tube runs right across the junction of HV wiring leading to the choke from the main reservoir cap.

    The inline fuse holders I found didn't look to be much better a solution. I tried to find a place to put chassis mount (through chassis) holders, but, again not much room.

    Any suggestions on where to put the heater wiring fuses, greatly appreciated.

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    No need to put fuses on the heaters. Normaly the wire is so thick as any short here will determine the fuse in primary side to blow first and will not endanger the heater winding even a bit.

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    Senior Member Pedro Vecino's Avatar
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    Just a week ago an American guitarist (here in my city) accidentally connected his Vox AC30C2TV 120V to an output of the autotransformer with 230V. I thought the worst but fortunately only the mains fuse and filament fuse melt.
    I think it's better to have more fuses than less

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    Quote Originally Posted by catalin gramada View Post
    No need to put fuses on the heaters. Normaly the wire is so thick as any short here will determine the fuse in primary side to blow first and will not endanger the heater winding even a bit.
    I think you may be wrong there.
    Problem being that shorts tend not to be perfect, such that any slight impedance limits current but allows the heater winding to deliver way more than its rated current without primary side fusing being stressed.

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    Supporting Member loudthud's Avatar
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    I've seen the heater wires melt the insulation back to the transformer on a Fender blackface when a short occurs at the pilot light. On some years there are small resistors in series with the pilot light.

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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Just to note that regular fuse types / fuseholders aren't really suitable to low voltage, high current applications such as tube heaters; with such a low 'wetting' voltage, any slight gunk build up on the contacting surfaces and things get way hot, drop voltage etc.
    Whereas automotive fuses are a very similar application, so see what you can source.
    Another option are fuses with leads that solder in place like a regular leaded component.

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    If the he you were referring to was me, I've faced down some of the same issues. You're right - it is quite difficult to place the fuses.

    I'm with pdf on the heaters - if you can find suitable blocks to mount them in, automotive blade fuses are the simplest to deal with. I googled "automotive fuse blocks" and found many hits. Here's one source: http://www.wiringproducts.com/fuse-blocks

    In a typical amp, you'd need to fuse the primary (already done for you, probably), two high voltage leads on the secondary for B+, and one to three heater leads. That accounting comes from the idea that there may or may not be a 5V heater used for a rectifier tube, which would get one; one or two for the 6.3V heaters, depending on whether there is a hard CT used. If the hard CT is used, you need two. If there is a resistor CT, you only need one.

    The four-place automotive block may be good for this. Use two for 6.3v heaters, which are held near signal ground by the CT, and one for the 5V rectifier fuse. Use your pliers and rip out one spot in a four-position block between the 5V heater fuse and the others, to get more HV insulation spacing as the 5V heater is sitting up at B+.

    The inline soldered fuse is a good suggestion for sidestepping the problems with 5V being high current and low voltage, but sitting on top of several hundred volts of DC. You could encase this in shrink wrap, soldered in line with one of the 5V leads over to the rectifier tube socket, and also solve one of the other issues with fuses - how to run the wires so as to cause the least hum and noise. The 5V leads really need routed away from anything that might pick up rectifier hash.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Thanks, yeah, mostly worried about a $260.00 transformer melting down from a bad tube, before the primary fuse goes out. I have fuse clips, Attachment 48814 for the HV, didn't know about low V high I problem, thanks for mentioning it.

    I tried to find good inline holders but most look really crappy. Thanks for the suggestion about automotive. I can mount the HV clips on the side of the chassis, pretty much right in the path where the wiring is routed.

    The heater wiring, akk, no good place to put holders, and the automotive blocks need some space as well. Will get dimensions on the smaller automotive clip.

    So, I guess I know why the manufacturers didn't fuse everything properly, they would never be able to make any money.

    The automotive pushin fuses have a really small footprint! Didn't know they could be used for heater wiring. How cool is that!

    If I can't fit anything else in there, the solder-in idea, with heat shrink sounds great. Easy to test. Since its a low occurrence fail.

    I think I understand the low V issue a little, the fuse holders e.g. the chassis mount, barely touch the fuse. The automotive fuses have a fairly high contact pressure between the blades and fuse. The chassis mount fuse holders I have, do have a fairly high resistance. I can't rotate the fuse once pressed in and it fits in fairly tight. Is this the main issue?

    As mentioned above, solder in: I could push the fuse in and solder a very small bit of wire from the lug on the fuse holder to the metal part.

    But, then, back to the wire routing/location issue. :O

    How close do I need to keep the heater wiring from other stuff (presence control, the components on the end of the board for bias, HV wiring? Is there some minimum distance that I can use and say double?

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    Last edited by mikepukmel; 05-11-2018 at 12:53 PM.

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    Quote Originally Posted by mikepukmel View Post
    Thanks, yeah, mostly worried about a $260.00 transformer melting down from a bad tube, before the primary fuse goes out.[...]
    I tried to find good inline holders but most look really crappy. Thanks for the suggestion about automotive. I can mount the HV clips on the side of the chassis, pretty much right in the path where the wiring is routed.
    Fuses are tricky. They're in that class of things that seem to be absolutely straightforward, but get more complicated the more you try to apply them. They're sensitive to how much excess current flows, and for how long, and for snap-in fuses, you get the whole range of complexities of metallic contact metallurgy and contamination. Pure fuse issues aside, it seems you're having the contact and mechanical issues now.

    Automotive fuses are intended specifically to interrupt low voltage currents. Fuses have a voltage rating, meaning that for voltages higher than their rating, they may fail to prevent a long lasting arc from forming as they melt. cartridge fuses are generally rated for 125vac to 250vac. Auto fuses may only be rated for 50V or so. One decent option is the 2AG 5x15mm mini cartridge fuse. It's not as small as automotive, but nearly. They also come in little single-screw mount blocks. See
    MetricOMNIBLOCK here. Here's another: Littelfuse_Fuse_Block_254. They illustrate the singles, but they usually offer a 2, 3, 4 and more fuses wide block. The better fuse blocks are rated at 400Vac as well.

    I think I understand the low V issue a little, the fuse holders e.g. the chassis mount, barely touch the fuse. The automotive fuses have a fairly high contact pressure between the blades and fuse. The chassis mount fuse holders I have, do have a fairly high resistance. I can't rotate the fuse once pressed in and it fits in fairly tight. Is this the main issue?
    It's back to that "details matter" stuff. Good contacts require enough surface pressure between metallic contacts that the little microscopic hills and valleys on the surfaces grind together to get points of real metal to metal contact. The more points of contact, the better and lower resistance the total contact is. Each microscopic point contact can conduct X amps, and the total current carrying area is the number of contacting hills N times the current ability of each X amp contact. X depends on the metals in the contact wire and how flat it is. N depends on both how flat the contact surface is, and the area of the possible contact. Dirt and grease, sand, smoke particles, etc. fill the valleys until they start holding the hills apart and eventually N goes to zero and the contacts don't meet at all. Metal oxide films also interrupt hill-to-hill contacts. High pressure between the two metal surfaces breaks through the metal oxides and dirt, and re-insertion scrubs the two surfaces together and scrapes dirt and oxides off. Gold and other rare-earth metals don't oxidize in air, so they're best for premium contacts.

    We know that all insulators have a voltage rating, where the voltage will literally punch through the insulation. Happens with dirt, grease and metal oxides too. High voltages can punch through minor contamination. Low voltages may not be able to. So the auto fuses use wide contact area and high spring force combined with long wiping action on insertion to make up for the lack of high voltage punch through. AC power line and higher, and expecially AC voltages can punch through any minor contamination easily enough. So AC line fuses get by with much less contact pressure and long through wiping on insertion.

    For your B+ fuses, you want high voltage AC fuses. For your heater fuses, you want auto-style or similar. Auto style fuses in B+ lines may not clear on a fault.
    As mentioned above, solder in: I could push the fuse in and solder a very small bit of wire from the lug on the fuse holder to the metal part.
    It's a PITA, but it would work.
    How close do I need to keep the heater wiring from other stuff (presence control, the components on the end of the board for bias, HV wiring? Is there some minimum distance that I can use and say double?
    1. Heater wires carry low voltage AC. They can also carry AC line-borne junk. It is usually best to run heaters on twisted-pair wires which are run right along the metallic chassis. Even better, run shielded, twisted pair for heaters. Ground the shield at one end to the chassis. Twisting mostly keeps the AC voltage and line-borne junk inside the pair. The shield further attenuates any radiated capacitive coupling from the pair.
    2. Coupling into signal wires depends on radiated magnetic field and capacitive radiation. Magnetic field radiation is maximum with high currents and big area enclosed by encircling currents. You can't necessarily lower the current in the heater loop except by star-wiring the heater pairs (!) but you can make the area between conductors smaller. Twisted pair is about the optimum possible way to do this. Shielded twisted pair heaters is about as good as you can do without going to DC heaters.
    Coupling of hum into signal wires also depends heavily on the impedance of the signal wire being coupled into. Grid wires are the worst, as they're very, very high impedance. Tone/volume controls are also high-ish impedance runs, and you want them away from heaters; also not near the rectifier and first filter cap wires, Ideally, PT, rectifiers and first filter cap are at the far end of the chassis from any signal wires, and most especially away from the input jacks (which includes the reverb return, remember).
    3.Parallel wires couple more than crossing wires. If you cross heater wires at 90 degrees, even directly touching, coupling is minimal, especially if the heater wires are twisted. Avoid long runs of signal wire in parallel with heater wires.
    4. Given that your heaters are in twisted pair between tubes, you can usually stop worrying about distance from the heater wires after 1/4" to 1/2" from them.

    At least that's what I think while going through my first two cups of coffee.

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    Last edited by Steve A.; 05-13-2018 at 11:25 AM. Reason: Added [ to QUOTE]
    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Supporting Member Steve A.'s Avatar
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    I always added automotive blade-type fuses when replacing 24VAC power transformers on HVAC equipment as added insurance... I used 1/4" crimp terminals to wire them in and would wrap them them with Scotch 33+ electrical tape.

    In the old days I would use glass fuses in plastic holders but would have to charge extra for them...

    Steve A.

    P.S. Modern HVAC circuit boards often have 12VDC automotive fuses in line with the 24VAC control voltage supply but see RG's point in not using them for higher voltages.

    P.P.S. I believe that "Immortal Mods" was the name of a Dan Torres article in Vintage Guitar magazine back in the 90's... I don't think that writers like him were actually PAID for their articles but would receive a free stack of magazines that they could sell in their shop.

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    Last edited by Steve A.; 05-13-2018 at 11:37 AM.

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    I don't know about the "Immortal Mods," but "The Immortal Amp Mods" was a short series of articles written by R.G.
    I don't know the publication for sure, but I believe it was Premier Guitar, as after having occasionally run across them online in part, I ran across several of them in the actual issues when a friend gave me a huge stack of his old GP & PG subscriptions... I may have cut them all out and filed away hard copies somewhere...

    Yeah, I'm one of those odd folks who still likes to read actual PAPER...

    Justin

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    Interesting facts about low voltage fusible protection here. I wonder how low is low enough to consider regular 240v ac. fuses as a bad designation for low voltage dc. rail protection ?

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    Last edited by catalin gramada; 05-13-2018 at 04:03 PM.
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    Probably depends on the current more than the voltage. Voltage is an issue for getting current into the fuse filament. The microscopic hills making metal to metal contact and having enough voltage to burn away small bits of crud still applies, but how low a fuse can go for current depends on the design of the filament alone, which is a different thing entirely.

    Fuses interrupt AC better than they interrupt DC because the voltage and current go to zero - possibly at different times - twice per AC cycle. Any arc that forms has two chances per cycle to extinguish. But low voltage DC is easy enough to interrupt, as the voltage isn't high enough to form a self sustaining plasma arc.

    Car fuses were specialized for conducting high currents. I can't remember an auto-style fuse lower than 5A, although they may well exist. Cars use 12Vdc, usually at 10A or more.

    My best guess is that for currents in guitar amps, where 10A of heater current is high, don't sweat it. Either style will do. For solid state amps, if you have to put in a fuse, put in a 2AG or 3AG for last-ditch hail-Mary attempts to save things, and do solid state current limiters. Solid state limiting is the only thing that has a chance to save a high current device from overcurrent - fuses are way too slow and way too imprecise.

    And the real answer is to do what I do: go find and read fuse application manuals.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Quote Originally Posted by catalin gramada View Post
    Interesting facts about low voltage fusible protection here. I wonder how low is low enough to consider regular 240v ac. fuses as a bad designation for low voltage dc. rail protection ?
    Often you can use a particular fuse in a higher voltage application than it is rated for, particularly if there are no suitable options for an application. I know littelfuse recommends contacting themto get a recommendation for an appropriately de-rated selection.

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    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    Thanks again everyone. Another great thread to get printed, and put in that big binder of great amp topics. The HV is covered, the line voltage, still don't have a perfect solution for the heater wiring but not for lack of your knowledge, its just that the blocks don't fit in this tiny chassis. Geez, this amp building stuff covers many very interesting topics. I know there are many old amps without all these fuses, but I do recall a few that went up in smoke back in the day.

    Justin, yeah, that's the one, RG Keene, Premier Guitar. I made a few hard copies myself for my big binder of "stuff I would like to keep if I lose internet".

    From all your links found 4 and up auto blocks, but haven't found a 2 block yet, still looking. Maybe a 4 block and a band saw with a fine blade would work. i think I have to get this one "right".

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    Why don't to solder on the wire in line and put a varnish over it? It will not be replaceable but bet you no need it for next hundred years.

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    I commend your efforts to add appropriate fusing on the secondary side.

    One half of the coin is to add the fuse - the other half is to use a fuse rating and type that aligns with your particular amp.

    For heater use, that choice for rating may be somewhat obvious in that valve heater currents are generally known, and you would add some tolerance margin to allow for heater in-rush which can be 4-5x and may take a second or so to get to 2x. That inrush normally points to a T (timelag) type of fuse, and that type may be better if a soft fault occurs, but that is when it can get a bit complex to sort out. You really need to identify what spec the fuse is made to - as that could be unknown (ie. automotive), or UL284 or IEC60127 if a 5x20 or similar style.

    For B+ use, PSUD2 is I reckon your best friend, along with a measurement or two, to start the selection process. Then it comes down to what fuse model and type and spec you can get. Of course you can wing-it with a value that you have to hand, but its good to appreciate what that means.

    Some additional fuse selection discussion and selection design is in link.
    https://dalmura.com.au/static/Valve%20amp%20fusing.pdf

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    NTC thermistors are a handy thing to remove start up current surges, and avoid the need to use T type fuses.
    They can be used in the primary and secondary circuits.
    Modern Fenders tend to have them in the primary circuit, and many of those use regular F type fuses.
    Their downside is that if the amp has been in operation but then get switched off accidentally, flipping it back on too soon (ie before the thermistor has had chance to cool) can pop an F fuse.

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    As a side note, I mentioned solid state current limiters. There is a suitable circuit here:
    Tube Amp Current Clamp

    This circuit prevents the current from exceeding the specific current setting - ever. Well, at least as long as the limiter circuit doesn't overheat, die, and let the current go to infinity. It is possible to integrate an opto isolator into this that would signal a circuit back down at ground voltage to shut things down if the overcurrents exceed a preset time.

    This may sound like a PITA, but it is dramatically more predictable in its protection than a fuse can be, and opens the door to even more sophisticated protection schemes.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Its worth appreciating how intrinsically fault tolerant a heater supply is nowadays.

    Vintage heater supplies were often chassis grounded on one side of the heater voltage - so a single fault to chassis would cause a direct short. Similarly, a CT taken to chassis was also a single fault situation. Wire insulation wasn't as good with higher temperature or age. Although one advantage was that octal bases had lots of clearance between pins.

    Valve heaters don't fail internally to a bolted short circuit - which is a major difference to B+ related faults due to internal valve faults, which can also cause collateral damage to an output transformer.

    A humdinger, or elevated heater supply provides fault tolerance, as one fault to chassis does not cause a short circuit.

    A modern production amp, or good diy effort, is likely to take enough care to make heater short circuit situations very rare.

    I should do some heater inrush measurements on common valves, just to check how spread the response is, and how influential the heater winding current rating is, and how limited a heater winding peak current is when fully loaded to its rated continuous level (both at turn-on and if a fault occurs during operation).

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    Thanks everyone, for the links, and info. Reading...

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    Senior Member trobbins's Avatar
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    I'd place a caution against using automotive blade fuses for heater supply protection.

    A check on the typical industry standards, and datasheets from major manufacturers, for the current-time performance characteristic shows a wide range in 'operate' time for current levels 1.6 to 3.5x the fuse current rating. It is likely that a blade fuse could blow during heater in-rush, even if the fuse rating was 2-3x the heater supply current rating. That's not to say that a particular fuse would blow.

    The link indicates the range of the peak current during in-rush for a variety of common audio valves. What is not shown is the time response of that in-rush current.
    http://www.gammaelectronics.xyz/audi..._filament.html

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    And I view that as another vote for active solid state watching for overcurrents.

    Fuses are extremely, extremely hard to predict blowing currents and times for. A fuse's rating is, as I've said before, not the current it will blow at. It's the current it's guaranteed NOT to blow at. A 1A fuse is guaranteed to carry 1A, and to open at some current above that. the manuals on choosing fuses for blowing characteristics rapidly get back to talking about the I-squared-T through the filament. That's the time function integral of energy delivered to the filament, and is related to the power dissipated by the filament resistance and the heating time until it gets to a melting temperature. That has to be compared to the details of how heat gets out of the filament to cool it. Estimating melting time rapidly becomes a complex heat flow calculation/modelling in three dimensions over time.

    The best mere mortals can get to is to use charts and graphs, estimate safety ranges, and pick one.

    The advent of $0.50 microcontrollers with many channels of A-D and easy to apply opto isolators makes this all get simpler. You can measure current in real time, do moving-average RMS calculations (even if they're table lookup) in real time, and make decisions based on several criteria when to stop the incoming AC power line, and when to allow it to come back on.

    $0.50 is really close to the ones' cost of a single fuse.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Just as I hit "submit" on that, I remembered something else.

    I did the math on whether you can count on a heater fuse to blow the AC line fuse. Turn out, it may blow it, but you can't count on it. The heater lines can get wildly hot and not blow the AC line fuse, so they cook for a long time, and eventually burn out the heater winding insulation.

    But that takes a long time. Heater windings in particular have a high thermal capacitance, so it can take a long time of overcurrent before they're damaged. Precision is less necessary for heater windings than for other, smaller windings.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Quote Originally Posted by Steve A. View Post
    I don't think that writers like him were actually PAID for their articles but would receive a free stack of magazines that they could sell in their shop.
    I have actually written tons of articles in Guitar Magazines so know the deal firsthand: they actually "pay" you, even if a symbolic amount, for copyright/legal reasons: that way they "own" the article , you lose rights so you canīt ask/sue them later for whatever $$$$ you want for it; also if later , say, you write a book including that article or want to republish it in another Magazine, you need their written permission.
    You *can* post a link to said article (or a scan) in your own Net page, but mentioning their Magazine.
    One usual payment type does not involve cash but they publish your Ad for free, actually in exchange for the Article and this is clearly mentioned somewhere, usually they bill you for the Ad but write: "paid for with xxxxx published in the same issue" or the equivalent.
    Don't ask for half or full page ads of course, usual is 1/8 to 1/16 page Ad ... which is actually quite visible.

    Magazines which are actually built around free lance writers, say 3 Guitar players explaining Scales or Licks or onstage setups , a couple tech articles (Electronics, instrument setups, Recording or Live sound tips, etc.) plus, say, a singer explaining Vocal exercises or whatever, or a band Manager explaining how to get gigs or transport cost or whatever , will often dedicate a full page, divided in 8/12/16 small "contributor Ads", or if not enough to fill a page, will spread them all over the Magazine, to fill white space.

    The real payment of course is that those little ads bring lots of customers and in general keep you in the Public eye

    FWIW I often get people calling me and mentioning some Tech article I wrote as far back as the 80īs , go figure.

    Similar to what Justin Thomas says about RGīs Premier Guitar articles.
    Guys here also keep clippings or photocopies and now and then upload some old scan ... it doesnīt hurt at all

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    Juan Manuel Fahey

  26. #26
    Senior Member trobbins's Avatar
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    Quote Originally Posted by R.G. View Post
    A 1A fuse is guaranteed to carry 1A, and to open at some current above that.
    Just to embellish a hassle with fuses - for the generic 5x20mm or *AG style miniature cartridge fuses, the fuses with IEC60127 standard compliance have that "1A fuse is guaranteed to carry 1A" rating, whereas a UL248 (ie. US centric) compliant fuse is only guaranteed for 0.8A continuous from a 1A fuse. And many people would I suggest have no clue as to what standard their fuses are compliant to (if indeed they are quality made fuses).

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    Quote Originally Posted by trobbins View Post
    I commend your efforts to add appropriate fusing on the secondary side.

    One half of the coin is to add the fuse - the other half is to use a fuse rating and type that aligns with your particular amp.

    For heater use, that choice for rating may be somewhat obvious in that valve heater currents are generally known, and you would add some tolerance margin to allow for heater in-rush which can be 4-5x and may take a second or so to get to 2x. That inrush normally points to a T (timelag) type of fuse, and that type may be better if a soft fault occurs, but that is when it can get a bit complex to sort out. You really need to identify what spec the fuse is made to - as that could be unknown (ie. automotive), or UL284 or IEC60127 if a 5x20 or similar style.

    For B+ use, PSUD2 is I reckon your best friend, along with a measurement or two, to start the selection process. Then it comes down to what fuse model and type and spec you can get. Of course you can wing-it with a value that you have to hand, but its good to appreciate what that means.

    Some additional fuse selection discussion and selection design is in link.
    https://dalmura.com.au/static/Valve%20amp%20fusing.pdf
    I spent quite a bit of time scanning for more detailed info on fuses, but couldn't find much other than the bare bones ratings. Yeah, definitely appreciate the issues. Would be bad to go through all this trouble, and still be able to smoke the transformers before the fuses blow. Again, great article.

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    Quote Originally Posted by R.G. View Post
    Just as I hit "submit" on that, I remembered something else.

    I did the math on whether you can count on a heater fuse to blow the AC line fuse. Turn out, it may blow it, but you can't count on it. The heater lines can get wildly hot and not blow the AC line fuse, so they cook for a long time, and eventually burn out the heater winding insulation.

    But that takes a long time. Heater windings in particular have a high thermal capacitance, so it can take a long time of overcurrent before they're damaged. Precision is less necessary for heater windings than for other, smaller windings.
    From your calculations, how far over the rated N amps (whatever that is) is the draw that takes a long time to burn the winding but not blow the fuse? its sounding more and more that, as others e.g. Juan has said, might not be worth it to fuse the heater windings?

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    The higher the current, the shorter the time the fuse will hold for.
    Find a manufacturer's fuse info sheet to see.
    It will be 'typical characteristic' rather than a spec.

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    Quote Originally Posted by mikepukmel View Post
    From your calculations, how far over the rated N amps (whatever that is) is the draw that takes a long time to burn the winding but not blow the fuse?
    The calculations were not general-case sorts of things. The context is that a particularly-adverse-to-learning guy in this forum decided that it was flatly impossible for a fault in an amp PT secondary NOT to blow the AC line fuse, and concluded from this seat-of-the-pants assertion of his that secondary fuses were never needed, and an AC mains fuse was always enough. I measured a few PTs and looked up wire resistances for some PTs on line, and then constructed a plausible example.

    When you short a transformer secondary, the current is limited by a few things. These include:
    - resistance of the short itself, as all "shorts" are not necessarily good, solid, welded-on metal connections; if it's a short to chassis, for instance, the resistance of the chassis itself is added in, and that may not be negligible, especially for an oxidized aluminum surface or a thin steel chassis.
    - resistance of the secondary winding and the wiring to the location of the short
    - leakage inductance of the PT from primary to secondary
    - resistance of the primary winding and the AC mains wires, AC mains fuse, switch contact resistance, all that minutae.
    Each of these numbers is unique to the PT and its installation. You can make realistic estimates of the ranges of each of these.

    With that as a base, you can calculate a reasonable secondary and primary current for a guessed-at short circuit, and then make a reasonable estimate of the change in primary current for a secondary "short". The heaters are the bad ones because of the extreme turns ratio between primary and secondary. The turns ratio for a 120Vac mains and a 3.15V (half of a 6.3V) heater winding is 38.1. So 38 amps in a heater half-secondary increases the primary current by 1A. So for a heater winding with a grounded CT, you can get a current that is limited to a significant fraction of 100A by the resistances of the short, wiring, and secondary winding that only bumps the primary up a little. Especially if the amp is simply sitting there, not being run at full output, and moreso if the primary AC fuse has been sized about 25% higher than max normal current, the primary current may well be an amp or two below its stated fuse rating, and the extra amp won't even push the primary fuse out of its rating, ever. Or the extra amp may just kick it a bit over the fuse rating, into perhaps 120-150% of its rating.

    Primary fuses are universally time delay types because they have to sustain the power-on inrush current. So they're designed with a slow response to sudden heating. As a result, the primary may take a long time to blow. As in hours for just a little over its rating, or it may never blow if the added amp or so has not even pushed it above its nominal rating.

    So the half-secondary can simply sit there and cook, heating just the heater winding and whatever's next to it, until the insulation overheats and the wires touch. At that point, smoke comes out and you get a chain failure that eventually opens the AC mains fuse, but by then the PT is effectively dead.

    its sounding more and more that, as others e.g. Juan has said, might not be worth it to fuse the heater windings?
    It all depends on what level of risk you're willing to take. It is quite true that the majority of guitar amps run for several decades, maybe for a century without sustaining a half-heater short. In concocting the "Immortal Amp" idea, I was struck by the thought that some things in a guitar amp regularly need replacing. Tubes are obvious, but electro caps do so as well, and then other things have semi-predictable failure modes. So I did the thought experiment native to engineers - if you can't make it last forever, make it easy to fix, and provide protection for things that are very expensive to replace.

    If you put in heater fuses, you can be reasonably certain that a heater short won't destroy your PT. But then the odds are that if you wire the amp correctly and use good quality tubes, it wouldn't ever short anyway. Shoot, there's even a mechanism that ages fuses; Repeated near-maximum currents flex the filament and can make it simply break. But fuses are cheap, PTs are not. You have to decide what level of risk you want to assume.

    If you think you'll never get a half-heater short when you aren't there to notice it and shut things down, fuses are unlikely to help you. If you want to make really sure you won't ever have the problem, fuses are cheaper than PTs. That is "worth it" is a calculation YOU have to make.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Senior Member SoulFetish's Avatar
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    You ever run across amps that fuse the bias supply?

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    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

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    Quote Originally Posted by SoulFetish View Post
    You ever run across amps that fuse the bias supply?
    That would be unwise...imo

    but im not going to fuse any 6.3vac heaters either.

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    Quote Originally Posted by SoulFetish View Post
    You ever run across amps that fuse the bias supply?
    As dstrat says, bad, bad, bad idea. The bias supply may be thought of as a separate power supply voltage that is itself a protective mechanism. It protects the power output stage from destroying the output tubes, output transformer and possibly the power transformer too. You never want the bias supply to shut down.

    It's generally unlikely that it will shut down, as the bias supply is a low-power, very low current thing, so it's easy to simply use massively derated parts to run up the reliability of the parts. It has a high impedance output to the tube grids, so there's not much chance that a shorted wire to a tube grid will kill the bias supply. Realistically, a fault in the bias supply rectifier or filter cap will let the output tubes run wild. The outputs serve as an "indicator" that the bias supply is having problems by getting too hot, overloading the main PT, sending up smoke signals and other amusing and interesting effects.

    In fact, along with the ideals of the Immortal Amp, it might be a good idea to have some circuit sense the presence of the bias supply and turn the entire amp off if it was not of sufficient magnitude.

    Quote Originally Posted by dstrat
    but im not going to fuse any 6.3vac heaters either.
    It's a personal risk assessment we all have to make.

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    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

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    Yes It will likely bite me sooner or later with modern tubes

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  35. #35
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by SoulFetish View Post
    You ever run across amps that fuse the bias supply?
    Doesn't SEMKO demand a fuse on the bias supply? (Safety certification for European Community + Switzerland = SEMKO)

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