Step 3: The Audio Circuit: What's Going On


The basic principle behind this amp is pretty cool and worth taking a look at. This amp is called a 6V6 Push-Pull amp, the 6V6 being the tube model, but what's this push-pull business? It refers to a special case of differential circuits, where a whole signal is made up of the difference of two voltages, a negative and a positive (technically speaking, only the output stage is push-pull, but both are differential). Simply put, one tube produces to the positive half of the AC audio signal, pushing the speaker out, while the other one produces the negative half, pulling the speaker cone in, and as a result sound waves are created. Take the two triodes in the preamp, for example. Their cathodes are connected together and the signal goes to the grid of only one of the tubes. So, when the signal goes high, the grid on the one tube repels less of the electrons jumping off the cathode, and so more rush across, and since the cathodes are connected together, electrons also rush out of the cathode of the other tube as well. This results in current flowing in one direction through the circuit and eventually through a transformer which produces a current forcing the speaker cone outwards. When the signal goes low, the opposite occurs. The grid repels electrons back to the cathode and these electrons go to the cathode of the other tube, sending current in the opposite direction as before. When this current goes through the primary coil of the output transformer, it induces a current in the secondary coil which causes the speaker cone to move inwards.

Before it does go to the speakers, though, the signal is filtered through the two .33uF capacitors (DC can't pass through these, since a capacitor is technically a break in the circuit, but AC can) and makes its way on to the output stage, or the main amplifier section. Then, everything happens all over again, except this time with the 6V6's and to a greater extent. The -20 DC voltage in the middle there is for the bias on the grids of the 6V6's. The shield of the 6V6's is connected through a resistor to the anode, so that it has a fraction of the anode voltage.
Thank God for William Shockley that we don't have to play with infernal tubes for much today. Certainly not for audio amplification. Tube audio amplifiers belong in museums next to the stone knife and bear skin display!
<p>Often times, technology is used by the rich to make more money and customers are convinced the new technology is a further advanced and more convenient. Just look at the frequency reproduction differences between CD's and LP's. Same difference between Tubes and Transistors and with tubes you can fine tune the sound by changing out the tubes. But, it is all about how involved we want to be in what we do and the end result we wish to enjoy. Though, there are people that can not tell the difference in the sound reproduction differences between tubes and transistors!</p>
<p>the thing is, ears and brains are notoriously bad at judging subjective sound quality. It's why I'm a firm believer in &quot;listen to what makes you happy in the manner that makes you happy&quot;! If you think LPs sourced through tube amps and paper speakers sounds better then a digital audio file then listen to it that way and be happy! For example, I personally think the blues sound horrific but plenty of people would call me insane and love the stuff. </p><p>That said, I think tubes as a technology are amazing and awesome. I mean, can you take a silicon based electronic component and repurpose it as an X-ray emitter and make your own simple Xray machine? You can do that with some old vacuum tubes! Of course, it's a fair bit harder to kill yourself with silicon diode then it is with a big old vacuum tube, but hey that's part of the fun!</p>
<p>What is stopping you from adjusting the sound by changing out transistors? Having character curved traced a number of transistors myself I can assure you they are <strong>NOT</strong> all the same! Ignorance like you're displaying now is what perpetuates the popularity of tubes today.</p>
<p>When someone is this closed minded, beware. The reason that many still enjoy tube amplification certainly isn't they are a bunch of tone deaf fools. </p>
<p>Only a fool would think that tubes have better signal handling characteristics than solid state does. Instruments far more sensitive than ears physically prove this too.</p>
I wouldn't mind a bear skin coat...growing up with -35&deg;C winters really makes you appreciate good ol furs and leathers<br><br>as to stone knives, dunno about those...but properly sharpened, even some hardwoods outdo the crap you see in stores these days<br><br>tube amps rawk
<p>Could you just connect a wire to a lightbulb filiament and get binary in light?</p>
<p>What are some safety tips for me when working on it?</p>
Hello! I live in Florida, any idea were can I learn electronics in order to build a beauty like this? Thanks!
<p>online!!!! lol look up &quot;Uncle Doug&quot; on youtube he literally takes you though the steps of building one including troubleshooting and how the whole thing works!! and by your definition of &quot;Learn&quot;, tube amps have a HUGE amount of allowed error in them compared to transistors thus trial and error is a VERY good way to learn... just pull up an old fender guitar amp schematic (ex: the simplest 5F1 that &quot;I&quot; learned with) and build one!!! and if you have any problems email ppl on forums if you cant find help I have a guy who used to engineer tube broadcast stations in korea and I can ask him questions so just email me (<a href="mailto:ltdavidm200@yahoo.com" rel="nofollow">ltdavidm200@yahoo.com</a>) or (<a href="mailto:VanessaYamiR6@yahoo.com" rel="nofollow">VanessaYamiR6@yahoo.com</a>) and id be glad to help</p>
<p>the transformer hum is literally from the 60Hz (mains electricity frequency) hum which vibrates the transformer quite a bit to the point where you can fell it by touching it (kinda feels like your phone vibrating) which just means your laminations for your transformer core isn't solid enough or youre using an air coil (saw it in a photo up there ^) second using copper sheets for the grounding plane will result in hum due to ground loops that can lead to hum or even modulation of the sound waves. By experience the unmatched tubes for the PP (Push Pull) system doesn't have that much of a significant effect although the PP will be a source of modulation if not correctly designed. Also the hum might be through the induced frequencies from power lines etc. Thus cross all wires at right angles and have everything neat and tidy (trust me.... i built tube amps for a while and It helps to be OCD when building tube amps...mods repairs and fighting hum just gets so much more easier) and one more point that not many people know.... have the transformers all on different planes meaning to put the power transformer upright then the output transformer laying down etc. so that the core of the magnetic field does not interfere with each other. happy building :)</p>
<p>the hum is <br>A: from the preamp circuit picking up on the AC frequency<br>B: bad capacitor lineup (Especially in the filter capacitors for the DC power)<br>C: unmatched output tubes in a push-pull style amp<br>D: poor placement of ground point or buss thus creating a &quot;loop effect&quot;</p><p>For the transformer hum IDK what the problem is. Most high end audio transformers are encased in tar for this exact reason.... new &quot;Better&quot; transformer maybe? switch from a toroid to a iron core? </p>
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<p>how are you determining the Vdrop resistiors?</p>
Hi great tutorial I was toying with the idea of building one of these. However I am in the UK which has a mains voltage of 240V as opposed the the lower american 115V. Would i just need to change the hammond 167G120 for one of the hammond 229 series transformers? <br> <br>Regards <br> <br>John
Slightly off topic here but can someone put my mind at rest and tell me the name of the piano music that plays during the video of the chap making hand made valves?
thobson - I am in the process of building this exact amp based on your instructable and info from Poinzie's DIY Audio Pages (Poindexter's Website with the 2002 design). I ordered mostly all the parts you used except I had Edcor make me some custom Audio Out Transformers and I used a high dollar caps for the coupling caps. I am planning on building Poindexter's Moebius Linestage that was made as the counterpart for this Amp once I finish the Amp and get it working. I have tried relentlessly to get a hold of Poinz but AudioTropic seems to no longer exist and all of the emails address I find for him come back undeliverable. I was hoping you could answer a question about the Amplifier that I have, as this is my first Tube Amp project (but not my first electronics project). The Hammond Power Transformer supplies 120V to the power circuit...how does the half-wave diode bridge and the couple capacitors increase the voltage to the req'd 320V for 'A' and 305V for 'B'? I have not finished wiring the power supply up yet so I have not had a chance to test the circuits to see what the voltages are but I am just curious because Poindexter mentions using Transformers with 275Vac or 300Vac HV Secondaries for the main power supply in his 2002 schematic. I just want to make sure that this circuit will achieve the correct voltages in order to power the amp to its fullest capacity. I am aware the Vdc is equal to 1.4 times the Vac but with that said - 120Vac x 1.4 = 168Vdc. Do the diodes and caps in this main power supply circuit somehow double this voltage again?? Thanks in advance for any insight you can provide. I figured you would be the guy to ask since you built this same design I am working on and have already tested the voltages and have seen what is going on.
Nevermind....Since I posted this, I have built this particular tube Amp and the Line Stage. I have learned many things about Tube Amp Building and Designing Tube Amp Power Supplies now. I am actually working on a modified version of the 2006 Musical Machine - a 12AX7 driven 6550 Ultralinear design. Obviously, the entire power circuit will have to be redesigned to power the &quot;bigger&quot; 6550's. Using Thobson's Walk-through posted above, I am achieving 9.2Watts on each channel for a total of 18.4Watts MAX with my 6V6 Musical Machine. This is a little more power than is typical with this design but my voltages are a little hotter but not out of spec from what is listed on the 6V6's Data Sheets. I am hoping with the 6550 Machine I can achieve an output power of double what the 6V6 Machine is capable of. For those wondering, this amp (the 2006 6V6 Musical Machine) has plenty of power to fill a large room or small house with sound. Currently I have two Definitive Technology 6.5&quot; Bookshelf Speakers connected to it. These are the rear speakers for my Home Theater Surround Sound System, but are doubling in use as my main speakers for the Tube Amp at this moment. I am using these as they are the highest sensitivity speakers I have at the house (93dB). In a 1450sq ft house, at full volume this amp is too loud to listen to when sitting in front of it, but perfect if I'm doing chores and cleaning the house up. You will want some sensitive speakers for this Amp as they will make it louder. I will be building some Folded Horn Enclosures soon to put a 6.5&quot; 97-101dB speaker in and these will become the dedicated speakers for my Tube Amps. Thanks Thobson for an awesome write-up....good luck to everyone else who is embarking on this journey and building their first Tube Amp....it is an educational experience and very exciting, mine was also quite shocking (literally), so be careful when testing the power supplies. I took 400Vdc to the wrist and elbow several times (grounded my arm accidentally while working on the amp). Fortunately it's not the voltage that kills but the amperage and in the case of this amp we are only dealing with 1Amp or less (not enough to kill but definitely enough to not feel good). Be smart and be careful! Good Luck!
Transformers can conduct large amounts of current. Current produces a magnetic field. A magnetic field affects current. Thus, if transformers are close enough to one another, each transformer's magnetic field will affect the current of the other one. That affectation will be evident in the output audio signal. <br> <br>How to avoid this? Mu metal. It has an extremely low magnetic permeability. This means that if you build a shield of mu metal around the transformer(sort of like a faraday cage), the magnetic field produced by that transformer will be conducted through the Mu metal instead of the air. It will also insulate the transformer from external magnetic fields (ie: the other transformers). <br> <br>In short: build a shield of Mu metal around each of the transformers, and each transformer will not be affected by any magnetic fields around them. No foreign magnetic influence = no audible distortion.
nice job I was looking online for a push pull schematic this one is the one for me, the others online were using parts that all together costed &lt;$500<br>thanks for posting,<br>John<br>p.s. when all said and done i will upload some pics of the amp
Sweet! Fantastic job! Well done, end to end.
AAA GOBBLE GOBBLE <br> <br> <br>me want that amp <br>to bad i cant order most of the parts (i am dutch) <br>and the shipping would be to expencive <br> <br>somone help?
Regarding tubes vs digital/solid state distortion discussion from early on in this forum: <br>Tubes certainly do distort the signals they amplify. But the two mediums distort the signal differetnly, and to many music lovers, the tube type of distortion is less offensive. For example, tube electronics generally produce more harmonic distortion than SS/digital. But, our ears themselves have harmonic distortion. The sound your brain percieves has some harmonic distortion produced by the ear drum and its likages. So, you could say that harmonic distortion is a sort of &quot;natural&quot; distortion and is less offensive than other types. Digital/SS electronics is good at throwing a lot of random hash into the signal. Think scraping fingernails on the blackboard. Not a musical or natural form of distortion. <br>MP3 is a whole other deal. These super-compressed audio flles mash the life out of music. (Never use the &quot;MP&quot; word around a self-respecting audiofile.)
It's not that simple! Harmonic distortion measurements typically are stated as &quot;THD,&quot; or &quot;Total Harmonic Distortion.&quot; What exactly IS harmonic distortion? It is called &quot;harmonic&quot; distortion because it is the addition - by the audio gear - of multiples or &quot;harmonics&quot; of the original fundamental tone. For example, an &quot;A&quot; of 220 Hz will be amplified as desired, but the amplifier will add its own artifacts or &quot;distortion products&quot; to the original, including the multiple of two - 440 Hz, the multiple of three - 660 Hz, four - 880 Hz, five - 1,100 Hz, six - 1,320 Hz, etc., etc. - all the way up to the limits of human hearing and beyond. Conventional tube and solid state amplifiers add greater or lesser amounts of harmonic distortion to any signal fed to them. Most people can tolerate higher amounts of distortion from tube amps because tube amps mostly add low-order, even harmonics. That is, the harmonic distortion consists almost entirely of second, fourth, sixth and eighth harmonics, which are musically related to the signal; they represent octaves and fifths above the original tone, which blend musically with the original tone. They make the original sound &quot;richer,&quot; to many peoples' ears, even pleasing. On the other hand, solid state amps - besides the low-order even harmonics - add more of the higher-order odd harmonics; seventh, ninth, eleventh, thirteenths, fifteenths, etc. These are often NOT musically euphonious; they represent dissonant intervals of seconds, fourths and sevenths. And the higher in frequency they are, the more AUDIBLE they are. SO, a person may listen to a tube amp with 2% low-order even harmonic distortion and not be irritated by it - may not even notice it. But that same person listening to a solid state amp with 2% high-order odd harmonic distortion will be driven from the room holding his ears! Double-blind listening tests have shown that a wide cross-section of people can detect high-order odd harmonic distortion at MUCH lower levels than low-order even harmonic distortion. In fact, the difference is greater than TWO whole orders of magnitude! A factor of ONE HUNDRED! And with all respect, your statement, &quot;Digital/SS electronics is good at throwing a lot of random hash into the signal&quot; is mostly hogwash! Whatever &quot;hash&quot; there is is certainly NOT random. Well... MOST of it isn't. As I explained, &quot;harmonic&quot; distortion&quot; is related to the signal in a very precise and mathematical way. Likewise, the other kind of distortion plaguing amplifiers, &quot;inter-modulation distortion,&quot; is ALSO related to the signal in a very precise and mathematical way. The ONLY random thing (technically, even this is not mathematically truly random) is the white noise that all electronic equipment adds to any signal. This white noise is all around us; it is literally the background echo of the Big Bang, the very explosion that created the universe as we know it today. As for MP3's, your statements about those are also a bit misinformed. &quot;Mash the life out of music&quot;? What the hell does THAT mean? I would agree with you that &quot;super-compressed&quot; MP3's sound bad. But not all MP3's are &quot;super-compressed.&quot; Most that one would buy online ARE highly compressed, with bit-rates typically 128 Kb/s to 192 Kb/s. But MP3's can be encoded up to 320 Kb/s. In controlled listening tests - on MOST musical material - even self-appointed &quot;golden-eared audiophiles&quot; can't reliably and consistently hear the difference between uncompressed audio and 320 Kb/s MP3's. I can hear the difference MOST of the time, but the difference is not that huge, not night-and-day - not even over a good audio system. Well-ripped and encoded MP3's CAN sound quite reasonably good except with the most critical program material that is particularly sensitive to digital compression artifacts. For example, classical music with lots of high percussion. Things like triangle, wood block, snare drum, xylophone, can display digital pre-echo very noticeably under some conditions. But at higher bit-rates, even this effect is usually unnoticeable except rarely. But I will give you that &quot;super-compressed&quot; MP3's (I've seen some music MP3's encoded at the abysmal 64 Kb/s) as well as typical Apple iTunes MP4's, are awful and for that reason, I don't buy stuff online very much. But there ARE places online that do sell decent quality MP3 downloads as well as lossless FLACs. Now, let's talk about digital electronics - specifically digital &quot;class D&quot; amplifiers. They introduce their own kinds of distortion, but most of them aren't &quot;random hash&quot; either. Like with all things human-made, some Class D amplifiers are well-designed and executed, others are not. They tend to get a bad name because they are used in things like cheap portable MP3 players and the like. Why are they used in cheap things like this? Because they are not only inexpensive to make, they consume very little power - an important consideration with battery-powered devices. Class D amplification is - by its nature - EXTREMELY efficient; for every 100 watts in, you get 95-97 watts out. I have listened to good class D equipment, built by dedicated DIYers who know what they are doing and use the very best quality components for their builds. These amps are VERY impressive, especially for their relatively (compared to comparable quality tube amps) low cost. Categorical and sweeping statements like yours are misleading at best, propagating even more mythology among the buying public than there already is. The bottom line is, LISTEN!! Regardless of what kind of media are being played, or what kind of electronics is in the box driving your loudspeakers, what SOUNDS good, IS good!
what was the total cost of this build? <br>
what was the total cost of build?
I'm sure I'm missing something here, but I just wanted to get a clear answer. This instructable appears to be geared towards using the tube amp for playing music from Ipods, CD players, etc.<br /> <br /> but would you say the sound quality would be suitable for guitar? sure, I'm open to the idea of altering the design to make it suitable for guitar (adding an EQ, distortion, etc) but what do you think?<br />
Unfortunately this amp is not suitable for guitar. The signal coming from an electric guitar pickup is very weak, and would not be sufficient. CD players and ipods have a little more oomph to them, so the amp has to do much less work than it would for a guitar. You will hear something, but it won't be very loud. There are some really good instructables on guitar tube amps, as well as a ton of information available from the Google machine. <br />
Actually, all that you would need is a preamplifier of some sort to bring the signal from the guitar up to line level. A small tube preamp can be built with instructions from this site, or you can buy a small solid state one for a rather trivial price.<br />
If you were thinking of putting a guitar through this amp,the stereo feature would get lost in most cases. Now if you had something like a guitar processor (some of the Line6 products for example come to mind) might be the ticket... tone shaping AND preamp - with adjustable output. I just might have to try this.<br><br>Roughly, as a stereo amp, how many watts per channel is this design capable of delivering, with a decent preamp?
Crap. Well, back on the hunt. There's another tutorial on here for building a tube amp for guitars, but the author was a little unclear on some parts. But hey, thank you anyway. You made a great tutorial.<br />
wtf people! tube amps have NO quality loss when amplifying audio signals thats why there so great. digital amplifiers turn nicely rounded sound waves into nasty square-like sound waves, tube amps do not do this, the sound reproduction is perfect.
I don't think that's necessarily true. There may not be quality loss, but I do think the sound is modified, in a good way though. I understand the whole thing about digital amps turning perfect sine waves into square waves, but each little step is so small that it's impossible to hear. It's kind of like the difference between a digital photo and an analog film photo. Yes, technically the resolution of the digital photo is limited, while the analog photo is infinite, but digital photos can be taken at such a high resolution that you can't actually tell. And if the vacuum tube is superior because it recreates the sound perfectly, then there is absolutely no point in listening to an ipod or any other digital media through a tube amp. The vacuum tubes cant turn that digital sound back into analog sound. Also, there are amplifiers that are analog but are not tube. So why do people still insist that tubes sound better? I'm not arguing that tube amplifiers don't sound better. I'm just saying that they aren't perfect. In fact, I think it's the imperfections that make it great.
Incorrect.<br />
I'm not gunna be dogmatic about it, but I think red is correct. Distortion and poor sound quality comes when tubes are overdriven. This doesn't usually happen with a quality stereo amp with plenty of headroom, but is intentionally done with guitar amps in order to achieve a specific sound.
There are many types of distortion - wave clipping is not the only kind. For example, if you look at any audio tube datasheet, you will notice a &quot;THD&quot; or &quot;Total Harmonic Distortion&quot; percentage listed as a property.
Thanks for the info :D So is it that tube systems reproduce certain frequencies that digital typically does not and so its a tradeoff between different freq. reproduction profiles, or is it that some people just plain like the distortion that tubes produce in all their music?? I guess it could also have to do with the sampling rate required with digital compared to the more steady current produced from a tube system?
Sorry if I missed it in the comments already, but I assume that I will need to use a preamp if I am to hook this up to a turntable. Is this accurate?
How much did this cost to make, along with how many hours did you put into it.<br>Thanks, Austin
The cost was somewhere in the region of $250 - $400 all told. It took me about a weekend to make.<br><br>thanks for your interest!
I have some questions about the power supply 1. Why does it need to be grounded to earth? 2. I noticed that the positive supply only uses 2 diodes. Why not 4? The negative Supply uses 4.
Very nice job.&nbsp; I'm looking into building a similar amp and have a question.&nbsp; The schematic shows 2x 5965 tubes and 2x 6V6 tubes.&nbsp; Where are the other 2 6V6 tubes used?<br />
That's just one channel of the amplifier. What looks like a pair of 5965 tubes is actually a single tube - 5965's are double triodes. The two 6V6s are a differential pair that together amplify a single channel. So, each channel has one 5965 tube (which contains two triodes) and two 6V6s. I hope that helps!<br /> <br /> thanks!<br />
I know that this might seem like a fairly obvious question, but I just built a vacuum tube amplifier and my tubes' plates glow a slight red when the music is playing. Is this normal operation, or am I putting too much power through them? The particular tube that I am using is the 13EM7, thank you!<br />
Well, all tubes are supposed to glow, but if the actual plates themselves are doing it that might be a little bit worrisome. If that is the case then you probably are giving them a bit too much juice. Just go carefully through the schematic and check your circuit. One way to troubleshoot is to plug in a wall wart type power adapter (~12V or so) and check your circuit at various points with a multimeter. There are also lots of good websites and forum threads online about this sort of thing. Maybe check out <a href="http://www.kbapps.com/tubecare.html" rel="nofollow">this</a> as a start. <br />
I think I've tracked the problem down - I didn't have the funds to buy any sort of output transformer, so I instead used a 120VAC step-down transformer - the voltage drop across the transformer is too low, so the plate gets too much voltage, the tube becomes overbiased, and the plate reddens.<br />
I always wanted to make a vacuum tube computer, but I know it would cost an arm and a leg not to mention a ton of time to make a calculator.<br />

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