How Amplifiers Work: Rectifiers and Filter Capacitors

greetings recently I posted a video describing two types of home that you might find an amplifier 60 cycle generally associated with tubes and 120 cycle associated with filter capacitors shortly after I posted the video a subscriber sent me a message and asked how can a hundred and twenty cycle hum occur I mean where does 120 cycle current come from so I thought I'd post this short and hopefully to the point video to explain how it happens to go into that we first have to discuss a little bit about how rectification occurs in an amplifier now there are several ways to rectify the alternating current a one is to use a single diode and diodes are peculiar little electronic devices that only allow current to move in one direction so here as long as the current is positive it allows the current to flow through the diode and then when it tries to go in a negative direction it eliminates it the next time we encounter a positive a type of wave we allow it to pass so you'll end up with if you put in 120 volts at 60 cycles end up with about sixty volts about half of your voltage because let's face it half of the voltage was eliminated and at about sixty cycles okay now this would not be very useful in an amplifier because it would really produce home that would almost be like a machine gun you can imagine as each one of these waves hits you're going to get a kind of a pop-pop-pop-pop sound it can be smoothed out but it's very difficult I think you'll understand the smoothing process a little better in a second let's look at the way most amplifiers are made and that is with our 120 volt 60 cycle input alternating current we can run it through what we call a full wave rectifier now there's two different types of full wave rectifiers we can use two diodes or a tube or four diodes and that is called a bridge don't be worried right now if this doesn't make much sense I will explain it in just a second now what happens is that with two diodes we have them arranged in such a way that we allow this part of the wave to pass and we allow this part of the way to pass but we invert it and make it positive so where we have one two three four Peaks two on top above and two below the central axis over here we will have all four peaks above and we can say and this is all rather simplistic we can say that they're all plus so this is all positive now negative is gone now if we arrange the diodes in a different direction if we reverse them we would have all of the humps down below and we would have negative direct current instead of positive direct current in almost every amplifier I've ever seen it's always there arranged so that it's positive like this now as you can see we have twice as many humps above the central axis so now instead of 60 cycle we have 120 cycles per second there's 120 of these little humps that will pass by every second it's still around 120 volts we did not lose half of our voltage we have it all again this is rather simplistic but it is at 120 cycles per second now and we have an area in between the humps where the voltage drops off and then comes back and this is called ripple and this will make the hundred and twenty cycle hum if this ripple is not eliminated this is almost like if you watch sort of like a alternating current wave up here involving the peaks and the our areas down here the intersections so that we have a varying alternating current which makes that home that mm kind of home that you might hear if your filter capacitors have failed now filter capacitors acts sort of like that backup battery you have on your computer or say in a hospital operating room if they have a power failure the battery takes over and starts to provide the voltage that's needed to keep things going well that's exactly what happens with your filter capacitors down here as the voltage is peaking it's charging the filter capacitor that's attached to this line then as the voltage Peaks and starts to drop off the filter capacitor says oh my lord look the voltage is dropping I better discharge and fill in that gap so we have a power failure here the voltage the capacitor fills the gap then it charges the capacitor again it fills the gap as the voltage drops off charges over here fills the gap and you see that instead of having this very jagged ripple that gives us the 120 cycling on we have a fairly smooth waveform here that provides a uniform a high voltage for amplifier and without any hum that you can hear now if your filter capacitors are failing if they're holed or dried out these are the electrolytic capacitors we will not have this discharge to fill the gap and we will have 120 cycle hum which you will hear from your speaker now while we're speaking about diode and tube rectification and also bridge let's take a look down here at two possible ways to do that number one will have the full wave rectifier where this is going to be our power transformer this is our primary 120 volt winding here's our secondary winding now one catch if you're going to use diodes or a tube to do your rectification you have to have a center tapped secondary on your transformer and all it takes is the two diodes now I'm going to show you how the tubes contain diodes we'll see that in just a minute but for now you'll see that each of the output wires from the secondary has a diode and we end up with a B+ direct current coming out here that will go to the filter capacitors to be smoothed out now in the bridge rectifier we have four diodes and they're arranged in this rather unusual pattern here and the end result first we don't need to have a center tap on our secondary of our power transformer so the transformer can be a little cheaper we end up with our B+ coming out here this is our direct current positive and we have our negative down below this is generally grounded at this end now what's the advantage of the bridge rectifier you end up with a little more voltage a little more current and you don't need the center tap on the power transformer so it's a little better okay it's a little superior definitely to this method now I've made a drawing here of the two different types of rectifier tube that you're probably going to find in your guitar amplifier if it is tube rectified now this is the typical 5 y 3 5 u 4 5 z 3 and the really old amps 5 r 4 I've seen 6 ax 4 6 ax 5s and some little more recent amps probably from the late 60s now notice it has here a heater and this is what's glowing when you look in at the tube is this and when it gets hot the electrons boil off of it and come here to the plates the plates are positively charged so the electrons are attracted to them the electrons are only going to go in one erection if they're heated and driven off from here and this is positively charged they're only going to go this way so what we have in this type of tube is two diodes one diode two diodes now the other and actually better way to make a rectifier tube and this will be the 5 V 4 5 AR 4 6 C a 4 GC 34 my favorite ah you have a cathode notice that here we have no cathode over here we do now what happens here is the emission from the cathode is secondary we're going to heat it up but the electrons are not going to go from here to the plates instead we're going to this is like turning the burner on the stove and setting a pan on it the cathode is heated by the heater filament and then once it gets hot enough it boils off electrons out to a dip will go to the plates again they only travel in one direction man there's two diodes within the tube but because there is time between turning on the tube and the release of electrons these type of this type of tube has what we call a slow startup and that is once you flip the on switch on to your amp there's not an immediate rush of high voltage to your amplification tubes instead the heater will heat up and then in turn heat the cathode and then the cathode will slowly just like water boiling on the stove slowly start to release electrons to the place this is much gentler on your amplification tubes and makes them last a lot longer this is why these tubes are superior to these tubes now let's take a look at a 5 u 4 tube and I think as you look in you'll see the 2 diodes that are present in the tube we have 2 separate plates and filaments the filaments are down here below there within these kind of gray metal surrounding wrappings and that's over here this is the filament that heats up and these are the plates the Reyes metal plates that are up here and then each of these then is a diode now this is a full wave bridge rectifier this is used in a lot of computer equipment and things of that sort the alternating current connections are made to these two diagonal corners it would be to this and this wire then the positive output has the corner beveled and the plus so this will be your B plus your high voltage that is positive and down here is the negative this would be your high voltage that is negative now here we have a computer power supply with 4 diodes arranged in a full wave bridge configuration notice that there's a silver band at the end of the diode this is the way diodes their polarity is indicated that band corresponds to this line so if you're using diodes in your circuits you have to be sure that the diode is in the right direction with the band in this case would be on the right hand side of the diode now these four then will rectify the high voltage for the power supply there'll be a ripple in that output voltage and that will be smoothed by these two electrolytic capacitors okay that's about it for this video I hope everything made sense and for those of you who didn't know much about how rectifier tubes function or how power supplies work I hope that this filled in some of the gaps just like the electrolytic capacitors do and then I eliminated some of your ripple okay so please stay tuned for future videos I haven't turned up any old amps lately but when I do you'll be the first to know okay I hope to see you soon thanks for watching

Channel: Uncle Doug

Views: 143,736

Rating: 4.9755354 out of 5

Keywords: Electrolytic, Capacitors, Diodes, Full Wave Rectifier, Bridge Rectifier, 5U4, GZ34, 5Z3, 5Y3, 6AX4, 5R4, 5V4, 5AR4, 6CA4, Ripple, Alternating Current, 120 Hz, Half-Wave Rectifier, B+

Id: dl729dEJ3To

Length: 13min 8sec (788 seconds)

Published: Mon Nov 25 2013