Jan Vigne Posted October 17, 2010 Share Posted October 17, 2010 If you've not seen me mention it previously, my experience is more with consumer high end audio (where tubed amplifiers still exist in abundance) than with Marshall amps. The only tubed guitar amplifier I've ever owned was a full amp/speaker combo back in the 1960's and '70's until I sold it for $50 when I was in grad school. (Saw three versions of it yesterday at the Arlington Guitar Show going for as much as $595. The seller with the version that had not been serviced and had a faulty foot pedal input and a non-working second input wouldn't budge beneath $300. ) I have owned numerous tubed home amplifiers for the last thirty years and I sold high end audio for twenty five of those years. From my perspective amplifiers should never distort or compress the music. Obviously, the opposite of that is more likely the aim of the average rock/blues player. However, one thing about "guitar amps" that I'm not understanding as I get back to using tubes for my guitars is impedance matching warnings. I've read on this forum and seen on numerous speaker/amp manufacturer's websites that matching the load impedance of your speaker system to the correct output tap from your amplifier is of "prime" importance. Many such cautions predict near immediate amplifier immolation if these warning are not strictly observed. Why? In home audio a tubed power amplifier is more than likely going to drive a multi-way speaker system with impedance and phase angle swings which are quite unlike (and far less friendly; http://stereophile.com/floorloudspeakers/805wilson/index4.html) those of a single (common model) driver in even a four-by speaker cabinet for guitar. OK, but even if you mix speaker models in a single cabinet total impedance load is still total impedance load and that's all the amp is primarily concerned with. Almost any manufacturer or sales person in home audio - if they know what they're doing - will suggest the user experiment with which output tap from the transformer will "sound best" with the chosen speaker system. The four Ohm tap will sound quite different than the 16 Ohm tap - but the amplifier will not destroy itself due to that loading. The output impedance of the amplifier will rise or fall given the the tap selected. This means a higher or lower damping factor in the system - the amplifier will better control the driver's motion and better resist the back EMF from the speaker system which allows for overall lower NFB specs (a good thing in home audio as this affects the amp's stability into low impedance loads or under excessive clipping conditions). A mismatch of output tap and speaker load will result in some alteration of the transfer function of the circuit meaning a higher or lower output power by a small percentage and a rise or fall of THD (which is important in home audio). But the average home audio amplifier would operate well even under severely mismatched load conditions, it just won't sound its best. So what about a tubed guitar amplifier is different that makes such dire warnings of Approaching Amplifier Armegeddon necessary? Looking at the schematics for a few tubed amplifiers I don't see a simple reason. Has anyone mistakenly loaded the 16 Ohm tap with a four Ohm system and had their amplifier go up in flames? Link to comment Share on other sites More sharing options...
wiggy Posted October 17, 2010 Share Posted October 17, 2010 Interesting post. I've asked the question both on here and other forums as to whether there is any significant difference in sound/voume/feel between say a 4 ohm tap and an 8 ohm (assuming all other variables are the same). I never got a definitive answer. The other question I had (and still have) is this. My amp (BC30) has several output taps running from 4 to 16 ohms. Now I assume that each socket is wired to a separate tap on the OT so my understanding is that each tap would expect to see a particular load. If this is the case why can't I plug a speaker (speakers) into every socket at the same time as long as each one matches the impedance for each socket. Sorry if the above isn't the model of clarity but hopefully you can understand what I'm asking Link to comment Share on other sites More sharing options...
Jan Vigne Posted October 18, 2010 Author Share Posted October 18, 2010 Interesting post. I've asked the question both on here and other forums as to whether there is any significant difference in sound/voume/feel between say a 4 ohm tap and an 8 ohm (assuming all other variables are the same). I never got a definitive answer. The other question I had (and still have) is this. My amp (BC30) has several output taps running from 4 to 16 ohms. Now I assume that each socket is wired to a separate tap on the OT so my understanding is that each tap would expect to see a particular load. If this is the case why can't I plug a speaker (speakers) into every socket at the same time as long as each one matches the impedance for each socket. Sorry if the above isn't the model of clarity but hopefully you can understand what I'm asking This is very much frowned upon. A severely stable amplifier(?) might deal with this sort of connection with only minor squabbles. However, most amplifiers will throw one hell of a hissy fit at just the suggestion of such goings on. It comes down to reflections from the higher impedance load of the speaker systems flowing back into the amplifier's lower impedance return leg of the circuit. Since the NFB is only taken from one tap - often the highest impedance tap - the reflections from multiple sources at several taps will eventually cause most transformer coupled amplifiers to slide into oscillation. Once there it's a short trip to the repair shop. The more global feedback the amplifier employs, the faster it will react to the connections. I'll see if I can come up with a more complete answer but it may take awhile. Link to comment Share on other sites More sharing options...
Jan Vigne Posted October 18, 2010 Author Share Posted October 18, 2010 This is very much frowned upon. A severely stable amplifier(?) might deal with this sort of connection with only minor squabbles. However, most amplifiers will throw one hell of a hissy fit at just the suggestion of such goings on. It comes down to reflections from the higher impedance load of the speaker systems flowing back into the amplifier's lower impedance return leg of the circuit. Since the NFB is only taken from one tap - often the highest impedance tap - the reflections from multiple sources at several taps will eventually cause most transformer coupled amplifiers to slide into oscillation. Once there it's a short trip to the repair shop. The more global feedback the amplifier employs, the faster it will react to the connections. I'll see if I can come up with a more complete answer but it may take awhile. "Signal reflection occurs when a signal is transmitted along a transmission medium, such as a copper cable or an optical fiber, some of the signal power may be reflected back to its origin rather than being carried all the way along the cable to the far end. This happens because imperfections in the cable cause impedance mismatches and non-linear changes in the cable characteristics. These abrupt changes in characteristics cause some of the transmitted signal to be reflected ... The most likely places for reflections are at joints in the cable, or at points of damage. Impedance discontinuities cause attenuation, attenuation distortion, standing waves, ringing and other effects because a portion of a transmitted signal will be reflected back to the transmitting device rather than continuing to the receiver, much like an echo. This effect is compounded if multiple discontinuities cause additional portions of the remaining signal to be reflected back to the transmitter. This is a fundamental problem with the daisy chain method of connecting electronic components. http://en.wikipedia.org/wiki/Signal_reflection Also consider each driver has a motor assembly (the voice coil/magnet) which acts to generate back EMF into the amplifier through the "-" return leg of the cable. While the amplifier's (let's call it "+" waveform) voltage/current moves the vc forward within the magnetic field of the speaker's magnet assembly the "-" side of the waveform and the suspension of the driver will cause it to move backwards. The action of the diaphragm and suspension will in itself create a voltage/current generated by the motor of the driver which is 180 degrees out of phase with the input voltage. This out of phase voltage/ current component must go somewhere, it cannot be absorbed into thin air. The back EMF is sent to the amplifier on the return leg of the cable thereby completing the amp/cable/driver circuit. When it reaches the amplifier the EMF voltage/current is then inputting voltage vs the amplifier outputting voltage/current to the driver's motor. (In effect it is the virtual short circuit of the speaker's motor by the amplifier which helps to control the driver's motion by resistively damping excessive movement and resonances; http://www.goodsearch.com/search.aspx?source=goodshopbar&keywords=damping+factor) Each output tap on the OT has a different output impedance which compounds the problems for the amplifier if multiple taps are being used. With the NFB taken from only one tap the multiple forces acting on the amplifier will generate multiple "messages" to the input of the NFB circuit which are then sent back to the front of the fedback loop. The higher the dB's of feedback employed, the less stable the amplifier (in many cases) and the more likely it will be to react poorly to the mulitle connection scenario. http://www.goodsearch.com/search.aspx?source=goodshopbar&keywords=negative+feedback+in+audio+amplifiers I'm sure a trained engineer could explain this better than I but I'm hoping you'll get the idea of what's happening here. Each of these individual images present at the input to the NFB loop will be slightly out of synch with the others as well as being out of synch with the incoming signal from the pre amp. Well, by the very nature of negative feedback that's not a good thing. If you look at how the feedback loop operates, you begin to consider how the amplifier might react to these individual out of synch inputs of out of phase signals (right? it's called "negative" feedback because it is [roughly] the polar inverse of what was sent to the driver). The very stable amplifier might just be unhappy, yet the less well designed and executed overdriven amp running at 20-100% THD and well into clipping; http://www.goodsearch.com/search.aspx?source=goodshopbar&keywords=clipping+in+audio+amplifiers will eventually have serious problems. Overheating and oscillation would be the first signs of trouble but, as I stated above, self-induced oscillation is for many amplifiers just the exit ramp which takes them to the repair shop. Should the amplifier survive oscillation, overheating will eventually wear both the passive and active components in the amp's circuitry. The overheating/cooling of components it a major cause for eventual failure (this would be most especially true for tube amplifiers which do not employ a soft start circuit; http://www.goodsearch.com/search.aspx?source=goodshopbar&keywords=soft+start+circuit+for+tube+amplifiers) just as it is the heating/cooling cycle of the filament in a lamp (light bulb) which finally causes its failure. As we often explained to those folks who brought in their equipment saying it was fine until they went to turn it on, how often have you had a light bulb go out while you're reading the newspaper? Most often it is the sudden jolt of a full 120 volts (or in the case of a tube amplifier possibly as much as 600 volts) which finally does the job. http://www.goodsearch.com/search.aspx?source=goodshopbar&keywords=back+electromotive+force Link to comment Share on other sites More sharing options...
wiggy Posted October 18, 2010 Share Posted October 18, 2010 Will take a couple of days to get my head round the technicalities of the reply but thanks for coming back with such a comprehensive response to my question. Ta Link to comment Share on other sites More sharing options...
RSDx Posted October 18, 2010 Share Posted October 18, 2010 Will take a couple of days to get my head round the technicalities of the reply but thanks for coming back with such a comprehensive response to my question. Ta LOL .... I feel the same .... also feel that way when I venture on over to sewatt forums .... every time I do, my mind is left reeling...whoa!!! Link to comment Share on other sites More sharing options...
Jan Vigne Posted October 19, 2010 Author Share Posted October 19, 2010 Will take a couple of days to get my head round the technicalities of the reply but thanks for coming back with such a comprehensive response to my question. Ta Good luck, I read all that on the back of a bubble gum card. :D Seriously, if you have any questions, I'll give it a try. Reading the links would be the first place to start. Link to comment Share on other sites More sharing options...
mtheory Posted October 19, 2010 Share Posted October 19, 2010 Hi-fi transformers are generally massive in comparison to guitar amp transformers, because they're engineered for minimum distortion and maximum efficiency. Guitar amp transformers don't require the same kind of over-engineered components. Inefficient transformers will tend to produce significant heat under load. Plugging into a mismatched speaker load causes additional stress, and thus additional heat, which can cause an output transformer to literally melt. Most older Fenders can handle a 100% mistmatch without anything more than some additional heat being produced inside the chassis. Some are more forgiving than others, obviously, and the size of the transformer will generally determine how forgiving it may be. Having said that, there's really no positive assurance that manufacturer's recommendation on load preferences for a given amp is really the most optimum choice for that given amp. The only way to be certain which tap and speaker load makes a given amp the absolutely happiest it can be, is to throw it on the bench, hook it up to a scope and signal generator, and try all tap and speaker load combinations. It's not inconceivable that an amp rated to operate from an 8 ohm tap into an 8 ohm load could possibly run its most efficient and happiest pushing a 4 or 16 ohm load. Link to comment Share on other sites More sharing options...
Jan Vigne Posted October 23, 2010 Author Share Posted October 23, 2010 Hi-fi transformers are generally massive in comparison to guitar amp transformers, because they're engineered for minimum distortion and maximum efficiency. Guitar amp transformers don't require the same kind of over-engineered components. Inefficient transformers will tend to produce significant heat under load. Plugging into a mismatched speaker load causes additional stress, and thus additional heat, which can cause an output transformer to literally melt. Most older Fenders can handle a 100% mistmatch without anything more than some additional heat being produced inside the chassis. Some are more forgiving than others, obviously, and the size of the transformer will generally determine how forgiving it may be. Having said that, there's really no positive assurance that manufacturer's recommendation on load preferences for a given amp is really the most optimum choice for that given amp. The only way to be certain which tap and speaker load makes a given amp the absolutely happiest it can be, is to throw it on the bench, hook it up to a scope and signal generator, and try all tap and speaker load combinations. It's not inconceivable that an amp rated to operate from an 8 ohm tap into an 8 ohm load could possibly run its most efficient and happiest pushing a 4 or 16 ohm load. One thread down (the toilet), we cautiously venture forward ... "Having said that, there's really no positive assurance that manufacturer's recommendation on load preferences for a given amp is really the most optimum choice for that given amp. The only way to be certain which tap and speaker load makes a given amp the absolutely happiest it can be, is to throw it on the bench, hook it up to a scope and signal generator, and try all tap and speaker load combinations. It's not inconceivable that an amp rated to operate from an 8 ohm tap into an 8 ohm load could possibly run its most efficient and happiest pushing a 4 or 16 ohm load. Clarification please. A test bench load resistor is not truly the equivalent of a loudspeaker. One - the load resistor - is, as you would expect by its name, almost entirely resistive in its load characteristics. Mostly inductive and possibly smaller capacitive values associated with the driver's motor - the reactance of the load (http://www.highbeam.com/doc/1G1-18081241.html) will vary with frequency and simple use. The heating of the driver's voice coil by excessive volumes and distortion components will raise the inductive and resistive values far above those more stable values of a large resistor. This will cause the amplifier to function quite differently into a bench resistor than into a real world loudspeaker, even one as simple as a single driver in an open box. What then are you looking at or for during this test you describe? And how is the test conducted? Does your final sentence imply there should be no problem driving a 4 Ohm speaker from the 16 Ohm tap other than the obvious issues of total RMS power delivered to the load? Or, are you only saying you feel an amplifier has a single "ideal" tap and the user should find a speaker that matches the impedance preferences of the amplifier? (As an aside, I feel your suggestion audio amplifiers are in someway more capable of withstanding the rigors of use to be yet another red herring on your part. True, most high end audio manufacturers design for vastly dissimilar expectations from their product when compared to guitar builders. However, if you - as a guitar amplifier builder - knew with an almost 100% certainty that your product was going to be driven into significant amounts of overdriven distortion and clipping for long time periods without rest, unless you wanted your repair shop to be full and your budget to be going to rebuilding the same amp several times, wouldn't you be choosing those parts which would withstand intentional abuse? IMO this says the astute guitar builder is actually the more likely to overbuild an amp while even the sturdiest of home gear manufactuers expects the user to turn down the system when the amp begins to clip.) Link to comment Share on other sites More sharing options...
Jan Vigne Posted October 23, 2010 Author Share Posted October 23, 2010 I didn't know "Cheeksy" got something like a twin-brother "Cheeksy" must be quite a guy. Link to comment Share on other sites More sharing options...
Jan Vigne Posted October 23, 2010 Author Share Posted October 23, 2010 Depends on the side of the view........ :unsure: He ended bad.... He's gone?! :o How will I break the news to mom? :P So, you can tell me, what happened? A run in with ... "you know who"? Sometimes you just give up trying to teach a pig to talk, you know? It only frustrates you and the pig is no better for the experience. Link to comment Share on other sites More sharing options...
Jan Vigne Posted October 23, 2010 Author Share Posted October 23, 2010 I could tell you - I don't want to do. All I know is, here aren't pigs! Most of us are "musicians" not engeneers, and we used to play guitar amps and not "home audio tube amps". Keep it easy man ! Do you really think anybody read those "Mega-Posts" with all the links ? Think of "helping" others - not "teaching" them..... it's a better way to make friends....... Yeah, I do expect there a some people who want to learn something they don't already know or actually understand. Not everyone wants to be totally incurious about things. Not everyone believes everything is simple black and white as told by someone who once knew an "expert". You have a problem with that? You sound like the kind of kid you sat in the back of the class and threw stuff or cut farts just to disrupt anyone else learning anything. If you or anyone else doesn't care to know anything, do you think it's best to complain about someone else wanting to? Did I hold a gun to your head and force you to read anything? That would be impossible, wouldn't it? My intent is not to force you or to lead you by the hand. If you like not knowing stuff, I really don't care - you can join the "huddled masses" and be as happy as that pig in the mud. If you think that makes you a better player - modes just to hard for you? too much reading and "understanding"? - then you go right ahead and ignore what I post and we won't have any problems here. My name's at the front of each of my posts so you can skip along to somewhere else if information bothers you. I said I didn't come here to make enemies but you and another fellow here are making it far more difficult than need be. I'll respect you if you respect me. What's your problem with that arrangement? You like causing trouble on this forum? I'm not noticing you being able to point to any flaws in what I've posted so far or, more importantly, to actually answer my questions. You and mtheory seem better at attacking someone than you do at actually knowing stuff. You both sound like a couple of los ... well, you two haven't put out the welcome mat, have you? I'm asking for the difference between home audio amplifiers like I've been around for decades vs guitar amps. Strict impedance matching isn't required on any tubed and transformer coupled amplifier I've come across other than all the warnings I'm seeing about it with the curent crop of guitar amps. I don't remember seeing anything about this when I owned a tubed amplifier back in the 1960's. It's an honest question. So back off the "home audio" BS, "friend". You have an answer to the question or are you just here to be annoying? As for not wanting to tell me? Why don't we just keep it that way and you stay away from me? Ignorance isn't catching as far as I know but I'd hate to take the chance with someone who has such a virulent case. Link to comment Share on other sites More sharing options...
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