Building a Stereo Tube Amp





Introduction: Building a Stereo Tube Amp

Ever wanted to build a highly dangerous, inefficient, yet awesomely retro piece of electronics? Well, I have. That's pretty much what a tube amp is. Vacuum tubes are old electronic components that act like transistors, controlling a lot of current with a little current. You usually hear about tubes being used in guitar amplifiers, because they distort in a way that suits guitar playing. However, tubes can also be used to amplify a stereo signal from another audio source such as a CD or MP3 player. Tube amps, unfortunately, aren't the most practical things in the world; they consume a great deal of power, get very hot, and are big. That being said, they look damn cool, and some people seem to think they sound pretty nice, too.

You can learn a lot about electricity and electronics from a project such as this. Going through the process of purchasing parts, planning, and executing is a useful experience for any maker. Keep in mind that I am just a dude on the internet - take everything I say with a grain of salt. Except, of course, for these next few sentences. This project is dangerous in a very serious way. It involves high voltages and a lot of current that can make you feel decidedly unpleasant or even decidedly dead. If you decide to work on it with the power on, be careful. Some of the capacitors in this amp will hold onto a charge for a long while after the power has been switched off. Discharge all capacitors through a resistor connected to ground, preferably with a voltmeter across it to be absolutely sure the cap has completely discharged. When testing the amp out for the first time, use something like a twelve volt power brick instead of plugging directly into the wall, just to be safe, as well as to prevent things from exploding or melting. An old trick is to keep your left hand in your back pocket all the time, so if you do get shocked, it hopefully won't reach your heart.

Also, you'll need to know how to read a schematic, solder, and use a hand drill.

Step 1: First Things First: Tubes

Quick Intro to Tubes

A project such as this one can teach you a lot of about electricity and electronics. In order to appreciate what more or less goes on in this amplifier, we need to take a look at how a few things work, and first up are the vacuum tubes themselves.

A vacuum tube is just a glass tube that's had all the air sucked out of it. A good example is the early lightbulb, inside of which a filament was subjected to a current which caused it to produce energy in the form of light and heat. If the interior of the lightbulb were not a vacuum, the filament would experience combustion due to the high temperature in the presence of oxygen and be destroyed.

The Diode

The most basic vacuum tube is a diode, so named because of the two electrodes which it contains. Vacuum tubes are said to be thermionic, meaning that heat is used to encourage the emission of electrons from an electrode. If a negatively charged electrode (as in it has an excess of electrons, in tubes it's called the cathode) is heated by a filament, the electrons in it become 'excited' and, if a sufficient electric potential difference or voltage is present, they will actually be emitted from the electrode. The potential difference is due to another electrode, called the anode or plate, which is positively charged and therefore exerts an attractive force on these enthusiastic little particles, causing them to whip across the vacuum from the negatively charged cathode. Since electric current flows from negative to positive and never the other way around, the current can only flow one way in a diode. However, this isn't much good for amplification, so let's move on to the next step in the vacuum tube hierarchy: the triode.

The Triode

In a triode, there is a third electrode in addition to the anode and the cathode, called the grid. The grid carries a negative voltage and is connected to an input source, which, in the case of this amp, is a fluctuating audio signal. The negative voltage on the grid repels a certain amount of the electrons leaping off of the cathode, and how many electrons it repels depends on how negative it is, which in turn depends on the amplitude of the audio signal. This is by definition amplification: using a small amount of electricity to control a large amount. The audio signal is quite small, and it is effectively controlling the flow of however many electrons we choose to saturate the cathode with. The base voltage of the grid, the voltage that is present whether there is a signal or not, is referred to as the bias voltage. The bias determines how much current the tube draws when there is no input signal.

The Tetrode

Add yet another electrode and you get a tetrode. This new electrode is called the screen or shield. Basically, the screen prevents the flow of electrons across the tube to be affected too much by the anode, and instead be totally controlled by the changes occurring in the grid. It has a smaller positive voltage than the anode. When the electrons initially break free from the cathode, they can only 'see' the screen, and so they rush towards it, and upon reaching it notice the anode and continue on towards that.

The Pentode

Now we come to the pentode. In a pentode there is, obviously, a fifth electrode that wasn't there before. This is the suppressor. The suppressor sits between the anode and the screen and is somewhat negative in charge. Its purpose is to make sure any electrons that might bounce off the anode don't escape for too long by repelling them back into it.


Pentodes are not the end of the road when it comes to tubes, there are many other kinds, and if you're interested in them, check out the links step. There are two kinds of tubes used in this amp: triodes and tetrodes. The triodes are used in the preamplifier stage, where the audio signal is stepped up to a certain level suitable for further amplification by the tetrodes of the main amplifier.

Also, check out this fantastic old-fangled movie about tubes and how they are made:

Step 2: Parts: What You'll Need


The guy who originally designed this amp, Poindexter of Audiotropic , has a wealth of information on his site about the parts he used. His device, called the Musical Machine, is made up of some very nice, but very expensive audiophile-type components (high quality specialty hookup wire, extra fancy solder). These components may well make for great sound, but somehow I just can't bring myself to spend $35 on a capacitor that I can have a reasonable facsimile of for two bucks, and my uneducated ears probably can't tell the difference anyway. So, for that reason, I have built the same circuit as Poindexter, but with cheaper and more available parts.

What You'll Need

The amp consists of two sections, the audio circuit and the power supply. Let's look at the parts in both.


You're going to need something to put your amp in, and there are definitely certain considerations to be taken when picking an enclosure. Transformers and especially the tubes themselves can get very hot, and so need to be in a well ventilated kind of area, preferably jutting out the top of the case. Unfortunately, in my case, I foolishly chose an enclosure purely for its looks, which ended up being a bit of headache. I picked an old wooden cutlery case, which just happened to have rather thick panels, making it painful to mount components that were designed to be mounted on circuit boards or thin metal project boxes. So, unless you want to spend the time making that kind of case work, I'd say go with something more conventional.


What to Buy

You'll be making the audio circuit twice over since this is a stereo amp, after all, and so you'll need two output transformers. Poindexter uses very fancy special order transformers that are way out of my price range, so I settled for a Hammond 125D, which, as far as I can tell, works fine.

There are four transformers in the power supply, one for the heating filaments in the tubes (Hammond 166N6), one for the main positive supplies (Hammond 167G120), one for the negative supplies (an Amveco toroid), and a 'choke' transformer (Hammond 158Q). A choke isn't really a transformer but an inductor. Basically, the function of a choke is similar to that of a capacitor in parallel, it resists sudden changes in current, and so acts as a filter.

Where to Buy It

I got these transformers from Angela Instruments which has a ton of old high quality audio components. Another good place is Parts Connexion . The Amveco toroid I ordered from Digi-Key (part no. TE62045-ND).


What to Buy

The tetrodes used in the main amplifier section are four Electro-Harmonix 6V6EH tubes, which are pretty easy to find. For the preamp tubes, we have two 5965 tubes which are double triodes, meaning each tube actually contains two triodes. If you look at the guts of the tube you will notice two separate metal sheaths; these are the triodes. Tube sockets are also essential for affixing the tubes to the case.

Where to Buy It

I got the tetrodes from [htttp:// Tube Depot] and the double triodes from Antique Electronic Supply . The sockets came from Antique Electronic Supply , eight-legged ones for the 6V6's and miniature nine-legged ones for the 5965's.


What to Buy

For the most part I just used regular metal film resistors, although there are a couple of high-quality Kiwame's in there just because I had them. The 60K and 62.5 4W resistors that you see in the schematic, called the plate load resistors are actually each two 2W resistors in parallel, two 120K's for the 60K and a 120 and 130 for the 62.5K. A few nice resistors are pretty cheap and most people insist on using good ones for the plate load, but they aren't necessary everywhere. The 100 ohm resistors connected to the 6V6 screen are rated at a 1/2W and I didn't bother with anything special, just regular old resistors. Same goes for the 100K bias resistors, 1/2W, metal film.

Where to Buy It

You can get metal film resistors at various ratings at any electronic supply store in your neighbourhood, or you can order them online from a variety of suppliers like Digi-Key . For the fancy stuff try Parts Connexion , Angela Instruments , or Percy Audio . Of course, there are lots of other good suppliers, as Google will tell you.


What to Buy

The two 0.33uF capacitors in the circuit are meant cut off any DC coming through and allow only the AC audio signal to pass. While expensive capacitors would be nice here, I'm cheap and so I just used 630V Mylar capacitors (the ones that look like big red rectangles). Actually, the ones in my amp right now are 0.47uF, but that just means I'll get a little more bass. All the capacitors in the power supply are standard components, except for the few that I replaced with some orange drop-style caps that I had.

Where to Buy It

I just bought the 0.47uF filter caps from a local store, but if you want the shmancy ones, you'll have to try places like Angela Instruments . The power supply capacitors were all ordered from Digi-Key .


What to Buy

I just used the UF1007 1000V 1A diodes recommended by Poindexter. Nothing expensive is really necessary I think, unless of course you choose to use tube diodes. Tube diodes will cost you extra and require a power supply for their heaters, though.

Where to Buy It

You can get them from Digi-Key or Parts Connexion and probably lots of other places.

Potentiometers, Switches, and Jacks

What to Buy

There are only three potentiometers in the amp, one for volume, and two for adjusting the bias on either channel. The one for volume is a double deck pot. There are two switches, one for the main power supplies and one for the tube heaters. Regular old single-pole double-throw switches are fine here. Since I only have one input in my amp, I don't have a DPDT to change between inputs like Poindexter. You'll also need two sets of speaker winding posts, so you have something to hook the speakers up to, as well as a set of stereo input jacks, and a plug for the power cord.

Where to Buy It

I personally didn't splurge on an expensive volume pot, but lots of people think it's an important part, so if you have the money feel free to splurge. Again, try Parts Connexion , Angela Instruments , or Percy Audio . For switches, posts, and jacks, try Radioshack or any other electronic supply store. The plug/fuse box for the power cord was from Digi-Key , and its part no. is Q201-ND. You will of course need a fuse for this, I think a 1A one, which you can get from any electronics supply store.

Other Stuff

You will need some screws, nuts, and bolts and those sorts of things, which you can get at any hardware store. As for hookup wire, Poindexter uses some pretty fancy stuff, but being the cheapo that I am I just used regular hookup wire form Radioshack.

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.

Step 4: Layout/Planning

This is probably the most important step of the whole project, as Poindexter points out. Just putting everything in the right place to begin with can make the experience so much nicer. Being new to the while tube amp thing, mine isn't exactly pristine, far from it. But it works, so there.

You'll want to keep the power supply and the audio circuit as two separate sections/sides. All the tubes should go together in one area, and the output transformers somewhere nearby. Putting transformers too close together can apparently cause hum, but I'm not picky enough to complain about it, so mine are pretty close together. Some of the hum issues can be solved by putting metal covers over the transformers and grounding their cases properly. In this amp, the bolts fixing the transformers to the case are firmly attached to the ground plate by nuts. Apparently, the positioning of transformers relative to one another also helps to eliminate hum. If you rotate the transformers 90 degrees, so that they are not in line with each other, you get a certain decrease in hum. The angle isn't always 90 degrees though, so if you're very picky and patient, carefully adjusting the angle until it is optimum is definitely an option.

The circuitry in many tube amps is implemented a little differently than in other electronic projects. printed circuit boards are not always used, and they weren't here. It's more of a free-form circuitry type thing, with components soldered directly to each other, and terminal strips as little hubs of connections. This isn't the most organized way to do things, but it's faster than designing and ordering PCB's, and proto-boards can take up more room than is available in your enclosure (although now that i think of it they might work just fine).

Step 5: The Build: Enclosure

The enclosure that I picked was an old cutlery box. Honestly, I wouldn't recommend this kind of case, since, as good as it looks, it is a hassle. The sides and top are quite thick, making it difficult to mount components meant for thin metal or plastic cases without tediously grinding away at the thing. Also, a metal enclosure makes it easier to ground things since you can just connect ground to the chassis and ground anything in the circuit by connecting it to the chassis. With the cutlery box, I had to put copper-coated plates in there to solve this problem. But, if you want a cool looking wood enclosure and you don't mind the extra work, go for it.

Step 6: The Build: Getting Started

So I put the power supply on one side and the audio circuit on another. On the inside of the case, I attached a copper-coated plate that would serve to ground anything connected to it. This makes it easy to ground the exteriors of the transformers and hopefully reduce some hum. The transformers aare firmly connected to the plate with nut and bolts, and so fix the plate itself to the inside of the case.

Step 7: The Build: the Power Supply

As long as you can read a schematic and solder, you should be able to put everything together. However, if you're like me, you'll do it wrong the first few times. As you can see, there are two copper plates, one for the power supply and one for the audio circuit. It would have been even better to have one big plate covering the entire surface, but I didn't have one big enough, so I had to make do. In this first picture, I was just getting a feeling for the layout of the rest of the amp, figuring out where to put the tubes and how to bridge the gap between the audio circuit and power supply.

In the later pictures, there's some of the 'free-form' circuitry I was talking about. There are these little clumps of components soldered directly to one another, which is a little more confusing than using a PCB, but it's quick and does save you the trouble of worrying about making space for and possibly designing a circuit board (and it's hardcore). The terminal strips keep most of them anchored fairly securely to the case, anyway.

The Amveco toroid is wired a little strangely, but never fear, there's a very handy diagram.

Step 8: The Build: the Audio Circuit

The audio circuit might seem a bit messy. I was soldering components to the legs of the tube sockets, there were wires criss-crossing, and so on. Thankfully, though, it works.This is a first-time kind of project, but it's also one that you can go back to again and again and improve and update, and I think I probably will.

Those red capacitors there are supposed to be 0.33uF, but they're actually 0.47uF, which isn't really a problem, it just means I'll get more bass. They also happen to hold on to a big charge, and so you should be extra careful with them and remember to discharge them. These components are ones that are probably worth replacing with high-quality parts at some point in the future, since most people agree they are very important to the sound of the amp. (Oh yeah, and don't forget, you're building the audio circuit twice - this is a stereo amp, after all!)

Step 9: The Build: Testing and Finishing Up

So once you've put it all together it's time to see if it actually works. However, don't plug it in directly into the wall at first, because if you do and you've made some wiring mistakes, you could end up frying some components. If smoke comes out of it, that's probably not a good sign. You could use a power adapter for a phone or some other low-ish voltage device to check everything first. Connect this to the terminals of the power socket and use a multimeter to test the points on the schematic where the voltage is indicated. With a power adapter like the one pictured, the voltages should be off by about a factor of ten or so. If everything looks basically right, go ahead and plug it in and hope that nothing bursts into flame. Once the power supply is behaving, hook up some speakers and see how it sounds. Keep poking around until it works, but be very careful (check the Intro if you don't know what I'm talking about).

Step 10: Conclusion

It works, dammit. And it's pretty.

(The big 167G120 transformer does hum a little I've noticed, any ideas on what to do about that? I think it's most likely just the construction of the transformer itself, so maybe I just need to upgrade)

Step 11: Links and Notes

Poindexter's Musical Machine

All About Circuits' Introduction to Tubes How Vacuum Tubes Work

Fun With Tubes is an amazing resource. (thanks satman!)

There's always Wikipedia

There are lots of good books too, such as Valve Amplifiers by Morgan Jones, which thoroughly covers all the fundamental tube concepts as well as amplifier design. Jones has a companion book called Building Valve Amplifiers which covers all of the physical design considerations that go into building a good tube amp, including component layout, enclosure design/construction, and performance testing and troubleshooting.

The Power Vacuum Tubes Handbook is a great, detailed textbook on tubes.


Poindexter has since updated his design, and was kind enough to provide the schematics, pictured below.



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    Looks fantastic, modeling my first tube amp off of this guide. My question is do you place the potentiometer before or after the 100KOhm resistors?


    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 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 "bigger" 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" 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" 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 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!

    A current of around 30 mA (0.030 amperes) is potentially sufficient to cause cardiac arrest or serious harm if it persists for more than a small fraction of a second. Therefore it is mandatory that household wiring is fitted with ground fault circuit interrupters. Are your sure it was "only" 1 ampere?

    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!

    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!

    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 NOT all the same! Ignorance like you're displaying now is what perpetuates the popularity of tubes today.

    As a former professional musician and lifelong audiofile, I promise you that IN GENERAL tube amps sound better than solid state. There's a reason that many amp manufactures have employed 'tube emulation' in their sound. Some even patented the name of their technology like "Transtube". Funny that tube amp manufactures have not tried to emulate the solid state sound.

    As a myriad of total harmonic distortion specifications plainly show vacuum tube technology cannot match the performance of solid state devices. So even if manufacturers of tube amplifiers tried to emulate solid state sound, they could not. The technical reasons why tube amps remain popular today are too complicated to get into here. It mainly boils down to a mixture of ignorance, and nostalgia though. With a healthy helping of simply being difficult thrown in, for good measure, of course.

    Wow ! Ignorance ?? Sounds like you need to finish reading ALL the specs. Your blanket statement regarding solid state out-performing tubes is embarrassingly ignorant and wrong. I challenge you to find 1 commercial broadcast station, radio or television, that uses "solid state devices" to broadcast a 100,000 watt or more signal. A walk in the park for tubes, been doing since before Marconi invented the dipole. I admit, this is of little use in home audio amplification but tubes do have a place and that place is well respected by the solid state design engineers.

    I would take a Tube Amp over Solid State any day of the week! I simply let my ears decide which sounds better. My experience, and I've had several high-end SS Amps in the past, has been that Tubes simply reproduce a much warmer more natural sound with a lot more detail and a much wider sound-stage that no SS Amp has ever been able to. So for my critical 2-ch Stereo and Vinyl set-up, I use 2 Mono Tube Amps. For my Home Theater I still use Solid State.