N
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
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.
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.