VFD Display Becomes an Amplifier

Born well into the transistor era of the late 80s, [Fernando] missed out on all the fun you can have with high voltage and vacuum tubes. He wanted to experience this very cool tech, but since you won’t find a tube checker down at the five and dime anymore, where exactly do you get a vacuum tube to play around with? [Fernando]’s solution was to rip apart the vacuum fluorescent display from an old radio (Google Translate) and use that as a triode.

Inside every VFD is a filament, grid, and cathode – three simple elements also found in the triodes of just about every tube amp ever made. By applying a small voltage to the filament, a larger voltage to the cathode, and sending an audio signal to the grid, this triode amplifies the electrical signal coming from a stereo or guitar.

[Fernando] built his circuit on a breadboard, and with a little tweaking managed to get a fairly respectable amount of gain from parts salvaged from a radio. While using VFDs as amplifiers is nothing new – we’ve seen it a few times before, tube builds are always great to see, and bodged up electronics even more so.

15 thoughts on “VFD Display Becomes an Amplifier

  1. The filament is just the heating element, or heater. The three things a triode has are an anode (plate), cathode, and grid. Everything except gas-discharge tubes have a heater.

    1. To clarify, in the link [Fernando] has his diagram backwards, which I think led to the confusion. The element labeled “Filament/Anode” should say “Filament/Cathode”. The cathode is the part that’s heated to emit electrons.

      Therefore, the article should say, “Apply a small voltage to the filament/cathode, a larger voltage to the anode…” I’ve noticed authors on here often reproduce mistakes from the work they link to. Remember, in an electronic device anode = positive, cathode = negative.

      1. Umm … your both correct sort of lol.

        I can’t remember if a triode had a separate cathode or just used the heater as the cathode but that doesn’t matter. A lot of simple valves used the heater as the cathode.

        One thing that is important was not so much the polarity of anode/cathode but the distances between them and the grid.

        The grid is always closer to the cathode than the anode. In these VFD’s the grid is closest to what is normally called the anode.

        So to use a VFD as an amplifier you have to reverse the polarity. This is why he has hearer / anode on his schematic.

        Amplification is a result of the ratio of the inverse of the squares of the distances between cathode / anode and cathode / grid.

        Keep in mind that it is the electrons that move. ie negative (cathode) to positive (anode) unlike conventional current flow which is from positive to negative.

        1. No, the cathode is always the heated one. Thermal emission of electrons whether from a the hot filament itself or from an indirectly heated cathode drives the process.

          I think you’re right about the amplification being related to the distances but remember, no matter what, the cathode-grid distance is less than the cathode-anode distance.

          1. My bad. I looked it up and the phosphor is on the anodes. One cathode, one grid and many anodes, one anode per segment. So obviously the grid sets a threshold current and the “tube” acts like a switch rather than a linear amp in its normal setup. But still the grid (from memory) is closest to the anode(s).

        2. We’re also the same person. HaD decided to use a different name for both of my posts.

          The power of the grid to affect the current flow is based on the relative distances between the elements. The maximum amplification is related to this, but it would be difficult to say “amplification is a result of the ratio of the inverse of the squares of the distances”. It’s a result of the grid voltage’s strong effect on plate current compared to plate voltage.

          And just as you said, electrons always flow from the cathode to the anode. Reversing their polarities doesn’t make a difference, because if it did, we wouldn’t have diodes. We would have weird silicon wires.

  2. love it.

    i’d put a resistor from the very input to ground of say 100K or more to bleed off (edit: charge up) any high voltage on the cap’s input. this is normally done when connecting tube equpiment the DANGEROUS WAY to an input.

    PPS: this is most likely not an issue here cuz he’s using only 20V but if it was a normal higher power tube at say 300V, it will zap your LINE-OUT circuit andor schock you when you touch the tip before plugging it in.

    and thats why people love cute little audio matching transformers.
    they keep the electric bite away :)

  3. “, where exactly do you get a vacuum tube to play around with?”
    Scores of places. Parts Express, for instance. Or the specialty sites that cater to musical instrument amp hobbyists. EBay, etc.
    For just fooling around you can use a 12AU7, a cheap dual medium mu dual triode that’s in current production, a socket, and a hand full of caps and resistors. High voltage transformers to generate the low-current 100VDC – 300VDC for a simple preamp circuit initially look difficult and expensive. Until you realize you need a 12VAC transformer to run the tube’s heater and you can connect a second 12VAC to 120VAC transformer ‘backward’ to the first one to get some isolated 120VAC to rectify for high voltage.
    Tube circuits are fun to mess around with.

    1. He is in Argentina (as so do I), and here is not easy to find (or at least cheap) vacuum tubes. Vendors treat them as royalty items. Transformers are pretty expensive too, and there is not much variety. But that comes with a bonus, here we are sometimes forced to hack with what we’ve got.

    1. The VFD in a Calculator and an VCR have everything in common except the filament voltage / gradient (Some specialty VFD heaters can run at DC-voltage, since the electron-emissive coating on the filaments is graduated from end to end) – the voltage from Cathode to grid and Anode is proportional to the scan rate of the segments, and is usually 40 Volts, but some times as high as 60V, and as low as 30V.

    2. Not really.
      The calculator has just a step up transformer built in. This makes 25V to 30V from a battery (sometimes a single AA), it also makes the filament voltage, something like 3 to 5 volts AC. The voltage fitted quite nicely to the requirements of the PMOS chips of the time. Theses had so high on-resistance in the transistors that even an old HP calculator with LED display had a step up to 25V, just for the electronics.

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