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Thunderhead

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  1. Whatever. Next time you blow a PA cab, don't worry, it couldn't have happened.
  2. Alex, just forget the theory of the whole waveform for a minute and think about what happens [i]within each half of the cycle[/i]. To quote the article linked above: "One of the most notorious myths regarding clipping is that it produces DC. This assumption is made because a square wave is seen to have flat tops and bottoms. In fact, it's incorrect to think of a squarewave as being made up of positive and negative DC components. The only way for it to be DC would be if there was a non-zero average value over long periods of time.[b] If the polarity changes at all within the time frame that you are looking at[/b], it is simply not DC." If the time frame you are looking at is less than half the frequency, the polarity does not change. For the duration of half the fundamental frequency (and remember that clipping the output stage effectively reduces the frequency to that because it obliterates all the higher components) what you have is DC. I agree that what you have is a square AC waveform [i]overall[/i], but it [i]can also be[/i] described as sections of DC separated by transients. The fact that they do alternate in polarity is irrelevant if you consider what is happening [i]in the duration of each section[/i]. Both ways of looking at it fit the waveform but the 'alternating DC' description explains more usefully why a clipped waveform can be so damaging to speakers, especially cabinets with crossovers. It isn't [i]just[/i] to do with the increased power when clipping occurs (although that is significant too). Sometimes it can be useful to throw away your full mathematical analysis and just look at things in a more simplistic way, especially in cases where a sudden change of operating conditions occurs (which it does when an output stage clips).
  3. Alex, a square wave IS sections of DC - each the length of half the complete waveform, and of alternating polarity - separated by vertical transients. Just look at it on a scope. Yes, it's [i]also[/i] an AC square wave. A clipped solid-state power section is quite capable of producing this (subject to the slew rate of the output devices, which makes the transients not quite vertical). Again, look at it on a scope. To the crossover, the sections of DC are 0Hz and so will be sent exclusively to the bass driver. I know the bass driver cone does not perfectly follow this - it can't due to its mechanical inertia - but it will try, and the lower the frequency the closer it will get. Connect a signal generator to a speaker and watch what happens to the cone as you lower the frequency with a square wave input.
  4. Speakers are in fact only ever blown by too MUCH power - you cannot blow one with too LITTLE - but the mechanism by which too much power reaches the driver is not quite as straightforward as you might think, and it IS possible for an underpowered amp to blow a higher-rated speaker. [quote name='Moo' post='313497' date='Oct 24 2008, 05:03 AM']The clipping amp destroying speakers thing is a Myth. - Do guitar amp cabs blow up every time the overdrive is switched on?[/quote]No, but that's because guitar cabs are very crude with only "low/mid" drivers and no crossovers, and the valve amps normally used to drive them have a limited frequency range too, due to their output transformers which block very low and very high frequencies almost completely. (Even solid-state guitar amps are designed to have this sort of frequency response, precisely to make them sound more like valve amps.) But in a cab with a crossover driven by a solid-state amp with a full-range frequency response, it's very different. A clipped signal starts to approximate a square wave, which is effectively a series of sections of DC separated by sharp transients - ie entirely composed of extreme low and extreme high frequencies - and a solid-state power section is quite capable of delivering this since there is no output transformer. If the crossover is really efficient, it will then send the full power of the amp alternately to the bass driver and the tweeter, instead of evenly distributing a range of frequencies... which may fry both units, particularly the tweeter (since in normal program music the power in the highs is substantially lower than that in the lows, so the tweeter does not normally have to handle as much), and might even damage the crossover since it's not designed to handle that sort of signal either. [quote]Too much power destroys Cabs, either mechanically with over excursion or Thermally by overheating and melting the voice coils. There are some circumstances where if the power of the amp EXACTLY matches the melting point of the speaker voice coils, the extra power given to them by clipping will take them over the thermal threshold and melt them, but this wont happen with a 50w Guitar amp clipping into a 100w Cab for example.[/quote]Exactly, but it CAN happen with a 500W PA amp clipping into a 1000W cab, because a clipping 500W amp can be putting out up to 1000W (the energy contained in a square wave is double that in a sine wave of the same peak voltage, and even if it's not a full square wave the energy rises drastically as soon as clipping is reached), and the individual drivers in the PA cab won't be able to handle this - especially the tweeter, which will more likely be rated around 300W. The bass driver may not like being hit with bursts of DC at that sort of power level either... DC is very bad for speakers because it does not allow the voice coil to cool itself properly as it is not actually moving while it's absorbing the power. [quote]The reason that 2x amp to speaker ratio is recommended for P.A. is to avoid distortion and sound nice. The max power rating of Amps is measured at the point where they reach 1% distortion. This means that if you run a 300w amp into a 300w speaker at its max level you will be amplifying 1% distortion as well as the sound you want. The harder you push them, the harsher they sound. If you want to have a nice clean sound in your P.A. then you need to run at a level which is less than the Max rated level of the amp and leave yourself some headroom. With a 600w amp into a 300w speaker, you can run it at 3db (Half the power) below its 1% distortion threshold and keep the sound clean. It is worth noting that High power P.A. systems will also be running with crossovers and limiters that will be set up so that the Amps output voltage will not exceed the thermal rating of the speakers and melt the voice coils. Unless you have limiters and have set the gain structure of the amps correctly, then it would be unwise to run 300w speakers with a 600w amp because if any 'helpers' who don't know your system turn up the amp and run it into the red then the speakers will be cooked.[/quote]Absolutely! The 'over powered amp' theory only applies if the user does not allow distortion to occur - if it does, the speakers will be toast immediately since the power applied to them is then even further over their rating. [quote]For bass amps the requirement is slightly different. There is only one manufacture of bass heads who routinely includes a limiter on the output of there amps (They also make P.A. amps). Unfortunately this limiter is pretty much useless for protecting the cabs, because it is not adjustable, and obviously not able to telepathically detect the thermal voltage rating of the cabs it is connected to. Many people prefer there sound slightly distorted and obviously some like it a lot distorted. In these circumstances it is wise to have cabs that have a higher rating than the amp they are connected to. If the amp is rated higher than the cabs, it could in theory sound very clean and 'Hi Fi', but one slip with the volume knob, or one slap too hard will result in fried cabs.[/quote]I agree - I prefer to treat bass systems more like guitar ones. I do use fuzz bass myself (which makes it essential) but even for clean sounds it's safer to go with more power handling for the speakers. In the end, there is only one truly safe method - make sure the speakers (including all the individual elements inside the cabs, for multiple-frequency cabs) can handle the full distorted power of the amp. This does mean using much heavier and more expensive drivers than you really need for a clean sound though, so it isn't a great way to build a PA system usually... although I have done this when working as a repair engineer for rehearsal studios (ie PA systems being run unattended by musicians!), which I mentioned in the thread about protecting speakers. Normally for single-range guitar or bass speakers, double the clean power rating of the amp is enough - an amp can't produce more than double its clean power even fully distorted (although you can still run into the problem that the speaker power rating doesn't necessarily apply at near-DC frequencies, and mechanical cone-excursion damage can also occur even when electrical damage doesn't). For PA, you may need three or four times the amp rating to be really sure you won't blow the tweeters, which obviously is going to require otherwise unnecessarily expensive and heavy cabs, so it's not the right solution for most users, when simply avoiding distortion will do the job.
  5. What is the exact way it doesn't work - Does the light come on when you press the switch? Does it pass signal at all, in bypass? If it doesn't power up at all, or if it doesn't pass signal in bypass, it's more likely to be something on the power or analog side of the circuitry and more likely to be fixable (though still not a certainty). If it works except that nothing happens when the effect is on, it's more likely to be something in the digital side and less likely to be fixable, or at least not for much less than just buying another one. Either diagnosis could be wrong depending on exactly what is faulty, too!
  6. [quote name='joegarcia' post='312874' date='Oct 23 2008, 12:04 PM']Yea I know all that but I'm sure I read it's legal to have them on vintage gear that would lose it's value if modified. No?[/quote]No. Electrical safety regulations do not pay any regard to the 'value' of an item. There's nothing to stop you using one personally, but you could possibly be in trouble if you sell it, and an accident then happened to the buyer. Technically these connectors are not legal for use with any voltage over 50V due to failing on several counts - cable retention, being able to be dismantled without tools (both for the inline type), and the socket holes being too large (AKA 'toddler's finger test', which applies to both types). Shops in particular need to be careful about selling amps with them, since as professionals in the trade they could reasonably be expected to know the rules and risks, and could possibly be held legally accountable. I don't know if it's ever happened though. [quote]I'm just saying that personally, if it was a choice between using a good amp with a bulgin or a bad amp with an IEC I'd choose the good amp.[/quote]Of course, so would I... and then fit it with an IEC . I'm not saying I would [i]always[/i] fit one - there are some vintage amps which are original enough that it's not really 'right' to do so, and mostly these will not be in professional use anywhere it could cause a problem. But any amp which you can expect to use at a gig needs a proper modern power connector, in my opinion - for safety, reliability and easy replacement if you break or lose it. I don't really understand the logic of keeping a crappy old part in use for the sake of originality, when you need to change other parts of the amp occasionally when they fail anyway. It is a real pity there isn't a proper 'in keeping' IEC socket available which would preserve the look of the amp without being obvious that a new connector has been fitted across the hole from the old one (it's difficult even to close off the gaps without it looking like a bodge in some way) - I'm actually more bothered by that than the idea of filing out a couple of corners from the chassis hole.
  7. [quote name='joegarcia' post='312674' date='Oct 23 2008, 01:24 AM']I really think you're worrying too much about the bulgin thing. As long as you unplug it from the wall before the amp and make sure you have a spare, whats the issue?[/quote]That they are unsafe, especially the 'inline' ones - it's possible for the cable to get pulled and partially disconnect the wires inside, maybe leaving you with no earth... and worse, the earth wire floating around loose where it could come into contact with either of the other pins - the design was always inadequate regarding cable retention. And that they are illegal, even the better right-angle ones. There are venues where this could cause a problem if they get officious about health and safety requirements (seriously). They are an absolutely terrible connector, always were, and should be consigned to history. It's a pity there is no way of making a IEC socket fit the same hole without cutting a small amount of the chassis away, but it's a small price to pay really. It would be better if someone made a round IEC socket that matched the outline of the Bulgin... but they don't.
  8. [quote name='Mr. Foxen' post='312430' date='Oct 22 2008, 07:42 PM']Wheres that issue come from?[/quote]They have problems with blowing output transformers, overheating, frying power valves, screen resistors desoldering themselves, and a few other things. Not one of Marshall's better designs... [quote]Tried a vintage modern (if thats what you meant by MV) didn't think much of it tbh.[/quote]No! A Vintage Modern is a totally different amp, and I didn't think much of the one I tried either (for guitar - but I doubt it would be any better for bass because it sounded very thin). The JMP or JCM800 Master Volume model (model 2204 50W or model 2203 100W) is a much simpler, single channel amp with no switching, reverb or any other features other than High and Low (which bypasses one gain stage to give a cleaner sound) inputs, and Preamp Volume/Master Volume/Treble/Middle/Bass/Presence controls. They're among the most bombproof amps ever made by any company (earlier models with iffy voltage and impedance selectors aside), sound absolutely great and even the 50W version will blow a "60W" JCM600 into the middle of the next street . The 100W 2203 is one of the loudest and most dynamic valve amps ever made. (The 100W rating is extremely conservative too, most measure in the 140W region even clean and will easily top 200W fully distorted.) I actually think - guitar purists will no doubt disagree! - that the 2203 is the best-sounding Marshall ever, early JMP ones especially. Marshall have reissued the 2203 now, but I wouldn't really bother when you can get originals for much less, even including a service.
  9. [quote name='dangerboy' post='312333' date='Oct 22 2008, 05:32 PM']I can't imagine the circuit will be that complicated. Probably just a blown component or two. Have you opened it up to look for scorch marks? The only problem is that it's often not worth paying someone to repair effects since labour is so expensive.[/quote]Most of the newer MXR, Dunlop, EH and many other modern pedals use surface-mount components, which make them unrepairable to the average tech... including me, I just don't have the equipment or techniques - or to be really honest, the inclination - to work on them. From the symptoms it sounds worse than something simple like a fried protection diode, that would usually just leave it totally dead. Try to find a tech who will at least open it and tell you if it's go or no go for free.
  10. [quote name='Mr. Foxen' post='312292' date='Oct 22 2008, 04:47 PM']Paired it with a Marshall JCM600 into a 4x10 for some top (make up for single guitarist). Filled the place with low end.[/quote]You're lucky that didn't fill the place with a nasty burning smell... I would seriously NOT recommend this for bass given the volume I'm sure you're playing at . If you want a decent Marshall guitar amp for top-end bass crunch, you'd be better with a JMP or JCM800 single-channel MV amp, they have less of a tendency to double as a smoke machine . (And they sound far better too.)
  11. [quote name='larrikin' post='311959' date='Oct 22 2008, 12:11 PM']I see, what double effect looper do you think it is??[/quote]Not sure, but they all do basically the same job. Some are buffered, some are 'true' bypass, that's about the only difference. You can't tell which that is unless you know who built it. [quote]And is a Boss buffer the dd6?[/quote]Yes, all Boss pedals (apart from the very early CE-1 type) have full buffering.
  12. Most of the models are worth a little more than Taiwanese ones to people that care, but you're not exactly going to retire on it . There are a very few (rarities like the SG-1 Slow Gear, SP-1 Spectrum and VB-2 Vibrato) which are worth fairly serious money, but most of the normal Japanese models are worth no more than about 50% more than a similar Taiwanese one in equivalent condition. I may be a little out of touch but I would say no more than about £50 to £60 for a CE-2, and it would have to be in pretty good condition (maybe a bit better than what you're describing) to even be worth that much. They're just not rare enough, or that much better-sounding than the Taiwanese one, for it to be a major factor really.
  13. [quote name='BassManKev' post='311882' date='Oct 22 2008, 10:52 AM']looks like a double effects looper to me[/quote]Yes, and the second loop goes to what looks like a Boss Tremolo, then to the Micro Amp (which IS connected the right way round) then back - you can follow the cables. This may be because the Boss TR-2 is notorious for a level drop (some popular mod companies offer a fix for this) and the Micro Amp is there to make up the difference. So it will be left permanently on and only the level needs to be adjusted, which is easier with it that way round, especially if you want to do it with your foot! The first loop contains only the Big Muff, probably because those Russian ones are horrible tone-suckers without either being modded to 'true' bypass or put in a loop. The other pedal looks like a Boss digital delay and the whole signal goes through that without a looper, which is actually a good thing if it's the last pedal because the Boss buffer is pretty good and it will drive the long cable to the amp well.
  14. [quote name='Waldo' post='311405' date='Oct 21 2008, 04:29 PM']Just noticed that it's in a shop on Denmark street and will therefore no doubt be an overpriced peice of junk.[/quote]+1 ... and probably not as described either. I've never seen any pic of Sid playing a sunburst P-Bass (it wouldn't have been 'punk'), and if he did it probably wouldn't be in that poor condition - he hardly had long enough . The white one that Steve Jones has now has obviously been cleaned up, but even so those 70s Fender 'Thick Skin' finishes are pretty bombproof and don't show minor knocks much at all. Sid's bass had a big dent in the front above the bridge, but it's covered by Steve Jones' arm in the pic so you can't see it. I think it is the same bass though, the pattern of 'brightness' of the neck dots is the same, which would be difficult to fake. I suppose it's possible it's one he owned from before the Pistols (and if already secondhand, maybe that would explain the condition), but I would want to see really convincing proof. It would be very easy to take a beaten-up punk-looking P-Bass of roughly the right age that was going to cost too much to put right as a normal playable instrument, and market it as 'ex-Sid' instead... at several times the price.
  15. Have a look at a pic of any P-Bass. Then wait until you have the strings on, tune the top two to a bit lower than normal (to allow enough slack to push them down but still tight enough that they don't flop about), rest the string tree on the strings in the position you saw in the pics, then push it straight down onto the headstock, taking care you don't force the strings sideways. Mark the screw hole position with a sharp object - the screw itself will do if you can get enough pressure on it with your thumb to press into the wood slightly - and drill the hole.
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