A Vacuum Fluorescent Display as a headphone amplifier

December 6th, 2008 · 19 Comments ·

After all, VFDs also have a heater, grids, anodes and are encased in glass. And they glow in tha dark.

Vacuum Fluorescent Display as a headphones amplifier

VFDs are common on VCRs. I have a few of them I took from some broken VCRs. Last night I was working on how I could use them as vacuum triodes. I don’t have much experience with real vacuum tubes so I had to invent some, possibly wrong, arrangements, but I finally got something.

I’m not going deep into the structure of VFDs as I wouldn’t add anything to what is available on Wikipedia or on manufacturer’s websites. Just need to know that the heater (or filament) is made of thin straight metallic wires that emit electrons (thermionic effect) when heated by current. The electrons are accelerated by the electric field generated by a voltage applied between the heater and the metallic anodes, the metallic plates shaped as digits and pictograms and that are covered with fluorescent paint which glow when hit by the electrons.

A number of thin grids are placed between the cathode and groups of anodes with the purpose of screening or letting go the electric field generated by the voltage at the anodes.
A negative voltage between one of the grids and the cathode will generate an electric field opposite in sign with the anode-cathode one, reducing or voiding at all the latter. The electrons will be stopped and will not reach the anodes behind the grid and those digits will be dark. A positive voltage at the grids is actually necessary to pre-accelerate the electrons.

That said, I reached this final layout.

Connection diagram, Vacuum Fluorescent Display as a headphones amplifier

The grids are connected together as well as the anodes. I connected a headphone between the anodes and the positive of the anode voltage through a decoupling capacitor.
The grids are polarized by a 100k Ohm resistor to the positive and the audio fequency is fed into the grid through another decoupling capacitor.

Vacuum Fluorescent Display as a headphone amplifier, grids and anodes in parallel

The heater generally requires 2-3 Vdc. When powered from AC they give a more uniform brightness but in our case DC is better, to limit hum into the headphones.
The anode voltage may vary between 20 to 40 Vdc, depending on the model. The connections to the filament are the only ones to be really careful about as misplacing them and feeding with the anode voltage wil blow the heater. Looking closely through the glass of the display the filament and their connections to the connection pins can be easily found.

Vacuum Fluorescent Display as a headphone amplifier, closeup of display

The power into the headphones is limited but I never expected more than this.
A novelty, nothing more, possibly.

Might behave better as low power preamplifying stage for a real vacuum tube power final stage.

That’s it, and I had fun.

Addendum (after comment from Hiro Protagonist) : The arrangements of the anodes in the drawing is simplified as the display I used has multiplexed digits, that is the segments and pictograms are paralleled inside the display to minimize connections to the driving IC is usual applications. This limits the use of the VFD as a multiple triode.

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