Electronics: Tube Amplifiers Explained

While watching the new Iron Man 3 movie a while back, I found myself thinking how cool it would be to have a "lab" of sorts in my house that was dedicated to research and building new technologies or even older, tried and true technologies with a Crow Twist on them.  Electronics have always fascinated me and now that I've gotten older I often wish I had gotten a degree in Electrical Engineering in College instead of a Business Degree.....but who knows at 18 what they want to do with the rest of their life anyways??? If you did, then good for you but like most of the people I know, it took a few years of REAL Life Hardwork to figure out what I DON'T want to do for the rest of my life.  Anyways, this is a totally different topic for another day....I would love to discuss more with you if you would like, but not here and not now.  So, back to the topic at hand.  Last year I started reading and studying about HAM Radio and other forms of Radio Communications, including Morse Code, Packet Radio, etc.  Radio really intrigued me, the more I read, the more I longed for my own Radio Station, the more I studied, the more I wanted to start building my own High Powered 2 Meter VHF Amplifiers and Long Range Yagi Antennas.

I bought two books, one, the "ARRL Operating Manual" and the other, "The ARRL Handbook for Radio Communications".  Both should be owned by anyone wanting to get into Ham Radio, I can't recommend them enough.  The first, the Operating Manual, takes an in depth look at all areas of Amateur Radio. It does not contain any math, nor does it get technical and scientific. There are kids under 12 years of age that are Licensed Ham Radio Operators in this country and they too have read and understand this book.  It is also a great reference for the experienced Radio Operator as it provides information on the newest modes and technologies available.  The second book, the Handbook, is really an engineering manual. It is correctly subtitled "The Comprehensive RF Engineering Reference". I will tell you straight up, I have NOT read this book all the way through and I WON'T ever. However, I have looked up a specific topic in the index and read a few chapters here and there, and I often reference it for information on non-radio projects.  This Handbook covers many different aspects of Electronics, not just radio, and it should since it is an "RF Engineering Reference" (RF - Radio Frequency).  RF encompasses way more than just radio, your iphone, laptop, home phone, alarm clock, and keyless entry to your car all had an RF Engineer on the design team.  With that said, the Handbook is a great reference for any electronics project. It is made up of simple and complex electrical formulas, circuit diagrams, projects, and tutorials. A 12 year old could not comprehend the things going on in this book, but someone who has a desire to learn and truly WANTS to know these things, will be able to read and understand whats going on in the Handbook. 

Reading through these books, led me to the internet, to research more about radio and electronics in my spare time.  Finally, one day toward the end of July I was thinking about Circuit Diagrams/Circuit Schematics. These are basically the blueprints of any electronic item.  With a detailed circuit diagram in hand, one can assemble any electronic device pretty easily.....that is, if you can read the damn thing! To me, these circuit diagrams had always been just a bunch of cool looking lines with abbreviations everywhere, I had no earthly idea what any of this meant or what the hell it was. Seeing more and more of these circuit diagrams during my Radio studies, I decided that I needed to learn to read one (which makes a huge difference in studying for the HAM Radio licensing test - more on this later).  Circuit Diagrams are readily available for any electronic technology, you can find them in books, magazines, the patent office, and of course my favorite encyclopedia of knowledge - the internet!  For example, this is the complete circuit diagram for the iPhone 3GS - now go build yourself an iPhone....seriously, you could, now that you have this.

iPhone 3GS Circuitry

But, this is a very complex and highly sophisticated circuit. It also requires programming software into chips, and tiny little fingers to grasp the tiny little components and solder them to the circuit board.  With that said, you won't find me ever building an iPhone 3GS in my garage, but this circuit does allow me to work on it if I need to. This is the main reason these circuit schematics are so readily available, so that technicians can service a device and replace parts, etc.  You'll find the circuit diagram for your  Washer or Dryer on the inside of the Washer or Dryer Housing, the same with your AC or Refrigerator. The repairman will use this circuit schematic to test certain parts of your appliance and to make repairs as needed.

So, I set out to learn these circuit diagrams, and I did learn how to read them, and fairly quickly I might add. Think of a circuit diagram as a map, lines are roads to different areas and the shapes are the different sights and landmarks to see along the way.  Here's a much more simple circuit than the iPhone 3GS circuit above, and one that is the reason for this post.

Poindexter's 2006 Musical Machine Audio Circuit Diagram

At first glance, it should be apparent how much more simple this circuit is over the iPhones. However, this circuit only represents 1/3 of the entire electronic device, a tube driven stereo amplifier.  This is Stereo Tube Amp's Audio Circuit was designed by a genius named Eric Kingsbury, aka - Poindexter. Poinz is a HiFi tube audio aficionado whom hails from the islands of Hawaii. He ran a company for many years named AudioTropic, and I won't say that he does not still run this company but, it's website no longer exsists and every contact email address I have found for him is no longer valid. Poindexter seems to have fallen off the web sometime after 2010, maybe he will resurface though at some point in the future.  Continuing on with the circuit, as I said this only represents one third of the entire device, it is in fact a single channel of the 6V6 Musical Machine Stereo Tube Amplifier that Mr. Kingsbury is so famous for designing and manufacturing.  There are many different variations of the 6V6 Musical Machine out there, with the earliest version, that I found, dated 2002 and the most recent, dated 2008. The circuit we see above is actually identical to the 2002 circuit in all respects except the addition of the 1ohm 10W cement resistor on the signal ground.  Each symbol represents a component in the circuit. The small Zig Zags are resistors, the round circles with dashed lines in them are the tubes, the up and down small parallel lines are capacitors, and the two smaller circles with the black dot centers on the far left are the RCA input jacks (the place you would plug your iPod or CD Player in).  I just Googled "Circuit Diagram Components" and found many different pages listing each and every component possible. The Europeans use a few different variations of some symbols but they aren't so different that it's difficult to decipher, these are readily available on Google as well.  Let's say we were going to build this Tube Amp, we would have to build the above circuit twice, one for the left speaker and one for the right speaker, and lastly we would have to give it some power so that it can actually amplify the sound and make music....isn't that the purpose of a Musical Machine?  That's where the second circuit diagram comes into play, the Power Supply Diagram shown below.

Poindexter's 2006 Musical Machine Power Supply Circuit

This diagram also comes from Poinz's 2006 design. He revamped the power supply design in '06 using these three fairly inexpensive Transformers so that the DIYer could easily obtain supplies and build this amplifier.  Now, you don't necessarily have to go out and buy these exact transformers, and that's why the voltages are listed as well.  I've found myself recently salvaging parts from old electronics that no longer function, such as the early large box style flat screen TV's, broken VCRs, powered 2.1 computer speaker systems, old video game systems, surplus PC power supplies, and other various devices that were taking up closet space and collecting dust like a 7 to 1 video distribution switching box, CD players and radios, and of course any type of electronic found on the roadside during heavy trash pickup day.  Back to our Tube Amp circuit - essentially, three things must happen in order to provide power to the Audio Circuit. First, the filaments within the Tubes must be heated prior to the Main Power (HT - High Tension power) being switched on. Different vacuum tubes take different voltages, some use 6.3Vac, some use 12.6Vac, some don't use AC at all, but use DC instead which would require some type of rectification built into the heater supply. That's another story and not required for this simple yet elegant build. You will need a 6.3Vac Transformer for this build. If you are lucky enough to have an old non-working Tube Amp on hand, your HT Transformer (High Tension - Main Power Transformer) may in fact have a filament winding on it as well and you would only need to use one Transformer in place of the first two Hammond Transformers listed above.  Poindexter offers a few other circuit designs for other various voltage Transformers at the only website of his that currently exists today - Poinzie's DIY Audio Pages. The site has not been updated since 2002 so the info may be old, but is still very relevant as Tube Amplifier design has not changed much since the 1970s. The second thing that must happen, is that the power circuit requires negative voltages of -160Vdc and -20Vdc, however you get there does not matter so much, as long as you have these voltages by the time you reach the Audio Circuit. The Amveco Toroidal Transformer can take care of this for a really good price if you can't salvage a Toroidal Transformer with dual windings on both primary and secondary sides. For my Houstonians out there, Amveco is a local Houston company, so if your all about buying local then I would definitely use Amveco over the other Toroidal choices.

As Poindexter stated in an article about the Musical Machines Design, he utilized a KISS (Keep It Simple Stupid) mentality while designing and building this amplifier. So, many fancy features, that would require lots of additional circuitry and wiring, were left out. For example, on most Super-High End Commercial Tube Amps made within the last 30 years, you will only have a single power switch. With this design we have two. Having two power switches actually allows the builder to split the power supply into 2 sections: 1. The Filament Heater PSU and 2. Everything Else (HT and Negative PSUs). This design is known as a Human Powered Delay. It is simple yet effective and removes excess circuitry from the amplifier. To power on such devices one would simply flip the first switch (the Filament Heater PSU) allowing power to flow to the Tube's filaments, warming them up. Within 20 to 30 seconds the user would flip the other switch, sending power to all other components of the amplifier. This is necessary in Tube Driven devices for a reason; tubes must be allowed to warm up at low voltages, if full power (usually very high voltages in the range of 400 to 1000 Volts) is applied to the tubes without them having warmed up, it can shorten their life span or worse case scenario - damage the tubes permanently, resulting in poor audio quality.
The third and final thing that must happen to provide power to the Audio Circuit, is the most important, providing a clean and stable, rectified DC Power Supply to the Output Transformer's B+ wire (typically a red wire on the input side of the OT). There are many different ways to rectify AC power before filtering it and sending it on its way to the Audio Circuit. When tubes were first invented and used for everything electronic, tubes were used to rectify power by using various styles of tubes such as diodes, half-wave, or full-wave rectification tubes. When semi-conductors became common place in modern electronics, power rectification could be achieved much more efficiently in less than a quarter of the space in used to take when using tubes. Some tube amp designers still prefer to use tubes to rectify the power, but for this amp, Poindexter chose to use silicon diodes. This is smart for this particular amp as silicon diodes have very little voltage drop, can be purchased by the 100s for a dollar, and put off very little heat.

The schematic symbol for a silicon diode 

Silicon diodes are actually used in three of the four sub-circuits within the PSU of this amp. For the HT sub-circuit, the circuit calls for only two diodes arranged in a voltage doubler configuration. It is in this configuration the diodes rectify the power exactly like the name states, the power is doubled in voltage and converted from AC to DC. After being rectified through the silicon diodes, the newly converted DC power must be filtered to reduce ripple and power spikes. The capacitors following the voltage doubler rectifier steady the power and reduce the ripple and spikes, essentially filtering it. Before it can leave the HT circuit and enter the Audio Circuit on the B+ wire of the Output Transformer however, the power MUST filter through a Choke (Chokes are essentially inductors but housed similar to power transformers in bell housings).  Using a Choke on the HT power is not 100% necessary but it is highly recommended if you want the cleanest, ripple free DC power possible, which is essential for great sounding audio. I use chokes in all of my Tube projects now, and I recommend you do to! If you decide to not use a choke, then a 2 - 5 Watt 60 ohm Resistor could be used in it's place for this circuit, but you won't get the same level of power filtration. Immediately following the choke you will see the largest electrolytic capacitor in the HT circuit. This capacitor is the last filtration seen before the power supply feeds the Output Transformer on the Audio Circuit. This capacitor is usually the largest as this is where the largest voltage fluctuations will occur as a result of high decibel level sound reproduction, extended bass notes, and loud highs. The larger this capacitor is, the more power reserves for feeding the Output Transformer during those moments of extended bass or loud highs, although you don't want to go too large here as it will negatively impact the entire HT circuit. So, power leaves this capacitor, in this case a 120uf cap, and goes two places - one being the B+ line of the OT in the Audio Circuit, and the second place being through a resistor, which drops the voltage down, and into a last electrolytic cap before it goes out and feeds power into the preamp tubes of the Audio Circuit (this is the 295V section labeled with a circled B on the Audio Circuit schematic). The preamp tubes don't draw near as much power as the power tubes being fed from the OT, so the electrolytic cap feeding power to them is typically half the size of the previous cap, for the 6V6 Musical Machine this caps value is 56uf, a little less than half the value of a 120uf cap.
So, our Power Supply is complete and made up by four sub circuits - an HT circuit that feeds the Preamp stage and the OT's B+ of the Audio Circuit, a Filament Heater circuit that delivers power to all of the tubes filaments, warming them up before HT power is turned on, and then two negative power circuits feeding negative DC power to the both the Preamp stage and the Power Tube stage of the Audio Circuit.

It may come as no surprise to you, but all electronics are made similarly. With a tube amplifier you have the two main circuits, the Audio and Power Circuit. With a solid state amplifier using silicon chips, you still have the two main circuits, the Audio and Power Circuit. Every electronic can be broken down into at least two sections, and then those sections can be broken down into even more sub-sections. If you're interested in learning more about electronics and how they work, I suggest starting small like I did with a simple electronic device, learn to read the circuit schematic and then following the schematic trace the circuit board and wires throughout the device if you have it handy, or build it yourself to learn how it works.  Building this tube amp, the 6V6 Musical Machine, taught me a lot about electronics that I never knew, and I've always considered myself to be pretty tech savvy. Even though I was tech savvy and could work just about any electronic device put into my hands, I was unaware of how the device actually worked. I now have a decent understanding of how the power flows through the circuit of an electronic device, how each electronic component functions, and how it all works together to allow that particular device to operate. Recently, I've gotten into working with resistors and LED's as well as powering actuators and small motors to move larger objects. I'm also looking into teaching myself some basic C++ programming and using micro-processors to control different sensors, solenoids, actuators, and motors (basic computerized machinery or robotics). Who knows what I will build next, now that I've conquered the Tube Amplifier the possibilities are truly limitless!