What is power supply sag and what to do about it? 30 Jan 2019 Mike at MDBVentures.com http://www.MDBVentures.com - Great prices on great tubes! Power supply sag is when the voltage supplied to the electrical circuits changes. Usually defined as voltage going down a bit or sagging. This is caused by the electrical circuit demanding more power than the power supply can fully provide. In reality, all power supplies sag to some extent. In most cases it is so insignificant that it is not noticable. In cases where the supply voltage stability is critical, great care is used to minimize the sag by using voltage regulators. One area where voltage sag is often discussed is in tube based amplifiers. This is because often the circuits are minimal and thus are susceptable to sag. In a guitar amplifier sag usually happens when a string is initially plucked. This is when the biggest demand is placed on the circuits as the volume is normally the loudest at that point after which it fades as the string slowly reduces its vibration intensity. I've seen some references to the sag being caused by the rectifier tube not being able to supply the power and then "recovering". That is not quit what happens. The tube never "recovers", rather it is the entire power supply circuit that recovers. More specifically it is the charge voltage on the filter capacitor that "recovers". Generally, the tube is the only part that is easily replaceable, so it usually gets the blame. Let's say the power supply can provide a continuous 100 milliamps at 100 Volts which relates to a continuous sinewave being generated at a sinewave amplitude of 1 Volt. With the volume control set to maximum and a string is plucked, the sinewave amplitude jumps to 2 Volts. This puts a demand on the power supply of 200 milliamps. This is beyond what the rectifier can keep the filter capacitor charged at, so something has to give. What happens is that the voltage across the filter capacitor will fall due to the excess demand on it. In the mean time at 120 times a second (assuming a fullwave rectifier circuit - 100 Hz for the 50Hz power folks), the rectifier tube will try to recharge the capacitor. The more current demand from the power supply, the more voltage drop across the rectifier tube. As a result, it will not be able to charge the capacitor to the same level it was at at the lower electric current demand. That means the voltage across the capacitor will be less (not to mention that it will be pulsating at 120Hz (or 100Hz) as each charge cycle comes around on the AC source). As the string vibration amplitude fades, the current demand on the power supply is reduced and thus the current flow demand from the rectifier tube to keep the filter capacitor charged is reduced, so the voltage drop across the rectifier is reduced, which means that the voltage across the filter capacitor and thus the power supply voltage increases. The rectifier tube hasn't really "recovered", it is just acting normally. If the tube is weak, the resistance acrross the tube will be higher, and that means the voltage drop will be higher, both at the normal volume level and at the higher current demand, but it will likely be more noticable at the louder volume level. So, what to do about sag? First keep in mind that sag is a component in what makes the distinctive distortion sound of an amplifier when operating at maximum volume level, along with the clipping of the tube circuits. So, you may not want to actually "fix" it. The one replacable component in the tube based power supply is the rectifier tube, so it is pretty much the only thing you can change to affect the power supply sag (other than turning down the volume). Since a rectifier tube increases its internal resistance as it gets used up, a new rectifier can keep the power supply charged up to a higher voltage under maximum load, meaning less sag. The same holds for trying a different type of rectifier tube. It is the internal resistance of the tube (as noted by the voltage drop across the tube) that determines how much voltage it can keep the filter capacitor charge to. The lower the resistance, the higher the voltage that gets to the filter capacitor. Note: The listed voltage drop and the internal resistance of the rectifier tube, while related, are not the same thing. The voltage drop specification is determined at a specific current level through the tube, which is dependant upon the tube design. Thus a tube with a lower voltage drop may actually have a higher resistance than a tube designed for higher current levels. To determine the internal resistance, check the specifications of the tube. If you can't find the resistance information, you can determine it by looking for the voltage drop rating, which is normally given at a particular current level. Divide the voltage drop given by the current given to get the internal resistance. Example: A 5U4GB has a voltage drop of 50V at 275mA Thus the internal resistance is 50/0.275=182 ohms. Why Arcing is Bad As well as causing damage to the tube itself, arcing can damage the circuits in which the tube is used. When a tube begins to arc, it starts to tear apart the tube, especially the cathode material. This can create a non-rectifed resistance path between the cathode and plate. This has similar effects as a short between the cathode and plate. Meaning bad things happen. What that means is that the tube no longer blocks the reverse electrical path. While that in and of itself is no worse of a problem for the tube (the damage caused by the arcing itself being the real damage to the tube), the rest of the circuits will not be happy to have reverse voltage being applied to them. In particular rectifier circuits pretty much always have a filter capacitor that is polarized. If a reverse voltage is applied to a polarized capacitor, it will damage the capacitor, often resulting in catastrophic failure (ie the capacitor blows up). If the arcing continues for any length of time, the parts of the tube that the arc is tearing apart can start to lay down a electrical path (sometimes called a carbon track), between the cathode and plate (and/or grid for non-rectifier tubes). This will permanently damage the tube, although the damage to the cathode will already have degraded the tube to likely being useless anyway. The more the cathode is damaged, the fewer electrons it can release, which is equivalent to the tube quickly being degraded to end of life condition. ------------------------------------------------------------------ Understanding Rectifier Tubes A simple way to look at a rectifier tube to better understand how it works is to see it as an ideal rectifier (a one way valve with no voltage drop) in series with a resistor. The resistance is fairly stable within the bounds of the normal current levels through the tube. It is the resistance of the tube that defines the voltage drop of the tube. Just as more current through a regular resistor increases the voltage across the resistor, an increase in current through a rectifier tube causes an associated increase in voltage drop across the tube. So the voltage drop is dependant upon the current through the rectifier tube. Note: Gas rectifiers are an exception to this as the plasma generated by the gas tube during operation causes the current flow to follow plasma rules rather than vacuum tube rules. Plasma based retifiers are generally negative resistance devices. That means they reduce the internal resistance as the current through the tube increases. This is because as the current increases, more plasma is generated which provides more paths for the electrons to follow. That translates to a lower resistance. The result is that a gas rectifier will tend to have a constant voltage across it, whereas a normal rectifier tube will have a constant resistance (meaning the voltage drop changes with the current flow through the tube). Of course as with all things about tubes, these are only generalized aspects about the operation. Tubes are non-linear, especially at the outer limits of their normal operating parameters. However within the bounds of their normal operating paramters, they will tend to be consistent. The construction of a tube defines the resistance of the tube and that determines to a large extent the maximum current through the tube. The type of material used in the cathode coating and the heater voltage and current determines how many electrons the tube can release. That together with the distance between the cathode and plate, which determine how far the electrons have to travel determines the tube resistance (gas rectifiers being the exception - see note above). The distance between the cathode/heater and the plate determines the maximum voltage the tube can handle. Another important part of the rectifier circuit is the filter. The rectifier itself is actually a part of the filter that is used to convert the AC voltage into a DC voltage. AC is used because it makes distribution of the electricity simple and efficent. But it also means that to be used in most electronics it needs to be converted to DC. The AC contains a full sinewave of positive and negative voltage. The purpose of the rectifer is to block the unwanted half of the AC power. Usually the positive component. In a full wave rectifier circuit, the other half of the AC cycle is inverted (via the transformer) so that it can also be used. This makes the power supply more efficient by using both halves of the AC signal. Halfwave rectifier circuits only use one half of the AC power source. It is less efficent, but the circuit is also cheaper and can be built without a transformer (which is why cheap radios only use half wave rectifier circuits). Once the AC is converted through the tube into a pulsing halfwave signal, it needs to have a capacitor to help smooth the power into a stable DC voltage. A note about AC power voltage measurement. AC power voltage is measured as RMS (Root Mean Square). What this means is that instances of the voltage are sampled over time (measuring an instant in time of the various voltage levels of the waveform) and each resulting measurement is then squared and the result added to all the other squared measurements. The total result is then divided by the number of measurements and the square root is taken of the result. The answer is then an idealized result of the voltage as if it had been a DC voltage instead of an AC voltage. The classic way to measure it is to apply the voltage to a precision resistor and measure the heat generated. That is then compared to the heat generated when an equivalent DC voltage is applied to the resistor. Since that method is not very accurate, it is pretty much never actually used. Modern meters just use the RMS calculation method. The peak voltage in an AC power source is 1.44 times the RMS voltage and conversely the RMS voltage of the peak voltage in a perfect sinewave is 0.707 times the peak measured voltage. This is important to note because the DC voltage derived from an AC power source will be 1.44 times the AC RMS voltage. Assuming a perfect conversion and there is little or no load on the supply. Since there is a pulsating DC voltage that comes out of a rectifier that has stripped off the unwanted half of the sinewave voltage, a device is needed to temporarily store electrons to smooth out the voltage to something stable (ie a DC voltage). That is the purpose of the filter capacitor. (A choke is sometimes used as well - we will get into that a bit later.) The voltage that comes out of the rectifier starts out at zero volts at the beginning of the half portion of the sinewave. The voltage then rises to the peak voltage in a little over 4 milliseconds (60Hz power), then falls back to zero volts again in a little over 4 millseconds (5 milliseconds for the 50Hz folks). It then stays at zero volts for a little over 8 milliseconds (10 millseconds for 50Hz) while the other half of the sinewave is being blocked. For a full wave rectifier, the other half of the rectifier tube is doing the same thing to the inverted half of the AC during the off time of the first section). So what the circuit does is that during the time that the voltage is available from the rectifier tube it pumps electrons into the capacitor (referred to as charging the capacitor) as well as providing the electrons to the rest of the circuit. Then when the voltage drops below the voltage present on the capacitor, the electrons for the circuit will come from the capacitor instead of from the tube. Trivia: Airplanes use 400Hz power instead of 60Hz or 50Hz so that the power supplies can be made smaller and more efficient. This is because at 400Hz, the amount of time between cycles is 2.5 milliseconds. That means smaller capacitors can be used and the recharge time is faster so there is less waste heat being dumped. The downside is that 50/60Hz based equipment won't work on aircraft power systems. The equipment has to be specifically designed for use on an aircraft power system. As a more visual description of what is happening, let's say we have a device that uses water (our electricity analogue) to power it and an assistant with a bucket (the rectifier analogue) to get water from the river (the AC power source analogue). So, our assistant runs to the river and gets a bucket of water then runs back and pours the water into our water powered device. Then runs back to the river and gets another bucket of water, repeating this over and over. The problem is that our water powered device is going to be running in spurts as it only gets power when the assistant pours the water into it (which is what causes hum in the electrical circuits). To make the device run continuously, we need a constant source of water (electricity). We need something to store the water temporarily. So we grab a bucket (the capacitor) and poke a hole in it. Now we have the assistant pour the water into the capacitor bucket and the water slowly comes out of the hole and powers our water circuit. By picking the right size bucket and making the hole the right size, we can have enough water to power our water circuit while our poor harried assistant is rushing back and forth to the river to fill the rectifier bucket with water and rushing back to pour it into the capacitor bucket. The amount of water and the size of the hole in the capacitor bucket determines how quickly the water drains out. Too small of a hole and we don't have enough water to drive our water circuit. If we don't have enough water in the bucket, the water going out the hole will slowly reduce as the pressure of the water (the voltage) behind the hole reduces. If the bucket is too small, we will run out of water before our assistant can fill it again from the rectifier bucket. So why not make the capacitor bucket bigger? We can, but that means that our rectifier bucket will need to be bigger to keep it filled. Our much put upon assistant might not be happy at having to carry a bigger rectifier bucket. It will wear the poor fellow down much faster because he is having to carry much more water (ie electrical current) on each trip. So how about making the hole bigger (less resistance) to make sure there is enough water to run the water circuit? The problem here is that we may run out of water in the capacitor bucket before our assistant (who is now thinking about finding a different job) can fill the capacitor bucket again. One trick is to get another assistant to help out (full wave rectifier). That doubles the amount of water being transferred from the river, and if the two assistants alternate, the capacitor bucket will be filled twice as often without having to make our assistant run twice as fast. Note: aircraft use 400Hz power, which means that we are expecting our weary assistant to run 6.7 times faster, so we will have to hire a strong young athletic type to do the work instead. But even though more expensive and having to use specialized hardware, we can make the system smaller and lighter. Since we don't have the money to create a whole new system, plus by now the whole town is using our old system, we have to work with what we've got. And we have a problem. Our water circuit is slowing down as the water level in the capacitor bucket drops because the pressure behind the hole in the bucket is going down as the water level drops and that results in a slower stream of water coming out of the bucket. So over lunch we talk to our friend the fireman about the problem and he says "Hey, you could borrow my firetruck and firehose for your experiment! That way you could fill the bucket in no time at all!" That seemed like a great idea, more water to fill the bucket more quickly. That way we could keep it near full all the time which would keep the pressure of the water coming out of the hole more stable. So we set it up, point the firehose at the bucket and turn on the water... and the bucket goes flying, destroying half our water circuit. Oh darn! The bucket just wasn't capable of handling that much water all at once. It needs to be be filled at a careful rate to keep from destroying it. (This is the resistance of the rectifier, or in the case of a solid state circuit, the actual resistor in series with the rectifier). Of course we could get a better bucket, but that will cost lots more money, and firehoses aren't exactly cheap either. So in order to keep costs down, we try to balance the cost of the parts with the actual needs of the circuit. Another trick that sometimes is used is to add a choke to the circuit. This would be like having our poor worn out assistant (who has just notified us he can't take it any more and is giving us his two weeks notice), instead of pouring the water into the capacitor bucket directly from the rectifier bucket, just gives it to another assistant who gives him an empty bucket and sends him off back to the river. So while our soon to retire assistant is collecting more water, our new assistant is slowly pouring the water from the choke bucket into the capacitor bucket. This helps to keep the water level in the capacitor bucket more stable. If we balance it just right, the choke bucket will run out of water just about the time that our retiring assistant manages to struggle back from the river with another bucket of water to give the new assistant to replace the now empty choke bucket. The assistant then gives the empty choke bucket to the tired and soon to retire assistant to go fill again. Then once again starts emptying the new bucket of water into the capacitor bucket. This works great, but we do have to pay for a second assistant for this system to work. But it does mean that if we balance things out properly, we can reduce the amount of water we need our overworked retiring assistant to drag back from the river each time, as we only need enough to keep the capacitor bucket near full rather than having to bring more than we need each time to keep the bucket pressure up if we didn't have the second assistant there to pour more water into the bucket capacitor to keep it full. That means our rectifier bucket assistant doesn't have to work as hard, so maybe he won't quit after all. So, why can't we just put a bigger capacitor into the circuit? That way it will hold more electrons and thus keep the voltage to the circuit more stable. There are two problems that approach. One is that capacitors don't like having large amounts of current driven into them. If you make the capacitor bigger, that means that yes it will keep the voltage more stable, but it also means that the amount of time available to pump electrons into the capacitor is reduced. So we end up with a current surge into the capacitor. In addition, the rectifier is going to have to supply that surge, which means more current though it for that short duration. That means it may not help much, because the tube resistance determines how much current can be supplied by the tube. So making the capacitor bigger can quickly become a effort of no improvement as the tube just can't supply the current needed in such a short time duration. In addition, it means that the charge current will only be pulled from the AC power at the peak of the curve, so you are trading time for current surge. This will distort the AC power and may actually cause an increase in hum in the rest of the circuits due to the surge caused by this approach. Not just because of the current surge, itself, but because of the magnetic field of the transformer seeing shorter pulses as the current gets pulled from the transformer in short bursts. This is not only reflected back into the AC power source, but also the magnetic field flutter can potentially be picked up by the rest of the tube circuits. The other problem is that with a bigger capacitor, when you first turn on the circuit, the capacitor has to be charged from an empty condition. The bigger the capacitor, the more power that is pumped into it very quickly at turn on. This stresses both the rectifier and the capacitor. The result is that both will end up having shorter lifetimes because of the stress. So a better solution if you really want to make the voltage more stable is to consider using a choke in the circuit instead. It is more expensive and bulky, but it is also a more appropriate solution to the problem (assuming there actually is a problem). But if you are using a tube rectifier, likely in many cases the tube resistance is enough as long as you pick the right size filter capacitor. The advantage of the choke is that it can provide power to keep the capacitor topped off while the output of the tube rectifier is in it's off phase. During the charge part of the cycle, the choke builds up a magnetic field. While it is doing this, it also adds a bit of resistance to the circuit so that the charge current into the capacitor is reduced. Then during the time the rectifier is in the off phase, the magnetic field around the choke collapses. This generates electrons to feed into the capacitor to help keep it charged up. The trade off is that to work best the coke and capacitor must be tuned to the average current draw of the circuit, and you will still see some voltage saging outside that range, but it is still better than a simple resistor in the filter as long as you don't mind having a big bulky expensive choke and that your load on the power supply is relatively consistent. In some cases if you don't need a lot of current, you can consider using a pi filter. A pi filter is just another resistor/capacitor stage added behind the tube to provide added filtering. This is sort of like what the choke does, except it is smaller and cheaper to implement. The trade-off being that it is not quite as good or efficient as the choke method. You can also use a choke in the pi filter instead for greater efficency, although at greater cost and bulk. Well, Our poor worn out assistant has left us, so we need to find a new one. Unfortunately nobody in the village wants the job. So we need to come up with a different method to fill up the capacitor bucket. The only thing that comes to mind is our friend's fire hose (ie a solid state rectifier). So maybe if we build a stronger capacitor bucket the system will survive. That is how modern power supplies do it without having to add resistors or chokes to the power supply. They just use more expensive capacitors that can handle the higher current peaks that occur with the solid state rectifiers. As an interesting side story, early in the 21st century (around 2003 to 2006), a company stole the recipe to make high current capacitors from Nichicon (a Japanese capacitor company). Unfortunately the recipe they stole was missing a key ingredient. The high currents that occur in the capacitor cause hydrogen to be released inside the capacitor (similar to the way hydrogen is separated from oxygen in water with electrolysis). At lower currents this effect is minimal and not a problem. At high currents the gas can build up and eventually cause the capacitor to blowup from the pressure. The missing part of the recipe was the chemical that recombined the hydrogen with the electrolyte. Since it was missing, the capacitors from the company who had stolen the plans were self destructing after a short period of time. Depending on use, anywhere from a few months to a few years. A lot of equipment that had used those capacitors (pimarily those built in Asia) failed in a very short time. You can tell when the capacitor has failed because the top of the capacitor will be rounded instead of a flat top. Also in some cases, especially where the current levels were very high, there may be a white colored coating on the capacitor. That is the electrolyte that has escaped the capacitor after it blew up. On the top of the high powered capacitors you will see some lines cut into the metal. These are there specifically to provide a weak point for the capacitor to pop open should too much gas build up inside the capacitor. Without the cuts, the capacitor could explosively self-destruct causing more serious damage as the capacitor case becomes a ballistic missle. However, back to dealing with the loss of our wayward assistant... By using a solid state rectifier (the fire hose analogy), we can eliminate the need to have a filament and make the device much more efficient because it has a much smaller resistance (less than an ohm). The only thing we have to deal with is that because of the low resistance, the solid state diode can supply much higher currents. So much so that even the diode itself can be damaged unless it is controlled, or at least a diode is selected that can handle the currents that will be encountered. If a resistor or choke is not used to control the current into the capacitor, then a capacitor also must be selected that can handle the peak currents that will occur in the circuit. The worst case time of the circuit is during poower on. That is the point where the capacitor is completely discharged, so the diode has to supply large amounts of current to the capacitor during the first sinewave or two when power is switched on. The amount of current is determined by the internal resistance of the capacitor, the internal resistance of the diode and the internal resistance of the power transformer. If more current occurs than any of the parts can handle, the power supply may fail. This is why it is common for equipment to fail most often when power is first applied. That is the moment of highest stress. Once the capacitor reaches full operating charge, the current demand is usually much less, however it will still be "peaky", meaning that unless a choke is used, the current will only be supplied to the capacitor during the peak parts of the AC power waveform. How much of the peak current is used depends on how much the capacitor needs to be recharged and the resistance of the various parts of the power supply circuit. While the simple rectifier/capacitor circuit works well for most things, for some circuits it is still not good enough. The village people have been using our water power system for a while now and have started to build more complex devices that use the water. But now are complaining that the water pressure is not stable enough. So you hire a new assistant to man the valve (a voltage regulator) you installed on the hole in the capacitor bucket. His job is to keep the water pressure stable by adjusting the valve to keep the water in a glass column the same level all the time. There are three types of voltage regulators. A shunt regulator (gas tube voltage regulator or zener diode), a series regulator (sometimes referred to as a linear voltage regulator) which is more common, and a switching regulator which is becoming more common as it is the most efficent type of regulator (but with it's own issues to deal with). The shunt regulator works by having the maximum amount of water (current) flowing out of the power supply plus a little bit more. The shunt regulator diverts any unused water away from the rest of the system. In our water system, this would be like having the valve installed on the capactor bucket dump any excess water on the ground. In an electrical circuit we just dump the extra current as waste heat. The method is not very efficient and rather wasteful, but for low power low cost circuits it can sometimes be the simplest and most effective solution. The series regulator is like a normal water valve such as the one that you use to control the water flow in your sink or shower. The series regulators are one of the more commonly used and have been in use for many decades. They particularly came into major use with the solid state circuits which often require larger amounts of current and very stable voltages. A simple water pressure management version would be to install a float in the column of water being used to monitor the water pressure and attach the float to the valve so that it causes the valve to be opened wider or narrower to keep the column the same level. We're still dumping the excess water on the ground, but it is under tighter management and we can change the water pressure more easily by changing how the float controls the valve. So it still is not very efficient, but it is more efficent then the shunt regulator, especially if the demand for water is highly variable but we still need to maintain the same water pressure. The switching regulator is the most efficient way to regulate the water/voltage. In this case, we replace the valve with a shutter that can be opened or closed very rapidly. We have our assistant watch the water column very carefully. When the water pressure starts to rise, we have him immediately close the shutter. When the water pressure starts to fall, we have him immediately open the shutter. The advantage of this system is that we don't waste water by dumping it on the ground. Only a little bit is used by our assistant when he gets thirsty. So the system is highly efficent. There is a down side though. That opening and closing causes the water to pulse through the pipes. That can make them ring similar to how you can sometimes hear the water pipes in your home go clunk when you open or close the water valve to your sink. To some extent the distributed capacitance in the circuit will dampen this out, but not completely. Plus the switching regulator actually needs the voltage to rise and fall a bit in order ot know when to open or close the valve. For most digital circuits this is not a problem as they are designed with an expectation of some circuit noise and can ignore it. However for sensitive audio circuits it might not be good enough. So what is done is that a low drop out series linear regulator is installed after the switching regulator to even out the voltage. Since the series regulator only has to deal with a very small variable voltage, the amount of wasted energy is much smaller than would be the case if it had to regulate the original voltage from the power supply. There is a downside to switching regulators. As well as the current surges that have to be delt with, they generate higher frequency wideband noise in the circuits that can create problems, especially with audio circuits. So a switching regulator is not always the best solution even though they can be the most efficent solution to power regulation. They are also more complex which can make them more expensive. It also means there are more parts that can break which means they have a shorter life expectancy. Although the greater efficiency can make them worth the added expense, and the life expectancy of electronic components cna improved a lot in the last few decades. A well designed switching regulator can often last the life of the device for which it is providing power. In addition, the greater efficency means they produce less heat which means that often the overall size of the device is reduced since there is less need to provide the needed space to dump waste heat and less heat means less thermal stress on the device, which usually translates into a longer life for the components. In the terms of our water device, think of what it would take to design a device that moniters the water pressure column float and convert that to a device that amplifies the changes in water level monitored by the float to open and close the shutter valve rapidly to maintain the water pressure. We've replaced our generic human assistents with specialized water manage equipment. That allows our poor tired assistent to find more productive work. So our townspeople are happy. They have a stable water pressure and minimal waste of the water, which is important as the town is growing and more and more water usage is being demanded from the limited water supply. And since we can't seem to find any more assistents to manage our water pressure system, the automated float control has solved that problem. So everbody is happy. Also see the companion files: http://www.fourwater.com/files/fullrect.txt http://www.fourwater.com/files/voltreg.txt http://www.fourwater.com/tubeinfo.htm