Introduction: VFD Amplifier: a Tube Amp From VCR Screens

By TannerTech

Everybody wants a vacuum tube amp, but when tubes are in short stock, the best way to build an amp is with a VFD. Vacuum tube amplifiers are rare today because they use obsolete components from the 1960's and before. Sadly, this makes tube amplifiers expensive and tubes hard to obtain. Luckily, there is still a type vacuum tube being made today; a VFD(Vacuum Fluorescent Display). In this Instructable, I will show how to build this vacuum tube amplifier from common parts found in VCR's, microwaves, and other electronic appliances. The YouTube video below will compliment this instructable with a visual demonstration of the building process and a demonstration at the end. Lets get started!


Step 1: How Is a VFD a Vacuum Tube?

To get started, I will give you some background on VFD's. These tubes are the successors of the popular nixie tube. They are meant to be used high contrast electronic displays commonly found in HiFi stero systems, Microwaves, and VCR's. They are commonly known for there blue green hue and brightness. Now though these tubes are modern and used to display information, they still operate on the same principals used in the vacuum tubes during the 1920's. As in the picture above, these tubes have a filament, a grid, and an anode. The only difference between this tube and a traditional vacuum tube is the fact that a VFD has a phosphor coated anode. this causes the number shaped anode to glow. These tubes can be used as a vacuum tube amplifier, the only difference is that these tubes have a smaller gain than normal vacuum tubes due to the resistance of the phosphor on the anode.

Step 2: Tools and Materials

For this project, you will need a few different tools and materials.

Tools:

  • Soldering Iron
  • Wire Cutters
  • Wire Strippers
  • Hot Glue Gun
  • Multi-meter

Materials(Refer to above picture)

  • Power transformer(With 277 volt primary tap and 6 volt secondary output)
  • Audio output transformer (LCD backlight transformer from computer monitor)
  • Input transformer.(1:2 ratio transformer from an old radio)
  • Wood slab
  • AC Plug
  • 2 VFD's
  • Speaker
  • Bridge rectifier
  • Resistors: 75k, 50k,100k, 65k, 270r
  • 100k Potentiometer
  • Electrolytic Capacitors: 100uF, 10uF
  • Ceramic Capacitors: 10nF, 120nF
  • Copper clad board
  • Wires
  • Audio Jack
  • Switch

Step 3: Base and Prototyping Platform

To start building this amplifier, you will need a wooden slab to glue all the large components too. Use hot glue to attach all the components in the most efficient positions to use the least amount of wiring. Also, glue a piece of copper clad board down to the board, this will be the ground plane for the amplifier. Also consider the aesthetic appeal of the placement of the different parts.

Step 4: Preparing the Tube

The VFD tubes have a lot of pins, so they need to be condensed into 4 pins: common anode, common grid, and the two connections for the filament. To find which pins do what, trace the lines on the back of the VFD (Picture above) to find the different pins. The filament pins will always be the farthest left and right pins. To finish preparing the tube, solder all the grid pins together and all of the anode pins together.

Step 5: Adding the Power Supply and Electronics

Now to building the amplifier. Solder all the connections and components together based on the above circuit diagram(Before doing this, read the next step!!) Once you do this, the amplifier can be tested. This certain schematic was my first prototype, so it did not work too well, I suggest you read the next step before starting to build the amplifier.

Step 6: Why Version 1 Did Not Function Correctly

So as I noted in the previous step, this amplifier did not work too well. It was quiet and had a small gain. This was in part because I was giving the power output tube not enough filament voltage and there were a few components that I hadn't added. Even though I failed at building the amplifier the first time, I eventually got it to work. In the next step, I will redesign the amplifier into the the final version. Failure is the best way to learn though!

Step 7: Redesigning the Circuit

After tinkering with my old design for a few hours: adding, subtracting, and switching out components, I was finally able to get a working design. This is the good thing about building the way that I did, components can be easily switched. This working design is shown in the schematic above. I also replaced the power output tube with another VFD.

Step 8: Building Version 2

To build version 2, use the schematic in the previous step to solder all the components together. This is my working design, so build this one. Make sure to follow the schematic an your VFD amplifier should work.

Step 9: It Works!

The after you have put the VFD amp together, it is time to test it. Plug in an audio source, and fire it up. It should amplify fairly loud and produce cool fluctuations in the brightness of the screen in correspondence to the beat of the music. One thing to note, this amplifier is not very loud and though it does have some gain, it can't be used by low amplitude input sources such as guitars. This amplifier will only work with pre-amplified sound. This is more meant to be a proof of concept for now. The video below is the same as in the introduction. Fast forward to the end to hear and see the result. Thanks for reading and good luck building!

Disclaimer: This is a dangerous project that deals with high voltages! Do not attempt to build unless you have an adequate knowledge of electronics. I am not responsible for any bodily harm inflicted by the making of this project.