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Thread: Power Transformers With Internal Thermal Protection Fuse

  1. #1
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    Power Transformers With Internal Thermal Protection Fuse

    Friends...
    As a novice tech, I had no idea that Power Transformers have an internal protection fuse. I thought this only existed in hair dryers!!

    I recently encounted a blown fuse inside the transformer on a Fender amp. It seems cumbersome to go in and replace the fuse, then reassemble the transformer and hope it will not short out internally.

    So my questions are:

    (1) How common is this?
    (2) What is the point of having an internal thermal fuse when the amp already has an external fuse?
    (3) When buying a replacement transformer, how do you know if that one also has an internal fuse?

    Again, it seems redundant to have the thermal fuse- especially INSIDE the transformer!

    Thanks for your comments... Tom
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    Sometimes you can replace the internal fuse. But generally you would just replace the whole transformer since it's often more trouble than it's worth or the fuse is not easy to get to. This from what I've read here. I haven't had the pleasure.

    The internal fuse should be rated higher than the external fuse. It's there as a fire prevention measure and probably a legal issue for the transformer MFG. Unfortunately there seem to be some transformers in current amps that are prone to popping their internal fuse. This is an expensive glitch for the amp owner.
    "I should have been born sooner. Of course, if I had been, I might be dead now." trem

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    Ok Chuck....

    That makes sense. I appreciate the reply.

    Something I learned long ago- LOOK at the fuse to verify it's value. It could be that the guy who serviced the amp before you decided to over-fuse!

    So I now understand that in cases like that, the internal fuse acts as a backup.

    In my case, the amp did have the correct fuse. But again, who knows, someone might have over-fuse, burned up the transformer, then stuck the original fuse back in.

    Tom

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    How on Earth did you find that.?
    I also was not aware of this fuse.
    Anybody know what model amps/transformers had this.?
    I have a SF Deluxe Reverb with a bad PT.....
    And as you say, it is probably advisable (for a few reasons) to just get a new PT.
    Thanks

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    It's a way to meet the requirements of the safety regulations. There are standards for fire and electrocution safety. An overheated transformer may short out and cause a fire, and the standards say "it can't do that". More importantly, they require it not do that even with any single, worst-case component failure in the box. The standards do not say "put in a thermal fuse"; they say "it can't cause fires or electrocution", and how that's done is up to the maker.

    A thermal fuse is a good answer to this in many ways that are important to the makers and safety testers. It's pretty sure to shut down things on overheating before fires get started. The problem is, musicians and manufacturers differ in their opinions of what constitutes an OK overload.

    To answer your questions:
    How common is this?
    Very, very common, and also unknown until you disassemble the transformer.
    What is the point of having an internal thermal fuse when the amp already has an external fuse?
    They protect against different things. The AC mains fuse protects from electrical shorts that may also be outside the transformer. The thermal fuse protects from internal shorts and fires in some cases.
    When buying a replacement transformer, how do you know if that one also has an internal fuse?
    You can be pretty sure that if it's made by the original manufacturer, it does. Another replacements source may not. However, pay attention to what follows:

    The manufacturer goes to the trouble to specify that inside the transformer at least partially to limit their legal liability in case of a failure. The way that works is that if they have complied with all the safety testing, it's hard for a judge to say they were reckless in endangering customers.

    By the same reasoning, if YOU replace it with a transformer without a thermal fuse, you are taking the liability on yourself that the replacement will not overheat, start a fire and injure or kill someone. Remember that.

    Next point: if you repair the transformer by putting in a new thermal fuse in the old transformer, do you then subject it to the necessary safety testing? The answer is no- you can't, because the tests are destructive. No non-destructive tests you can do on it will do. And it is unlikely that you'll be good enough at transformer repairs to meet the thermal, electrical insulation, and creepage-and-clearance requirements inside it. If you did, it would involve a complete tear-down and rewind. A rebuild would take enough time to cost more than a replacement in bench charges.

    This is just my view of it. In my opinion, you order an exact replacement and put it in. But then I've been trained to be very, very afraid of lawyers. Of course, you may be less concerned with this. Up to you. You have been warned.

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    let me guess, is that Blues Junior PT? I've replaced a few of those (Fender Tech). Those amps tend to get run very hard, and they always sound awesome (just before they blow). Do not try to repair that. Replace with an exact Fender part, supplied by your local friendly Fender Dealer. Be sure to shop around and spend some cash while you are there!

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    Short example: your transformer may work regularly, for ages, at, say, 45C with an outside temperature of 25C. To consider very hot days, many hours of hard rocking and a basement or closed, soundproofed rehearsal studio with no wind at all, they *may* find acceptable that it reaches, say, 65C, which is very hot (70C is unbearable to the bare hand).
    Suppose the wire insulation can stand 120 or 140C *but* the nylon bobbin starts to melt or deform at 85C.
    The manufacturer may decide to split the difference and fit a 75C thermal fuse , exactly midway between 65 and 85.
    You can't complain to them: "your thermal fuse killed my perfectly good transformer at 75C", because if you follow the series of temperatures (under different conditions) I mentioned, you had no business letting it reach that temperature, which in anybody's book *is* a gross overload, well beyond any reasonable working condition.

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    RG and JMF. Would it be possible to have some sort of resettable thermal fuse? It is a crime to have to replace the PT if the internal thermal fuse blows for no apparent reason. Things DO fail.

    In some instances, a repair might be the best or only option. I purchased a non-working Fender Dual Showman Reverb head (the much maligned red know model) for close to nothing as a project. It was immediately apparent that the PT was not working although it did not show any signs or smells of abuse. As it turned out, the reason that the head was so cheap is that it is virtually impossible to find a replacement PT and I assume that the seller had already discovered this. I pulled the PT and sent it to Heyboar and they repaired it for much less than a new PT would have cost if I could have found one (about a quarter of what a custom build would have run). They also confirmed that there were no signs of a PT meltdown when they pulled the bells. The head works fine and the PT doesn't seem to run any hotter than other amps I have.

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    Quote Originally Posted by Kazooman View Post
    Would it be possible to have some sort of resettable thermal fuse? It is a crime to have to replace the PT if the internal thermal fuse blows for no apparent reason. Things DO fail.
    It's a game of weighing imponderable quantities.
    -The manufacturer have to pass safety inspections because of the possibility of huge legal awards in the unlikely event that an amp fails in just the right way to cause a fire or a death.The monetary losses for that are big enough to wipe them out.
    - It is unlikely that very many of the transformers will *ever* blow their thermal fuse. Understand that "not very many" is way down in the sub-1% range.
    - Of those few, only a tiny fraction will be traced to the thermal fuse.
    - Of those, only a few owners will be told the cause, and only a few of those will be angry.
    The net result is that sales will be a lot smaller if it's even a few bucks more expensive, and only a very, very few customers will be unhappy, and that will be years later. It's a good reason not to spend the extra money. Manufacturers who spend the extra money have lower sales, stay smaller, and go out of business more often. It's a Darwinian situation - only the manufacturers who pinch pennies survive and become big. The little ones fail more often, and this selects for the big ones, too.
    - It is possible to have a resettable thermal fuse. It is just much more expensive than the non resettable kind.
    - resettable thermal fuses are more prone to failure themselves because they're more complex than the non-resettable ones; as you note, things DO fail
    - Whether you know it or not, you are a very rarified, unusual, informed consumer of amps.

    In some instances, a repair might be the best or only option.
    You have to look at this from the viewpoint of the government, legal system, and manufacturers. In their view, "repair" means "replace the failed transformer". It does not mean that someone should hack in a fix and compromise the safety features and testing.

    There's another side to this. If you own an amp, and it fails with a thermal fuse in the transformer, then YOU insist that the tech repair that transformer instead of replacing it with the approved, tested replacement, and later sell the amp to someone, who is harmed by the amp, the courts can - and have - held that YOU would be partly responsible for the harm and according to the principle of joint and several liability, make YOU responsible for 100% of the eventual damages, even decades later.

    I am very much aware how much this chain of events is like trying to stack marbles. Still, very similar things have happened.

    ... it did not show any signs or smells of abuse... there were no signs of a PT meltdown when they pulled the bells. The head works fine and the PT doesn't seem to run any hotter than other amps I have.
    Yep. I believe you. However, from the maker's viewpoint:
    - the amp lived out its warranty time; the fulfilled their obligations to the customer
    - there were no internal signs of overheating, so the thermal fuse (if that's what it was) did its job and *prevented a potentially bad situation from getting worse*; the whole point of protective devices is to sense the edge of a damaging situation coming, and stop things before it gets irrecoverable; viewed in this way, the thermal protector (if that's what it was) did its job perfectly
    - the bad situation may have had nothing to do with the transformer; it could have been a high ambient temperature outside the transformer; note that a high temperature *period* is what softens and degrades insulation, so even if the transformer would have survived the temp, the protector should have opened, and was doing its job; or the protector could be faulty
    - a rebuild by Heyboer is probably a solidly built, reliable thing; that's not what the government, safety inspectors, legal system and manufacturers think of when they hear "repair a failed transformer"; they think of some junior tech with no experience or training, wrong materials and no idea about safety issues jury rigging a transformer in a clumsy, sloppy way

    In some countries it may well be a crime to repair and refit the same one. In the USA, it's not. But there are big legal penalties for being wrong and unlucky if the marbles do stack up.

  10. #10
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    *Excellent* analysis.
    In my particular case, I live in Argentina where thermal fuses the type we are talking about are not available.
    To be more precise, since they needn't be fit by Law, 99% of electronics suppliers don't even know what I'm talking about, and only a couple very tiny shops carry them.
    I'm talking, for example, a guy who repairs plane electronics, controller boards for monster newspaper printers and stuff like that.
    He has, for example, tiny toggle switches or 1/2" single digit LED displays for which he charges U$50, each.
    Why?
    Because they are the exact toggle switch or Led digit or whatever needed to repair a Collins HF9070 airplane radio, and they need it *now !*.
    Well, this guy does carry thermal fuses of all kinds, along tons of wondrous stuff, but at his prices .... forget it!!
    At the same time, I can buy tons of coffee percolator type bimetallic switches, which of course are self-resetting, for peanuts.
    Guess what I use.
    Before stoning me, remember that:
    a) I'm not in the USA
    b) Thermal fuses are not mandatory here, so I'm in fact *increasing* reliability or safety compared to the masses.

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    Thanks for the great analysis. I'm in the "fix it for fun" camp and don't plan to resell the head so I guess I needn't call my lawyer just yet.

    Having said that, I really should look again to see if a replacement PT has become available. I tried my best at the time and there were none to be had.

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    Quote Originally Posted by J M Fahey View Post
    Before stoning me, remember that:
    a) I'm not in the USA
    b) Thermal fuses are not mandatory here, so I'm in fact *increasing* reliability or safety compared to the masses.
    I'd be the last to criticize that. It is a very rational response to your situation.

    The funny thing about the USA situation is that thermal fuses are not mandatory here either. But if you guess wrong, and are legally unlucky, the possible losses are very big. Not only that, but if you have a lot of unhappy customers *with* the thermal fuse (for instance) then those customers can sue you for putting them in, on the theory that you made a defective product that fails too much.

    What's really going on is that the product liability law "industry" in the USA has become a kind of legal-industry lottery. A very few people get lucky in huge ways and the publicity on that is enough to get nearly everyone sued for something at some time, even if it's completely irrational and unfounded, and the vast majority of these suits are thrown out of court. So there is some customer, somewhere, that will search diligently to find anything you *might* have done wrong, or failed to do right enough. I have had to adapt my views to that upside down state of affairs. Ugh.

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    Senior Member Enzo's Avatar
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    You really don't want resetting breakers. At least not the self-resetting kind. Those will open when a failure arises, but then they sit there clicking back on every so often. Once powr is removed from the failed circuit, we don't need it powering back up for a few seconds once a minute.

    Your mains fuse opens when current is excessive. The thermal fuse inside the PT opens when the PT gets too hot. Why might you want both? That mains fuse represents an amout of power - in other words a 4A fuse on a 120v main means 480 watts can flow through that fuse all day. Your transformer can get extremely hot without using all those 480 watts. You don't want your transformer melting down while the fuse continues to handle it. SO a thermal fuse protects against that.
    Education is what you're left with after you have forgotten what you have learned.

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    The circumstances on a transformer are different. Most transformers of any size have a thermal time constant of half hour or more because of their high mass. A resettable thermal fuse inside a good sized power transformer would take at a guess 10-30 minutes to cool enough to click back on, and the same to get hot again if it was not massive overload causing the heat rise. I've done this test before. Things like heat sinks and small circuits with shorter time constants do the on-off cycling. Transformers are thermally ... slow...

    My choice would be to have the thermal breaker cause a "thermal overload" indicator come on to provide info so the user could make an informed decision about when to turn it back on. I've seen some guitarists where no amount of information would help, but that's an other issue. We can't sue their parents for that.

    Yet at least. 8-)

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    The couple times (no more) it happened I got the amp back on Monday, with the comment: "don't know what happened Saturday night, the amp turned off, we checked everything but couldn't turn it on again, had to use a backup, yet on Sunday, back at the rehearsal room, it turned on and worked perfectly well, please check it just in case".
    Obviously after 5 "endless" minutes it had not yet cooled enough, and the show *had* to go on.
    Oh well, being an impatient Teen again !!
    My only explanation is that they connected "just" an extra speaker, and the lower load impedance was enough to overheat it but not enough to trip the short protection.
    Mine triggers an SCR which disconnects the speakers through a relay, you have to turn the amp off 1 minute for it to self-reset and, of course, remove the short or it will trigger again.
    I've had complaints (even spread locally on the Internet) about my amps "turning themselves off for no reason, 10/20/30 times in a row"
    Oh well.
    EDIT: the "thermal overload" sign would be a good thing, can be backlit with a neon lamp across the thermal switch contacts.
    I *used* to put a "short-check speakers" labelled Led across the Relay coil/bobbin (it's energized to *cut* sound) but Musicians didn't like it, they were angry that it cut their sound at all.
    Soundmen (which are supposed to be Professionals) were angry that with my amps "they could not add/pull an extra monitor in the middle of a song because that killed that entire monitor net"
    The idea that you don't do such things in the middle of a song fell on (literally) deaf ears.
    Oh well [2]
    Last edited by J M Fahey; 06-21-2011 at 01:16 AM. Reason: Read comment

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    I know best practice is to replace the transformer as it's safety may be compromised but when its encased in metal on a metal chassis with the correct value fuses in the device it powers I consider it to be safer than a lot of the domestic cheap heaters (of the "warm the house" variety).

    The main culprits I gather were the under rated wall wart plug packs powering answer phones faxes etc that were left on all the time.

    It seems in order to be competitive and keep the price down the less copper
    the better.. they always seem to get remarkably hot and when on skirting boards and around curtains well it's um curtains for a lot of them causing house/office fires.

    I guess this is why switchmode are leglislated in by their nature ..and yes RG 1 spot adapters are now in kangaroo land !
    Deluxe Guitars - South Melbourne VIC Australia.

    HTH's post re Fender Stage 1600's 105 ($166) transformer
    and thermal fuse issues help with SS amp repair (Fender Stage 1600)
    + some pics of typical therm fuses and symbol
    Last edited by oc disorder; 06-21-2011 at 07:23 AM.

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    All of the aforementioned points are extremely valid. If you own or operate a repair shop, here's one more: replacing the thermal fuse in a transformer voids all safety ratings (UL, CE, CSA) and puts you in the direct line of fire (no pun intended) should an accident occur as a result of the transformer tampering. And rightly so. Severely overheated transformers should NOT be put back into service.
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    I have had 3 classic 30's with blown therm fuse transformers they pop at about 144C....
    144C? Wow!!
    That's too much!!
    Well into the plastic bobbin melting area, which by definition renders the transformer useless, even if (still) not shorted.
    Not forgetting that temperature inside the transformer is not uniform, there may very well be some "hot spots" 20 or 30C above the average.

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    Let's not forget that these fuses also blow at a certain amperage, so a quick over-current event might be to blame. That fact is what actually bugs me the most about these embedded fuses - if they were purely temperature-sensitive that's all well and good, but in fact they can blow due to current draw so in my opinion should be more accessible for replacement.

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    Wasn't sure about this, so I checked it up:
    Wiki says (in "Thermal_Cutoff"):
    Unlike electrical fuses or circuit breakers, thermal fuses only react to excessive temperature, not excessive current, unless the excessive current is sufficient to cause the thermal fuse itself to heat up to the trigger temperature.
    Cantherm Thermal Fuses from Cantherm says:
    In order to repair the circuit, the complete thermal fuse must be replaced. Thermal fuses have a solid, dust and dirt- tight housing. They react to ambient temperature and are generally insensitive to current at rated levels.
    So in general they state that although in an indirect way they may have some current sensitivity, it's only a byproduct of the thermal issue.
    EDIT: just checked Maplin Thermal Fuses : Thermal Fuses : Maplin : the ones they sell blow at 40A or 20A , independent from temperature.
    That's *way* above any reasonable fuse specified in a regular guitar amp.
    Last edited by J M Fahey; 06-21-2011 at 10:40 AM.

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    Yep. They're designed to be as insensitive to current as possible. However, there is some current that will melt *anything*. The self heating in a thermal fuses is pretty negligible.

    Another issue is that to work well, a thermal fuse has to be in contact with the thing it's protecting, and ideally imbedded inside so that air cooling does not keep it cooler than what it protects. There are ways around this, of course, but they're expensive and require more care - and money - in making the transformer, so they don't as a rule get done. The thermal cutouts are put in because they're *less undesirable and expensive* than other ways to meet the letter of the specifications on fire safety.

    Fundamental to the idea of protection devices is the concept that they in effect decide to stop things before damage actually occurs. So if a protective device operates, it is likely that things aren't damaged. It would be a reasonable attitude to say "Whew! I'm glad that operated and kept me from having to pay to get actual damage repaired.". But that's not what happens, with musicians at least. Their reaction is "Hey! It just quit. Nothing is damaged, so it should not have quit. Must be that $#&*() protector keeping me from doing my music."

    Back in my power supply design days I once had a field problem with an installation where a 5V, 25A supply was intermittently going down, but not having any permanent damage. The customer was very frustrated that these "false trips" were giving them problems. One of our power supply group came up with a good solution. We'd put a 50A relay in there and wire it to the shutdown signal so when that was detected, the relay would reverse 5V and ground and burn out 100% of the logic in the thing. This would convert an intermittent into a solid failure and make the customer happy. Right? >8->

    Took replacing the whole machine, and when we finally found the bug, there was a logic flaw turning on all the logic gates opposing each other at times, tripping the overcurrent. They never let us use the relay.

    If nothing is damaged, customers think that all protection is worthless, not saving their butts before damage happens.

  22. #22
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    A hobbyist who uses the gear only himself might thin bypassing the thermal protection is a reasonable risk....but is it? By not having UL CE or other compliance, your home owner's insurance is also voided for damage caused by your disabling it. Any shop that does it, should be closed down as a threat to the community. Look back through archives of newspapers before UL fire certification and you will see that house fires were very common in the early days of electric service to homes, it was a major cause of death. It still is in countries that overlook regulations. More people where I live die each year from electrical fires even now than any other form of home event. In the US, there are 310 deaths average a year according to FEMA, only 1/100th the shooting deaths. The introduction of strict safety protection regulations has dropped the electrical fire death rate by a factor of 100.
    The thermal switches are not current fuses, they open when the transformer has gotten so hot that it can no longer be safely put back into operation. The transformer is heat damaged. You can replace the thermal switch if for your own unit but not for a customer.

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    Yet (with my limited experience at least) these thermal fuses DO seem to pop with inrush current. Case in point: our kitchen blender, cost well over $100. Worked great for about 2 months then went dead on turn-on. Motor winding with thermal fuse tested open. Returned under warranty. Replacement worked about a month...then same thing. Died when turned on, motor winding tested open. Returned AGAIN under warranty...guess what! SAME THING. The 4th unit has lasted a few months now. (Internet searches showed a number of complaints on same model blender with similar symptoms)

    My guess is that the motor manufacturer bought a batch of thermal fuses made under poor quality control. So maybe in theory they should not have been current-sensitive, but certainly the inrush current did all of ours in, and the blender in each case was cold, it had not even run yet.

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    Maybe, why not ?
    Although there is also another possibility: all thermal fuse manufacturers *insist* on not soldering them, for obvious reasons, but crimping or similar "cold" mounting methods.
    A poorly crimped one, coupled to a strongly vibrating environment (blender motor) is not a recipe for reliability.
    Just as a side thought, it might very well have failed from any other cause, including a purely electrical one.
    Unfortunately, pouring $100 into a blender is not a guarantee of quality anymore.

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    Quote Originally Posted by nashvillebill View Post
    Case in point: our kitchen blender, cost well over $100. Worked great for about 2 months then went dead on turn-on. Motor winding with thermal fuse tested open ... (Internet searches showed a number of complaints on same model blender with similar symptoms)

    My guess is that the motor manufacturer bought a batch of thermal fuses made under poor quality control. So maybe in theory they should not have been current-sensitive, but certainly the inrush current did all of ours in, and the blender in each case was cold, it had not even run yet.
    I don't see any reason for "and yet".

    It should be self evident that there are cases where an overcurrent inrush will melt anything. I've melted solid steel rods about 1/4" thick accidentally. I was using a welder set to limit current at 120A. The locked-rotor current of a faulty motor can get to that in the milliseconds before the wall-breaker trips. In the absence of more information, it would be just as reasonable to say that the motors were faulty and were killing properly made thermal fuses.

    Of course they're current sensitive. Everything is. But that's not what to focus on. If I were responsible for a design that was burning out thermal fuses on an inrush to a cold transformer or motor, I would carefully examine what the inrush current was, and whether the thermal fuse even had a chance to survive.

    Hmmm. A little thought leads me to most devices needing some way to prevent primary overcurrent with a current sensitive fuse, and then possibly a thermal fuse to protect against overtemp, that being a similar but slightly different issue. Both thermal and current fuses rely on the melting point of a low-melting alloy. For current fuses, the idea is to make it melt at a specific current under self-heating. For temp, you make it bigger to carry more current to minimize the self heating and be sensitive mostly to the heat coming in from outside.

    Nothing says that somebody would not try to get clever and combine both of those into the same device, and still call it a "thermal fuse". As an engineer, it occurs to me that if it's tricky designing an overcurrent fuse right, and it's tricky designing a thermal fuse right, it's tricky-squared to design an overcurrent/thermal fuse right.

    That's the problem with anecdotal evidence, no matter how many times it's repeated. It can offer a place to start digging, but there isn't really a gold nugget in one's hand until the digging is done.

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    As a tangent to the whole safety ratings issue, performing a voltage conversion mod on a unit by swapping a domestic or export transformer ALSO voids the safety ratings. The only "good" voltage conversion is one where a multi-tap transformer exists.

    When customers ask us to do voltage conversions, we sell them step-up/down transformers. We will not modify any internal power supplies. If they piss and moan about it, we tell them to go elsewhere. Particularly in this litigious society we live in, one good lawsuit can close your doors for good. It's not worth the money that is turned down initially.
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    RG, ain't that what a properly rated thermistor is supposed to do?

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    A thermistor starts out with some rated resistance when cold (room temperature), then as it heats uo, the resistance goes DOWN. That is why they are useful as inrush limiters in the amplifier mains circuit. The couple amps of current flowing through it heats it up over a few seconds so it drops to a low resistance that essentially vanishes from the circuit.
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    Thermistors to reduce inrush current? Sure. That's what a properly rated thermistor is supposed to do.

    The problem is that all of these - power line fuses, secondary fuses, thermal cutoffs, inrush limiters, etc. - are there because someone predicts something bad will happen, based on either pessimism or experience, and the problem is then to make the vast majority of all of the units work correctly for their useful lives. It's absolutely certain that they all won't work right; some of them *will* fail. So the designer is in the business of divining the future, and putting in enough to keep the customers happy without losing his job because the boss' nephew who runs accounting thinks he put in too much.

    If all this could be calculated, it would be no problem. Because of the real nature of the Real World, it's always a prediction, and there is a penalty for guessing either too high or too low. In fact, in some companies, there is a penalty for guessing exactly correctly, because there's always some newly-minted hotshot who'll second guess it and do the analysis that they could have had more profit by allowing more of them to fail.

    Things that are certain to work forever cost too much for the economics we live under.

  30. #30
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    A postscript to my last observations. One of the personal joys I get out of living like a technology monk, spending a preponderance of my adult life learning technologies, is that I can put the little niceties into my stuff that makes it more bulletproof and resistant to trouble. I recently got two dozen 75C resettable thermal cutouts from a surplus outlet for a buck apiece. That happens to be the perfect temperature cutout to put on a power amp heat sink to shut things down safely. I can do that because I can find them, know which ones to buy, wire them up safely, and know what's happening when one trips.

    But learning all that took a long time. I wonder sometimes if I should have just practiced playing the guitar.

  31. #31
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    Thanks Everyone....
    I truly appreciate the great replies here- absolutely informative.

    As for the old PT, I will keep that for "project / testing" purposes. One last questions... since we are talking AC, does it matter which way you connect the thermal device?

    Tom

  32. #32
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    Quote Originally Posted by TomCarlos View Post
    since we are talking AC, does it matter which way you connect the thermal device?
    Not at all.

  33. #33
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    Sorry to revive such an ancient thread, however I've just taken a Fender Blues Junior in for repair where the thermal fuse has blown.

    The owner of this amp left it on overnight in his house.

    In my view the thermal fuse has done its job, ie prevented a potentially lethal fire.

    Incidentally, all the new(ish) Fender amps I see in the UK are wired to the incorrect voltage, which means they all run on the toasty side.

  34. #34
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    Quote Originally Posted by jpfamps View Post
    Sorry to revive such an ancient thread, however I've just taken a Fender Blues Junior in for repair where the thermal fuse has blown.

    The owner of this amp left it on overnight in his house.

    In my view the thermal fuse has done its job, ie prevented a potentially lethal fire.

    Incidentally, all the new(ish) Fender amps I see in the UK are wired to the incorrect voltage, which means they all run on the toasty side.
    It does seem like a very poor design if an amp cannot be safely left on overnight.

  35. #35
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    Quote Originally Posted by jpfamps View Post
    Sorry to revive such an ancient thread, however I've just taken a Fender Blues Junior in for repair where the thermal fuse has blown.

    The owner of this amp left it on overnight in his house.

    In my view the thermal fuse has done its job, ie prevented a potentially lethal fire.

    Incidentally, all the new(ish) Fender amps I see in the UK are wired to the incorrect voltage, which means they all run on the toasty side.
    It does seem like a very poor design if an amp cannot be safely left on overnight.

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