Prevent No-Load Failure of OPT (Outputs transformers) without speakers hooked
- Thread starter mroboto
- Start date
Well folks, I guess this is it for me.
50 years of owning, repairing and operating stereo equipment, and now I find out that I am stupid.
It's a lot of bother anyway.
Ordering an Alexa tonight to listen to music.
Foolproof, I hear.
Retrovert
Addicted Member
The industry papers, from which I quoted at length, are clear. Google will turn up piles of papers on the same subject. The sole benefit of the MOV is price. So when an ANSI standard or safety certification (UL, CE, etc.) requires some form of arc suppression the easiest and least expensive solution is an MOV. That meets the letter of the specification, but not its spirit.
Should the speakers become disconnected then the arcing cycle will continuous, as the audio is continually pumping the output transformer like an ignition coil. This is not "abuse" this is how physics works. It's just basic. If the volume is set at a normal level and the speakers are disconnected arcing can occur. That is the normal occurrence, including guitar amplifiers. It isn't the case that abuse happens, just that a normal sound check blows the amplifier.
Flyback diodes are routinely used in every application with an inductor to prevent arcing. It's not for foolish people or those with flawed perceptions. It's a good idea which could not have been affordably implemented at the time. Adding arc-suppression tubes would have totally blown the price point.
AmadeusMozart
Well-Known Member
Dale Carnegy once mentioned that if you keep on repeating the same stuff then eventually people will accept that as the truth. Trump applied this ad nauseum, in the end he even started to belief (some of) his own misunderstandings / falsehoods.
If an amplifier has been designed, build and operated properly then no protection is required and the gradual action of a MOV is perfect. (rather than the ringing in the transformer that a fly back diodes causes)
edit: don't bother to reply, I am done with this thread.
If an amplifier has been designed, build and operated properly then no protection is required and the gradual action of a MOV is perfect. (rather than the ringing in the transformer that a fly back diodes causes)
edit: don't bother to reply, I am done with this thread.
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trobbins
Super Member
Flyback diodes were used in PA valve amps from at least 1960 from examples I have seen. Ampeg started using them in circa 1970, and Fender circa 1990 from schematics I have come across - but Marshall didn't use flyback diodes. Commercial and music industry equipment was likely more prone to accidental speaker disconnections or no connections, and had to work in a more robust manner by way of industry reputation. It is the (edit: primary inductance and) leakage inductance, especially of PA and guitar amp OPTs, that means the 'high voltage flying' anode terminal is not directly protected by flyback diodes - and there are scope plots around that show how well a guitar amp can cause anode voltages to fly.
Note that a disconnected speaker may not be the only fault a valve amp has to endure during its lifetime, so if the amp has valuable or unobtainable parts then thinking about protection is well worthwhile 'a priori'.
Note that a disconnected speaker may not be the only fault a valve amp has to endure during its lifetime, so if the amp has valuable or unobtainable parts then thinking about protection is well worthwhile 'a priori'.
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trobbins
Super Member
What a load of twaddle Retrovert. Your mind works in mysterious ways indeed if you can't work through multiple posts on a subject that is not black and white.
The R3000 (3kV 0.2A) is the ubiquitous flyback diode used by Fender - interestingly it is not uncommon for that part to fail (and the amps typically don't have B+ much over 500-600V, and there is not enough evidence as to whether such a diode fails from PIV stress, or current conduction/thermal stress, or ...). Some speaker loads also seem to reflect back and cause anode voltage to fly in guitar amps - many ways for stress to occur.
The R3000 (3kV 0.2A) is the ubiquitous flyback diode used by Fender - interestingly it is not uncommon for that part to fail (and the amps typically don't have B+ much over 500-600V, and there is not enough evidence as to whether such a diode fails from PIV stress, or current conduction/thermal stress, or ...). Some speaker loads also seem to reflect back and cause anode voltage to fly in guitar amps - many ways for stress to occur.
Funny thing- I work around many techs that repair guitar amps- and I have seen FAR more Marshall amps with blown output transformers, than Fender, and more blown transformers in Fender amps, than Ampeg amps.
Coincidence? Maybe. Correlation? Very possibly.
Regards,
Gordon.
mroboto
Super Member
Important point, I just found this article where he said "Flyback diodes require the amp to be fitted with a high voltage (B+) fuse as they have a tendency to degrade over time and eventually short. No B+ fuse & shorted out flybacks = SHTF situation."
https://paul1179.wordpress.com/2016/03/27/flyback-diodes/
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Not sure why I'm being quoted in this thread since I haven't even contributed to it until now. Equally unsure what thread my quote was pulled from or why a comment to it was necessary, but my comment appears to be one based on a discussion of theory rather than practicality, which is the subject of this thread.
Beyond that, my only comment would be that back in the day when the flyback diodes first started appearing in guitar amps, my observation from being in a band myself was that the problem was rarely if ever due to the often (if not well over) cited "oops" scenarios of this thread, but rather, voice coils that were either burned open, or damaged into an open condition from mechanical failure of the speaker -- all due of course to high volume levels. Then, the inevitable failure of the OPT often followed. With the changes in "music appreciation" and social engineering to deliver it (loud music festivals) that were quickly ramping up at that time, it was (again) my observation (at the time) that speaker drivers were the last thing to catch up, with the near bullet proof drivers of today's world simply unheard of. The drivers of that day were still little more than glorified hifi speakers, and therefore much more prone to failure.
Today, the need for all the insane power vacuum tube guitar amps of that time is all but gone, with massive SS sound support systems taking their place. For all those old massive pieces that are still around (that inherently had to use external speaker interconnects), using today's high power rated drivers and positive lock speaker interconnects all but eliminates OPT losses from open speaker lines. Still, the best flyback diodes of this day make for best design practice in vacuum tube guitar amps of any power level, as things can always get dicey during overload conditions (loaded or not), where so many musicians prefer to operate at. That likely leaves the most oops possibilities today within the realm of operating vintage home stereo systems (where the use of flyback diodes were unheard of). Cranking such a system up to high levels when faced with a no sound condition without first ensuring a load is properly connected means all of the warnings and common knowledge (common sense?) of the day when the equipment was produced either went unnoticed, unheeded, or (today) un-researched, while driving the system into clipping (with speakers attached) and only then tripping over the speaker leads seems to be a bit of a stretch.
Dave
Beyond that, my only comment would be that back in the day when the flyback diodes first started appearing in guitar amps, my observation from being in a band myself was that the problem was rarely if ever due to the often (if not well over) cited "oops" scenarios of this thread, but rather, voice coils that were either burned open, or damaged into an open condition from mechanical failure of the speaker -- all due of course to high volume levels. Then, the inevitable failure of the OPT often followed. With the changes in "music appreciation" and social engineering to deliver it (loud music festivals) that were quickly ramping up at that time, it was (again) my observation (at the time) that speaker drivers were the last thing to catch up, with the near bullet proof drivers of today's world simply unheard of. The drivers of that day were still little more than glorified hifi speakers, and therefore much more prone to failure.
Today, the need for all the insane power vacuum tube guitar amps of that time is all but gone, with massive SS sound support systems taking their place. For all those old massive pieces that are still around (that inherently had to use external speaker interconnects), using today's high power rated drivers and positive lock speaker interconnects all but eliminates OPT losses from open speaker lines. Still, the best flyback diodes of this day make for best design practice in vacuum tube guitar amps of any power level, as things can always get dicey during overload conditions (loaded or not), where so many musicians prefer to operate at. That likely leaves the most oops possibilities today within the realm of operating vintage home stereo systems (where the use of flyback diodes were unheard of). Cranking such a system up to high levels when faced with a no sound condition without first ensuring a load is properly connected means all of the warnings and common knowledge (common sense?) of the day when the equipment was produced either went unnoticed, unheeded, or (today) un-researched, while driving the system into clipping (with speakers attached) and only then tripping over the speaker leads seems to be a bit of a stretch.
Dave
Well- I see a lot of musicians, that are still using very old Fender, Ampeg, Marshall, Traynor and many other brand tube guitar amps. In fact, the older ones are VERY sought-after. And bring HUGE money.
Most of these amps and speaker cabinets still contain the original vintage drivers (that's what the musicians want, in most cases). And most of them still have the original 1/4" phone plug speaker cable connections, too.
I recone speakers- and I will say- we still get TONS of work (probably literally, each year) of drivers from guitar amps, of all ages (these guys can blow even the modern heavy-duty stuff quite readily, too- all it takes is someone who doesn't care how much distortion is being produced), And, we see the accompanying amp failures- probably one or more a month, with a damaged output transformer or other issues caused by overload into an open circuit condition. I even see people blow drivers in a multi-way cabinet, and then blow up the crossovers (due to the flyback voltages produced by the loss of the speaker load, ostensibly).
If I was making a tube guitar or other MI amplifier, even in this day and age- you can bet I would have fused B+ and flyback diodes in it. In fact, if I were gigging with an older tube amp- I would seriously think about retro-fitting these type devices into it, if it didn't already have it. Try finding an original output transformer for an Ampeg V4 or 1959 Princeton, these days...
For home amps, though- if someone is driving them into that kind of clipping- they are going to pay the price in blown tweeters and mid drivers (the woofers will probably survive enough to avoid an open-circuit condition), probably well before the probability of damaging an amp will come into the picture. It's only the ones that absolutely don't care about how distorted the sound is (which is an exceedingly small minority of home audio listeners, fortunately!), that will risk this type of failures in a home...
Regards,
Gordon.
Most of these amps and speaker cabinets still contain the original vintage drivers (that's what the musicians want, in most cases). And most of them still have the original 1/4" phone plug speaker cable connections, too.
I recone speakers- and I will say- we still get TONS of work (probably literally, each year) of drivers from guitar amps, of all ages (these guys can blow even the modern heavy-duty stuff quite readily, too- all it takes is someone who doesn't care how much distortion is being produced), And, we see the accompanying amp failures- probably one or more a month, with a damaged output transformer or other issues caused by overload into an open circuit condition. I even see people blow drivers in a multi-way cabinet, and then blow up the crossovers (due to the flyback voltages produced by the loss of the speaker load, ostensibly).
If I was making a tube guitar or other MI amplifier, even in this day and age- you can bet I would have fused B+ and flyback diodes in it. In fact, if I were gigging with an older tube amp- I would seriously think about retro-fitting these type devices into it, if it didn't already have it. Try finding an original output transformer for an Ampeg V4 or 1959 Princeton, these days...
For home amps, though- if someone is driving them into that kind of clipping- they are going to pay the price in blown tweeters and mid drivers (the woofers will probably survive enough to avoid an open-circuit condition), probably well before the probability of damaging an amp will come into the picture. It's only the ones that absolutely don't care about how distorted the sound is (which is an exceedingly small minority of home audio listeners, fortunately!), that will risk this type of failures in a home...
Regards,
Gordon.
Pio1980
AK Member
Folks spend tens of thousands of bux to sound like someone else's cheap guitar on an overdriven cheap amp when the time would be better spent working on their own chops.
I can't emphasize enough how refreshingly gratifying it was to hear Mark Knopfler's clear fluid tone on the Dire Straits debut.
And, how about turning it down a ways from 11?
I can't emphasize enough how refreshingly gratifying it was to hear Mark Knopfler's clear fluid tone on the Dire Straits debut.
And, how about turning it down a ways from 11?
The circuit used to be called a tweeter saver, and it is a series RC circuit placed across the common to 16 ohm tap on the OPT. It was something like 47 ohms with a 0.22uF cap in series or thereabouts. It prevents HF runaway when the amp has no external load connected. The power it consumes in the audio range is to small to worry about.
AmadeusMozart
Well-Known Member
How did this thread go from protection (or not ) an OPT in a HiFi Sansui rfeceiver to protection speakers in a guitar amplifier? Different requirements altogether!
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Pio1980
AK Member
Precedent.
Retrovert
Addicted Member
Wow.
I realize that not everyone is an electrical engineer, or has even studied basic electrical engineering as a hobby. But it would be beneficial if you would perform some basic research or reading before hurling your excrement around like an angry chimpanzee engaged in thread crapping. If you do not like the thread, elsewhere read. Simple solution.
The issue at hand is simply stated: how to prevent arcing from an unloaded output transformer which acts as an ignition coil.
Guitar amplifiers routinely experience speaker connection and disconnection, greatly increasing the risk of disconnection of the speaker, either prior to use or during use, and therefore greatly increasing the need for protection against destruction of the output transformer, and output stage.
I clearly explained how flyback diodes protect the output transformer, regardless of whether it is HiFi or guitar. This functioning has been confirmed by others. No detrimental sonic effects are experienced. Problem solved!
I simply cannot fathom why this basic solution is contentious or disputed. It simply boggles the mind.
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AmadeusMozart
Well-Known Member
Perhaps you missed a posting from me in the past that I studied electronics, the last class to be taught valve technology, and worked for Philips afterwards before I changed careers. I've forgotten more than you'll ever know. Amplifiers that are continiously clipping are a total different proposition than HiFi amplifiers that should never be run into clipping.
Retrovert
Addicted Member
The filter you describe prevents amplifier oscillation; it has nothing to do with protecting the output transformer against disconnection of the speakers and is 100% guaranteed to not prevent the output transformer from arcing.
Retrovert
Addicted Member
Nope.
The flyback diodes protect against inductive kickback, aka "flyback". The circuit behavior is explained by Physics 101 and Electrical Engineering 101.
An unloaded output transformer functions exactly like an ignition coil: a collapsing field results in a rise in voltage until current may flow, i.e. an arc event. The flyback diodes thereby prevent a rise in voltage beyond twice B+, a value which the winding's insulation ought to safely handle. Flyback diodes are widely used on inductors to prevent dangerous transients and the ensuing destruction. This has been explained!
Faraday's Law of Induction and Lenz's Law fully explain the physics. These fundamental principles of physics ought to be covered in the first semester of freshman year! It's not arcane knowledge! Even Wikipedia will explain this! No need to pluck out one of your eyes like Odin to purchase true wisdom! It's out there, freely available! You just need to read!