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VFD as an audio/RF amplifier?

Discussion in 'Electronic Design' started by N9WOS, Jul 29, 2005.

  1. N9WOS

    N9WOS Guest

    Has anyone ever tried using a Vacuum fluorescent display as a tube type
    amplifier? In case anyone is interested, I have been toying around with one,
    trying just that. Here is my findings.

    VFD was pulled out of an old VCR. Made by NEC model number FIP14KM6. I
    soldered all the grid leads to one wire running the length of the display.
    Same with the anodes. That created a tube with basically one long grid
    running the length of the tube, and one long anode plate under that.

    All test was done with 5V on the filament.

    I could easily obtain 60mA of emission current from the filament with less
    than 30V tied to the grid and plate. I didn't try to push it any higher
    because the filament started showing noticeable extra heating near the side
    connected to the negative of the filament supply. (ie) the filament on the
    left side of the tube glowed a noticeably brighter orange than the right
    side at the 60mA emission level.

    Space charge around the grid with the grid and plate disconnected was
    about -2.2 to -2.4Vin reference to the most negative side of the filament.

    Now to get useful output.

    Hooked the plate up to a 0V to +60V supply with a current meter in line.
    Hooked the grid to a +15V to -15V supply with full zero crossing capability.
    I found that the grid in the tube is almost 100% effective. With the grid at
    zero volts in reference to the most negative side the filament, the plate
    current was only about 0.25mA. When the grid voltage dropped below -2.6V (a
    little bit lower than space charge) the current level dropped below readable
    levels. (0.01mA). The crossover from cutoff state to operating range is
    almost exactly 0V.

    It appears that the grid pretty much acts along the lines of a space charge
    control grid. Efficiency in that mode of operation is about 50%. For a yield
    of about 1 to 1 current gain. When the control grid is positive enough to
    pull 2mA current, then the plate will also be subjected to about 2mA as long
    as it is more positive than the grid. If the plate drops below the grid
    voltage then plate current will start to drop and grid current will rise. As
    long as the plate is at a higher voltage than the grid, then plate voltage
    doesn't have much effect on plate current. Because of that, I was easily
    able to obtain a voltage gain close to 100X with a large value plate

    Grid/plate curve was almost perfectly straight from 10V to 60V(cutoff) plate
    voltage with a 1V P-P input.

    Because of it's 1 to 1 current gain, normal cap/resistor inter-stage
    coupling networks are useless. The driving network will basically have to be
    designed like a cathode driven tube circuit. Except that the output is 180
    degrees out of phase with the input, instead of in phase. For audio
    applications, that means that all inter-stage coupling will have to be
    transformer based. Current gain would be performed by the transformers, and
    voltage gain would be performed by the VFD's

    And audio amplifier was easy to build on the workbench. Hooked the plate up
    to 60V through an output transformer, with the transformer driving and
    speaker. The input was via another transformer with a positive grid bias
    supply hooked to it. I adjusted the positive supply to set the plate current
    to the middle of my desired operating range. It worked quite well.

    On the frequency response side of things, it has a hefty inter-electrode
    capacitance. But it still provided good amplification at 2MHz. As long at a
    tuned plate circuit was used. Even with the large inter-electrode
    capacitance, self oscillation was still impossible even with a tuned plate
    and grid, because of it's close to 1 to 1 current gain. On a standard tuned
    grid, or Hartley oscillator you have to have more turns in the plate
    circuit, than in the grid circuit, or there won't be enough drive to drive
    the grid to sustain oscillation. The only working types of oscillators you
    can use the tube in is circuits with transformer feedback to allow current

    It makes it a very harmonically stable tube for RF amplifier applications.
    That gives me a few ideas for a full QSK QRP amplifier. Run it with zero
    grid voltage. It will be operating in class B. It would be good for CW or

    Another application, if you had two identical ones, is a push pull class B
    audio amp. Have the center tap of the grid input transformer fastened to
    ground, and the center tap of the output transformer connected to HV. Just
    two VFD's Two transformers, B+ , filament supply, and nothing else.

    I have no idea what the maximum voltage is, that you could run on the plate.
    The audio amplifier I had was driven by a 60Vsupply, through a transformer
    output, with the lowest peak plate voltage of around 10V and the highest
    peak plate voltage of about 140V to 150V. It had about a quarter watt peak
    output, with 6mA Q current. I estimate I could push it up to about 15mA Q
    current with about 1W, to 1.5W peak output with 100V+ B+, and a transformer
    that had the right winding ratio.

    Other VFD's probably have totally different specs than the one I am working
    with. But they should still perfectly viable for amplifier implications. If
    the current gain is over 1 to 1 then you may have a problem with VHF
    harmonic oscillation in amplifier circuits. But I don't think you will find
    any like that, because they would exhibit VHF oscillation characteristics
    even in their intended applications if they had a current gain over 1 to 1.
    A VFD display on an clock radio that generated a birdie on the FM broadcast
    band when ever the "PM" label was lit wouldn't make the FCC very happy.

    Now to figure out how to make a complete tube type radio out of VFD's. Or
    maybe, just a single tube regenerative receiver.

    Another neat thing about VFD's as amplifiers is, the display flashes in step
    with the music beat. :)

    Now if we could get a VFD manufacturer to produce a display with 4 or 5
    totally individual sets of elements, then you could produce a totally
    integrated tube type AM receiver with just one VFD in it. I have seen some
    figures factories that give an ~$5 to $8 per unit price on custom VFD's in
    quantities of over 10,000.
  2. Andy Cuffe

    Andy Cuffe Guest

    I actually have played with a few old VFDs. I didn't make any real
    measurements, but I pushed the plate to the full 400v of my power
    supply with no ill effects. The only limiting factor was power
    dissipation. When the tube got hot enough, the glass cracked. With
    enough negative grid bias it was happy to sit there all day with 400v
    on the plate. I was surprised it didn't arc with that much of an
    Andy Cuffe

    <-- Use this address until 12/31/2005

    <-- Use this address after 12/31/2005
  3. N9WOS

    N9WOS Guest

    I actually have played with a few old VFDs. I didn't make any real

    The first graph on the 12DL8 pdf will give you a good idea what it's
    operating characteristics is like. The current scale, and voltages are off,
    but it will give you the general idea.
    That would be easy to solve. Cover the anode side with thermal grease, and
    use a clip to mount it to a big aluminum heat sink. You would have something
    that would look almost like a solid state monolithic amp. :-0
  4. John  Larkin

    John Larkin Guest

    It's fun to wave a magnet around a VFD. I bet you could make a fairly
    sensitive magnetic-field detector from one.

  5. N9WOS

    N9WOS Guest

    The best way I could think of doing that is feeding an AC signal through the
    filaments so that they distort in some way when in the presence of a
    magnetic field, then monitor the electrode current in some way to determine
    the amount of filament displacement that is happening. That would give you
    an indication of magnetic polarity, and intensity.

    Second idea. Just have one grid positively biased, and monitor the current
    of just two anodes behind it. When the electrons move through the magnetic
    field, they will be shifted to one side or the other. electrode current will
    also shift. That would also allow you to determine polarity and intensity.

    Hmm... The second idea would also work with standard vacuum tubes that have
    more than one anode. Or even two or more electrodes of any type that is
    separated from each other by a small amout, that can be used as anodes. The
    magnetic field would shift the electron flow from one electrode to the
    other. I'll have to do some experiments on that subject.
  6. N9WOS

    N9WOS Guest

    Hmm... The second idea would also work with standard vacuum tubes that
    I have looked at over a 100 different tube types that I have on had. Ranging
    from the 4 pin 01A to a bunch of different compactatrons. I haven't found
    one with the correct electrode structure to do a good job at detecting
    polarity and intensity. The only two that I found, that would possibly make
    a good intensity detector is the 6AV6, and the 6JH8.

    You would have to have a tube with two anodes about the same distance from
    the cathode, with both anodes on the same side of the cathode.

    (ie) the electrons leave the cathode in one direction, headed for the group
    of two electrodes. The amount of current arriving at each electrode will be
    a function of the magnetic field along the path. The magnetic field will
    curve the path to one electrode, or the other, depending on magnetic
    polarity. And the current distribution between the two electrodes would
    indicate intensity.

    The only thing that draws even close to that is a cathode ray tube with
    deflection plates. Operate it with the focus out of adjustment enough that
    the deflection plates are pulling a measurable current. Then the current
    distribution between the plates will depend on how the magnetic field is
    shifting the electron beam.
  7. Just such a valve is illustrated in Fig 143 of "Magnetic Tape recording"
    by Spratt (Heywood & Co. 1958). It appears to be a glass-based type but
    no part number is given.

    Perhaps the reference will shed more light:
    Daniel, E.D. Figure 2 'A flux sensitive reproducing head for magnetic
    recording systems.'
    Proc. Inst. Elect. Engrs, 102B,4 (July 1953)
  8. flipper

    flipper Guest

    The Trochotron has multiple plates, e.g. 10, circularly around a
    central cathode with an outer magnet ring causing a spiral like
    trochotron electron pattern that's electrostatically deflected, by the
    "spades," to one of the plates as a nixie driver.

    It might be possible to adjust the normal operating parameters/ring
    magnet so it's sensitive to outside magnetic fields distorting the
    intended pattern, with the spades as a sensitivity/balance adjustment,
    and the operating instructions do warn about not mounting it near
    something magnetic or MuMetal.

    This site has data on them
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