VFD as an audio/RF amplifier?

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 resistor.

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 gain.

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 AM.

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.

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

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

John

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.

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.

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)

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