Introduction: DIY Single Ended Vacuum Tube Amplifier

Picture of DIY Single Ended Vacuum Tube Amplifier

For those of you who want to have the warm sound of a vacuum tube amplifier, when playing guitar or just listening to music, but cannot afford to buy an expensive tube amplifier, this instructable is for you. This instructable will show you how to build a vacuum tube amplifier using parts salvaged out of old electronics and 2 purchased vacuum tubes. The tube amp sounds really nice and it amplifies music and guitar really loud. I estimate the output wattage to be about 10-15 watts. For an explanation and a demonstration of the tube amp, watch the video below. Lets get started!

Step 1: Gathering Materials and Tools

Picture of Gathering Materials and Tools

For this vacuum tube amplifier, you will need many different tools and materials.

Tools:

  1. Soldering Iron
  2. Wire Strippers
  3. Wire Cutters
  4. Jig Saw
  5. Pliers
  6. Drill
  7. Screw Driver

Materials: Most of the materials for this project can be salvaged from old electronic devices. The 1/4 watt resistors were bought in bulk on Ebay. The high wattage resistors were salvaged. The capacitors, chokes, power transformers, speakers, wood, and metal slabs were salvaged from old electronic devices. The tubes were found in a lot that I purchased off of Ebay.

  1. Various Resistors
  2. Various Electrolytic Capacitors
  3. Various Film Capacitors
  4. Inductive Choke
  5. Solder
  6. Various Lengths of wire
  7. 12AX7 Vacuum Tube
  8. 6V6 Vacuum Tube
  9. Wood
  10. Power transformers: 120:12, 120:9, 120:2, 0:120:277(Primary)
  11. Metal slab
  12. 4 Ohm and 8 Ohm Speakers
  13. Audio input/output jacks
  14. Volume potentiometer
  15. AC mains switch

Step 2: Setting Up Chassis

Picture of Setting Up Chassis

For this tube amp, I am using the breadboard technique. First, I take a wood slab, and set a metal casing down on the the wood. Then, I plan out where I am going to bolt down all the power supply transformers. After that, I add a back plate to the end of the chassis. In the middle of the metal cover, I glue down a piece of copper clad board and ground it to the metal part of the chassis. The chassis should be done now.

Step 3: Adding the Tube Sockets

Picture of Adding the Tube Sockets

The tube sockets were made by drilling holes in a wood block according to the octal formation of the tube pins. Then, wires are inserted and glues into the holes for easy soldering to the electronics. The tube sockets are then glued down to the copper clad board near the places they will be used. For example, the 12AX7 is placed near the audio input because of it's use as a preamp tube, and the 6V6 is placed near the audio transformer because it is used as the output tube.

Step 4: Wiring the Heaters

Picture of Wiring the Heaters

To wire the heaters of the two tubes, run the two wires for each tube under the ground plane(copper clad board) to the corresponding power supplies that have the recommended voltage for the tubes. Tturn on the power supply and you should see the tube heaters warm up and glow red.

Step 5: Drilling Faceplate

Picture of Drilling Faceplate

The face-plate can be made using an old piece of metal from a power supply. Then, holes should be drilled in the face-plate with the diameters of the switch, audio jacks, and volume potentiometer. When the different components are inserted into the face-plate, nuts can be used to secure them to the face-plate. The face-plate should then be grounded for safety reasons.

Step 6: Building the Power Supply

Picture of Building the Power Supply

To build the power supply, you will need many transformers. For the b+ voltage power supply, you need to use a transformer with primary inputs for 0 volts, 120 volts, and 277 volts. The 0 volt and 120 volt pins are then wired in parallel with all the other power transformers, and the 0 volt and 277 volt pins are wired to the AC pins of the bridge rectifier. The collective 120v wire is then wired through a switch to the AC mains plug and the collective 0 volt wire is wired to the AC mains neutral.

The cooling fan transformer outputs 9 volts AC, and when put through a bridge rectifier, it powers the cooling fan.

The filament transformer outputs 12 volts AC to power the filament on the 12AX7 tube. In parallel with the tube, there is a rectifier and a buck converter module that step down the 12 volts into a usable 6.3 volts to power the 6V6 tube.

There are a total of 3 separate power supplies that are all turned on by one switch.

Step 7: Soldering the Audio Circuit

Picture of Soldering the Audio Circuit

To wire the audio circuit, you mostly have to just follow the circuit diagram. Follow the lines in the diagram and connect the different shown components together by way of the the lines in the diagram. Wire all the ground connections to the the copper clad plate on the bottom and isolate any connections and solder joints from the ground using nails hammered into small wood cubes. When wiring, pay special attention to the voltage and wattage ratings of the capacitors and resistors. Make sure all the solder joints are secure and make sure that all the necessary components have been used. As long as the circuit diagram is closely followed, this tube amp should work.

This tube amplifier uses the dual triode 12AX7 tube to first amplify the small signal voltage that comes from audio in. The voltage then comes out of the tube through a interstage capacitor into the 6V6 tube that drives the audio transformer.

The volume potentiometer controls the grid bias voltage on the 6V6 tube, when the volume is all the way down, the grid is grounded.

Step 8: Audio Transformer

Picture of Audio Transformer

The audio transformer that I found was a power transformer from a radio. It has an input impedance of 1k ohms and an output impedance of 12 ohms. The input impedance is not the recommended impedance for the 6V6 tube, but it works just fine anyways. To find the impedances, measure the coil resistance. Resistance and impedance are not the same, but as resistance ratios match impedance ratios, this method should work fine. When adding speakers, remember to match the speaker impedance to the the output winding(secondary) impedance. The use of the output transformer is to step down the 300 volts at a low amperage that the tube uses into a more usable low voltage high current that drives the speakers.

Step 9: Cooling Fan

Picture of Cooling Fan

The cooling fan is just a squirrel cage type computer fan that cools the 6V6 tube when it gets too hot. It is quiet and blows straight on the 6V6.

Step 10: Building the Cabinet

Picture of Building the Cabinet

The cabinet was built using plywood and particle board. It was built so the two speakers were in the front and the sides and back were open. On the back, two 2 inch wooden dowels support the top. This cabinet was built so the actual amplifier could sit on top. The speaker holes were cut using a jig saw.

The two speakers are connected in series to get the 12 ohms of impedance to match the audio transformer. They are then connected to the speaker jack that I added to the side of the amplifier.

Step 11: Troubleshooting

Picture of Troubleshooting

While building this amplifier, I ran into a big problem: When I turned on the amplifier, smoke started coming out of the power transformer. After problem solving, I realized that I had AC ground connected to chassis ground. Because of the autotransformer setup of the power supply the transformer was shorted out. This issue was fixed by disconnecting the AC ground from chassis ground.

Step 12: It Works

Picture of It Works

The vacuum tube amplifier works! When you plug it in, the tubes take about 45 seconds to warm up. When they do, you can plug in your audio source. There will be a slight 60hz hum until the sound actually starts playing. The tube amp also works well with my DIY electric banjo.

Good Luck Building!

Disclaimer: This project uses very high voltages. Do not build unless you are experienced with building electronics and familiar with electricity. I am not responsible for any harm caused to yourself or others during the building of this project.

Comments

robot797 (author)2016-10-02

tubes dont need to be cooled

why did you cool it?

datacomm (author)robot7972016-10-20

I would be willing to bet it's because he used wood for the sockets and the tube would heat that wood past it's kindling temp or at least char it.

robot797 (author)datacomm2016-10-20

how hot do you think tubes run

datacomm (author)robot7972016-10-20

'how hot do you think tubes run': It depends on the tube, it's condition, and the work it's haveing to do. First, the cathode is heated to a nice glowing temp. Then, High voltage is placed between the cathode and the plate and electrons are pulled across the space between them with the flow controlled by the grid or grids in the tube. Since there is current and voltage drop across that tube, there is heat generated there as well. In old tv sets with 3 tube IF sections, you could find a weak tube by touching the 3 and replacing the coolest tube. It was doing less work than it's partners in line. I've had damper tubes fail due to the glass envelope getting hot enough to flow into the plate. What's the temp of molten glass?

robot797 (author)datacomm2016-10-21

dont lecture me about tubes
i dont use anything else then tubes
in my newest projects

datacomm (author)robot7972016-10-25

Sorry! I wasn't lecturing. You asked so I answered.

robot797 (author)datacomm2016-10-25

sorry it wassnt ment that harsh

datacomm (author)robot7972016-10-25

No worries, robot797. It's good to know there's someone else out there valve oriented!....and, thank you!

datacomm (author)robot7972016-10-20

You're almost correct. Usually, tubes don't need to be cooled but in some cases, they do need to be cooled. Some tubes are made with cooling fins and heat sinks.

robot797 (author)datacomm2016-10-21

duh
transmitting tubes as they are called
had to be small and powerfull
they have to be cooled
if you look at old transmitting tubes
big and no cooling becaus they got space to relece their heat

datacomm (author)robot7972016-10-25

"dont lecture me about tubes

i dont use anything else then tubes
in my newest projects"

EdgarD21 (author)2016-10-02

Sound tend to be better as tube becomes hot............... I must have forgotten it....

gm280 (author)2016-05-17

Nice project. Seem you went through a lot of effort to built it. They still make tube sockets for both those tubes which would have made your efforts a lot easier. And you could have also used a LM7806 series regulator for the drop down voltage from the 12 volt circuitry for the 6V6 tube filament, or even a power resister. But there is nothing wrong with what you made as well. Just making suggestions for an easier to built project.

Are you going to built a tone circuit for it in the future? Maybe a Bass, Treble, Volume type preamp setup. And then work in some Tremolo and Reverb as well. Now that the basic amp is working, add on. Thumbs up!

tanner_tech (author)gm2802016-05-17

Thank you for the feedback! I used a buck converter for the filament voltage drop because it dissipates less heat, where a power resistor or a regulator would have heated up quickly in a very short run time. I might make a tone circuit in the future, there are only a few components needed to adjust tone. Thanks!

DonaldS87 (author)tanner_tech2016-09-11

I want to take this design and put it into my squire champ 15 enclosure. I'll probably have to rethink the power stage and wiring for the 10 inch speaker. I'm curious to see if I can reuse some of the original components because they are for solid state and not tube.

gm280 (author)tanner_tech2016-05-17

TT, I have three Heathkit guitar amps I'm refurbishing and an old tube MusicMan amp as well. I like rebuilt such things. The Heathkits are one of each of their offerings back in the days with the single 12" speaker, a dual 12" speaker dual amp and the combo head unit for their 4 12" speaker models. The MusicMan amp is the head unit. But all are really vintage and nice.

SzabolcsK (author)2016-08-05

WTF? cmon, buy some tube sockets they go as cheap as $3!

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