How Tube Amplifiers Work, Part 2: The Pre-Amp and Power Amp

greetings and welcome to part two of this video series in which we discuss how amplifiers work from an electrical standpoint in part one we talked about the primary circuit here 120 volt 60 cycle we talked about the 5 volt filament circuit the 6 volt filament circuit and the high voltage circuit that produced the 325 volts of direct current that we were going to send up to the rest of the chassis of the amplifier let's pick up where we left off ok now we're going to pick up where we left off with the 320 volts DC this is what we found our B+ and it looks like this a bunch of little humps side by side all the positive humps we're going to send it two ways first some of its going to go up here through the primary winding of the output transformer and then it's going to come over here to the plate of the 6v 6 now earlier I said that when the electrons are boiled off of the cathode down here like popcorn we want to have a strong vacuum up here to really attract them and nothing attracts negative electrons more than a nice positive three hundred and twenty volts of direct current on this plate now I think you can see that we've established a really strong vacuum here for the electrons that are leaving the cathode secondly we've accomplished something by sending it through this coil now we're not sending this direct current remember it can't pass through the transformer but the coil will act to smooth out these humps the humps will be resisted somewhat and smoothed out it's sort of like we're taking some of the kinks out of those of the humps here in the the rip of the direct current so we're smoothing it out and sending it to our plate remember whatever didn't go up above we're going to send over in this direction now we're going to have a filter capacitor in this case I think it's 16 micro farad's and what that will do I have a separate video on filter capacitors that you might want to look at and it explains how it's going to do this but it's going to smooth out the ripple then we run it through a resistor and that also smooths out the ripple so look here big pronounced humps but the filter capacitor and the resistor smooth it out to where the ripple is not nearly as bad let's put another filter capacitor and another even higher resistor here and now we've got that B+ or the high-voltage direct-current very smooth and flat here with no ripple hardly to speak of just to play it safe let's put another filter capacitor okay so we have a total of three and you notice what they're doing they're compressing that rippling down to where we get a nice flat smooth direct current and we're going to send that up here to the plates of the 12ax7 remember there's two of them because the 325 volts has gone through one two three resistors one for each plate here and these are 100k it's dropped down now to about a hundred and fifty volts DC it has the same purpose to be like a vacuum for the electrons here that come off of the cathode but in a preamp tube there's not as many electrons and there's not as neat a need for as much vacuum if you will so we run them at a lower voltage also we have to have an absolutely smooth buthe 150 volts at DC or else it will be very noisy your preamp tubes are very very prone to if there's any noise it will be amplified over and over again and come out where it's really a racket coming out the speaker a buzz a hum or something like that so you want to have extremely clean direct current coming in here to the plates and that's why we weren't so fussy with the plate of the 6v6 but we ran through three capacitors and three resistors to really filter that direct current for the plates of our 12ax7 okay just a quick review then our high voltage comes up to this stage with all sorts of ripple and it is sent through the primary the output transformer to smooth out a lot of the ripple and then sent to the plate of the 66 and then it's filtered through three separate filtering stages to get it really smooth and really flat and then it's fed into the plates of the 12ax7 now this is the final diagram and I have the high voltage still marked in orange here and we're going to take a look at our guitar signal and we're going to feed it in now and this will be the final circuit of the app and that is what is the guitar signal circuit in an amplifier and how does it end up coming in here with just a bear fluttering of strings and end up over here blasting till the neighbors call the police okay something really important is going to happen in here and I think we have all the building blocks to understand how that happens but first let's take a look at how a guitar generates that signal and what the signal looks like what is its magnitude how many volts is it and what is its frequency okay let's measure the voltage of the output signal of a guitar no amplifier is involved here it's strictly the pickups of the guitar connected to a voltmeter I'm going to turn the voltmeter to alternating current strike a few chords and see what kind of readings we get okay the highest I saw is 0.78 volts so let's say that the output from the guitar is around 0.8 volts of alternating current let's take the a string here for example when it's plucked in the open position it vibrates back and forth at a hundred and ten cycles per second as it vibrates back and forth over the magnet that is located beneath it in each pickup it will generate within that pickup an alternating current signal of 110 cycles per second then leave the pickup go to the amplifier through the cable and there the amplitude of that signal will be greatly increased okay now that we know what the guitar signal looks like let's feed it in here to our circuit here's our guitar signal and as you notice it's a very small little weak signal at 0.8 volts of alternating current and 110 cycles per second because that is the a string we're going to feed it in over here to our input jack we run it through a 68 K ohm resistor largely to cut down on feedback and we feed it into the grid of the 12 ax7 now I've split the 12 ax7 apart just like I took a chainsaw and cut it down the middle it just makes it easier to understand if I move the two halves apart okay they're absolutely identical so we're going to feed into the grid here of the left half of the 12ax7 now remember that the popcorn the electrons are boiling off the cathode just boiling vigorously off of the cathode and we've been kind of tricky here and that we've created our fence our grid to where it effectively blocks most of the electrons the way we do that is we charge the in negative the electrons come boiling off of the cathode all excited looking for someplace to go they say oh my lord look your ugh a negatively charged fence how repulsive and they're repelled back down into the tube they don't pass through so by negatively charging the grid and that's the way when we talk about grid biasing that's how it's done is we give this a negative charge it's like an electric fence to repel the electrons and keep them under control now up here remember we have a 150 full a volt DC vacuum that wants to really absorb every electron that that cathode can give off but this pesky grid is interfering with that now what happens next is I'm going to make a really crude analogy here but imagine that if you went out to your tire on your car and you took off the valve cover cap off the stem ah now inside you have what 30 35 pounds of air now imagine that every air molecule in your tire is an electron and you're going to go out in the garage and you're going to touch the valve just touch it very lightly with like a little pin or a needle and all of a sudden a blast of electrons are going to or air molecules you're going to come out of the tire well imagine that that's exactly what's happening here when the guitar signal comes in here it's going to alter that negative charge on the grid and it's going to alternately sort of open and close the grid in the rhythmic fashion exactly the way the string was vibrating and we're going to say open-close open-close open-close and we're going to each time that we get the guitar signal hits and alters the charge here of the grid millions of electrons are going to come flow out of that cathode and blasting into the plate and they're not just going to do it just like in an overwhelming blast oh no they're going to be modulated exactly like the guitar string so if I have the guitar signal looking like this then on my plate suddenly I'm going to get big blasts of electrons oh no electrons Oh huge blasts of electrons and as you can see the tiny signal that I applied to my grid becomes a huge signal over here on my plate as a million electrons slip through for every say ten or twenty electrons that pass through as part of my guitar signal I hope that makes some sense now here remember the rule that DC and AC can coexist in the same wire so we've got 150 volts DC here creating our vacuum and our plate but we have our signal remember that great big signal that we have because the grid was modulated a little bit and let millions of electrons come blasting through exactly in rhythm with our guitar string here comes that signal and it comes out and comes over here and the last thing I want to do is apply 150 volts to the grid of the second stage of my 12 ax7 it would just destroy it so let's put a capacitor here now this is what a coupling capacitor is we've all heard of them well this is what they do remember the rule that alternating current which is the guitar signal can pass on through direct current boom it hits a brick wall right here it cannot proceed so this capacitor keeps all of the direct current on the plate and it's sort of like the bouncer at the club when the guitar signal which is amplified now much larger than it was tries to come up here the Kappa says oh go right on through come over here and let's put a volume control in let's have some variable resistance here so that we can control how loud our signal is then let's feed it in to the grid of the second stage here of the 12ax7 and exactly the same thing happens except even more so because we're going to tilt the bow stem a whole bunch now with this high signal it's coming through and when we tilt that bow stem a whole bunch even more air molecules or in our analogy electrons are going to come blasting through from the cathode and into the plate so we're going to have an even greater signal now down here I've drawn sort of and this is really crude but let's look at our guitar signal we can barely see it it's really wimpy down here then after the first stage of amplification you notice that we've got a whole lot more amplitude to our signal or second stage of amplification we've got a whole lot more amplitude and then we're going to use a coupling capacitor here to keep the DC on the plate and we're going to let this much larger signal pass through and we're going to feed it into the grid of our 6v6 tube now this is where big things are going to really start happening this is the power amp part of the amplifier and let's look look up here 320 volts of DC on this plate don't you think that this tube is capable down of a whole lot more amplification than either of the stages of the 12 ax7 with their measly little 150 volts okay so we're going to heat up this cathode and we're going to get 5,000 pounds of popcorn popping down here and we've got a 320 volt DC vacuum up here trying to suck it through and we've got a really strong signal coming in here and really pressing that bounce down and really allowing all sorts of electrons to pass through huge hundreds of millions of electrons are streaming through to create a huge signal now that signal we're going to send out along with remember the 320 volts of DC is present here the signal can coexist with it remember the rule they both can coexist in the same wire here comes the signal now that's a transformer remember about transformers this is the output transformer DC can't pass through can it but here comes an alternating signal alternating current signal is coming through and this is just like the bouncer at the club it says oh an alternating signal from the guitar and look how nice and big it is and lets it pass straight through to the speaker now the guitar signal is been amplified way up to around 300 320 volts but at a very low current level it's only around point O 3 5 amps that's 35 milliamps so it's a very high voltage low current signal which doesn't work well with speakers so the output transformer is going to do what transformers do and it's going to transform this from a high voltage low current signal into a relatively low voltage high current signal it's going to change the 0.035 amps into something more like 2.5 amps and it's going to drop the say 320 volt signal down to around 5 volts now these numbers are really rough but they're sort of in the ballpark okay so we're going to feed that then to our voice coil of our speaker and we're going to allow the guitar signal remember positive negative positive negative positive negative to push and pull by changing the polarity on the voice coil which is inside of a magnet it's either repelled by the magnet attracted by the magnet repelled attracted on the speaker cone we'll move back and forth with exactly the same pattern of motion as the vibrations of the guitar string that we started with in the beginning except it's going to be doing so with a much greater force behind it because of the amplification that's taken place the 0.8 volt AC signal that we put in that pitiful weak little vibrating string has now produced a very powerful signal up around 2.5 or 3 amps to drive the speaker cone well rustier you've thoroughly thrilled with the video so far now guess not now for a study in contrast let's just feed the output from the guitar directly into a speaker to see what our puny little 0.8 volt output signal does with the speaker absolutely no sound whatsoever the speaker just laughs at this puny signal now let's run that puny little signal through the champ guitar chassis that we've been studying and for a little change of pace I'm going to hand the guitar over to Rusty and let him play us a little tune Wow not too bad rusty looks like chasin tennis balls isn't your only Talent well what about does it for this two-part video series in which we investigate the mysterious goings-on in an amplifier chassis all those weird voltages and different types of current hopefully now we have a pretty good working understanding of how the chassis functions from an electrical standpoint and how it changes the laughable cuny guitar signal into something that can make your neighbors cringe in horror I really appreciate the time you spent watching the videos I hope you will subscribe if you haven't already but most of all I hope you stay tuned thanks so much bye for now

Channel: Uncle Doug

Views: 391,677

Rating: 4.9704733 out of 5

Keywords: Amplifier Chassis, Plate Voltage, Plate Current, Coupling Capacitor, Output Transformer, AC, DC, Voice Coil, Smoothing of Ripple, Rusty Solo, Rectifier, 5Y3, 6V6, 12AX7, B+, How Amps Work, Circuit, Guitar Signal, Amplification, Rusty the Wonder Dog, Pre-Amplifier, Power Amplifier, Basic Electronics

Id: 901iaPVVzY0

Length: 20min 52sec (1252 seconds)

Published: Sat Mar 15 2014