I found a datasheet on the web:
http://www.dpr-dpm.narod.ru/8LO7I.pdf
Problem is that google translate has issues with the font, and it reads only weird characters.

The tube appears to be a fairly standard oscilloscope CRT

Pins 1 & 14 are the heater - standard 6.3V

Pin 9 is the final anode. This should be at or near ground potential. Having this pin at high potential will cause inage distortion if a grounded object is brought near the screen (This includes any form of case!)
Pin 2 is the cathode. This should be at 1500-2000 volts negative of pin 9.
Pin 3 is the grid. This should be at an adjustable voltage in the range 40-80V negative of the cathode. The voltage on this pin controls the image brightness. Feed this pin via a resistor of 100kohm to allow coupling in the video signal.
Pin 5 is the focus electrode. This should be at an adjustable voltage in the range 150-350V positive of the cathode.
Pins 7 & 8 are the Y deflection plates. These need a differential scan signal of aprox 200V pk-pk amplitude. The average voltage on these pins need to be the same as the voltage on pin 9.
Pins 10 & 11 are the X deflection plates. These need a differential scan signal of aprox 200V pk-pk amplitude. The average voltage on these pins need to be the same as the voltage on pin 9.
The deflection plates can each be connected to pin 9 via a 1Mohm resistor. Deflection signals can then be connected to them via capacitors.
Pin 12 is an electrostatic screen between the deflector plates. I believe it should connect to pin 9.

The supplies for the grid, cathode and focus anode can be derived from a high-impedance voltage divider fed from the EHT supply as the CRT needs only a few hundred microamps total current.

See if you can find the circuit diagram for the Mullard Servicemans Oscilloscope anywhere. The timebase section uses an EF80 and an ECC81. Two such circuits would be needed to give you your X and Y scans for a monitor. You would then need to amplify your video signal to aproximately 50-60V pk-pk and feed it to the CRT grid via a 0.01uF capacitor rated for 2500V! An ECC81 would probably be suitable for this amplifier.

Unless the CRT has at least a 5 inch diameter face, you probably won't be able to focus the spot finely enough for monitor use as the spot will be bigger than a single pixel even at a 256x192 resolution - I've tried it!

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Thanks for explaining the pins, that is what I was looking for.
If you didn't post that I would probably end up damaging the tube.

Actually this is a fun project, and I won't mind if the picture won't be in HD quality or such.

Just got some questions:

Why 2500V if the signal is 50-60V?


So it should be -1500V?
On which potential is then pin 9?

And how many voltage sources do I need in the end?
I build a 1500V DC source out of a laptop backlight inverter, and I got 200-250V form another inverter. Also I got 6.3V for heating. Anything missing?

Now the boss has gone home, I can spend a bit more time with some advice!


Your signal amplifier will probably have its output at or near ground potential. The grid will be the most negative pin on the CRT at aproximately 1500 - 2000 volts negative of ground. The coupling capacitor between the amplifier output and the grid needs to withstand this voltage, hence a 2500V rating.


Pin 9 should be at ground potential. (0V) The cathode should then be at aproximately 1500V negative of ground.


These should do you.
The 1500V supply can supply all the voltages needed for the CRT. You connect a potential divider made of a chain of resistors and potentiometers across it and tap off the appropriate voltages.

A suggested chain would be

from 1500V supply negative

100k potentiometer - slider connects to CRT grid via a 100k resistor. This is the brilliance control.
68k resistor
- - - CRT cathode connects to junction of 68k and 220k resistors
220k resistor
500k potentiometer - slider connects to CRT pin 5. This is the focus control.
510k resistor
510k resistor
510k resistor
510k resistor
To Ground - 1500V supply positive - CRT pin 9

4 510k resistors are used instead of a single 2M resistor because these resistors will have about 1000V across them and most resistors are not rated for this high a voltage.

If the CRT won't come to a sharp focus, note which end of the focus control gives the best focus. Reduce the resistor connected to this end of the focus pot by 100k and increase the resistor connected to the other end by 100k. This shifts the focus pot up or down the resistor chain whilst maintaining the total resistance, and thus the other voltages, at the same values.

Decreasing the value of the 68k resistor will allow you to increase the brightness of the image. Don't reduce it below 22k however or there is the danger that the grid will be driven positive of the cathode by the image signal - this will damage the CRT very quickly!

The 250V supply can be used to power your deflection circuits.
The 6.3V supply is needed to supply the heater. Note this could pull as much as an 1.5 amps! Connect one heater connection to the cathode via a 10k resistor. IMPORTANT - the entire 6.3V supply will be at 1500V with respect to ground. Ensure your mains transformer can take this - many can't.

The deflection plates are connected to ground by 1M resistors. The deflection signals can then be coupled to the deflection plates via 0.01uF capacitors. For a monitor application, you shouldn't need X and Y shift controls.

I suggest you look up schematics of valve oscilloscopes. These will show you the sort of circuit you need to drive the CRT

With a simple design such as this, the overall brightness of the image will probably change as the white / black ratio of the image changes. I've not found an easy way to rectify this.

One final point - unless you have a magnetic shield for the CRT, be sure to keep all your transformers as far away from it as possible or their magnetic fields will cause the image to wobble at mains frequency. When I played around with CRTs, I found a distance of 3 feet (1 metre) was needed to avoid magnetic fields affecting the CRT.

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[quote]The 6.3V supply is needed to supply the heater. Note this could pull as much as an 1.5 amps! Connect one heater connection to the cathode via a 10k resistor. IMPORTANT - the entire 6.3V supply will be at 1500V with respect to ground. Ensure your mains transformer can take this - many can't.[/
quote]
Why connect the heater to -1500V?
There is no way I can get 6.3V at -1500V, even if I order a custom transformer, they will only make it go for max 500V.

Also if brilliance control is at -1500V how can my EF80 tubes control it?

Could you draw me a simple schematic that would use EF80 tubes, and if needed I have lots of various ECC like triodes.

I was thinking about ordering a custom transformer once I get the tube working with the laptop inverter, so I don't have any silicon based electronics in the device. But how do I shield the tube from the transformer?

Why? The cathode carries the high voltage, and the heater is in close proximity to the cathode. The insulation between the heater and cathode isn't capable of withstanding 1500V. Therefore the heater (and the transformer winding supplying it) are biased to carry more or less the same voltage as the cathode. This protects the cathode insulation (but places a stringent demand on the transformer insulation.)

Notice that the transformer supplies 6.3VAC to the heater, and 6.3VAC is the voltage measured from one heater terminal to the other. You only see 1500V if you measure from either leg of the heater circuit to ground.

I suggest you look into a "power transformer" as used in old vacuum tube ("valve") equipment. These have multiple secondary windings. Get one that includes a 5V winding for the heater of a rectifier tube. Rectifier tubes use 5V heaters, not 6.3; don't ask me why! Don't substitute a 6.3V winding; it's not acceptable. The 5V winding will have extra good insulation -- maybe not rated quite as high as 1500V but it would probably be OK to exceed the insulation rating somewhat. Of course you want 6.3VAC to appear on the "5 volt" winding, so you need to boost the voltage applied to the primary by 26%. See the "boosting" configuration near the bottom of this page: http://www.allaboutcircuits.com/vol_2/chpt_9/5.html They show a resistor as the load, but for you the load will be the power transformer primary. You'll need an additional (smaller) transformer, but even so it could be easier/cheaper than a custom unit.

Whether or not you use an old tube-type power transformer, the 1500V supply will probably require special measures -- you won't likely find the ideal transformer (except from an old oscilloscope). Most transformers for tube gear have HV secondaries of only a few hundred volts. But a "voltage multiplier" circuit using diodes and capacitors can produce very high DC outputs from this. Luckily you require very little current.

Actual magnetic shielding is not easy. Instead, distance and physical orientation of the transformer(s) are the variables that are easiest to control. Ideally you'd mount the transformers behind the CRT, and far away. In any case, twisting them this way & that will reveal which orientations produce least interference. It's best simply to experiment with this.

cheers,
Jeff

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This sounds like one very unusual tube! Even though it's a CRT, I would expect it to be in many ways like the tubes that used to be in radios and TVs when I was a kid, whose cathodes were at or near ground, the control grid below ground, and the plate around 250V above ground. (I know CRTs' plate voltage will be around 1500; but I would still expect the cathode to be at or near ground.) The old tube testers that used to be in electronics stors and drug stores where you'd take your TV's tubes in and test them generally tested two things: cathode emissivity, and leakage between the heater and the cathode.

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What's an additional VIA among friends, anyhow?

I know at least two examples of oscilloscopes that have both a high negative voltage on the cathode and a high positive voltage on the anode. Still, I defer to PaulF in regard to this particular CRT; I have a feeling he's better qualified to comment than I.
That makes sense. So, do you suppose my Tek 465 has some sort of transparent electrostatic shield on the screen? The anode is at +15.5 kV, but I never noticed any image distortion from having a grounded object near the screen. (BTW the cathode runs at -2500V. I also have a Telequipment scope that uses approx. -1000V and +1500V)

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TV CRTS, which use magnetic deflection, usually have the cathode at or near ground potential as then the other electrodes are near ground potential as well except for the final anode which is at 10,000V to 25000V positive of ground. Thiss allows easy DC coupling to the grid from the image amplifier. In an oscilloscope CRT, the deflection plates need to be at the same potential as the final anode. It is usual to put the final anode at ground potential. This put the deflection plates at ground potential as well, making it easy to couple the deflection circuits to them. The cathode is then at a high voltage negative of ground. Also, many older oscilloscope CRTs would suffer electrostatic deflection of the beam if a grounded object were brought near the screen and the final anode was not at ground potential. This made it difficult to put the CRT in a case!

High performance oscilloscope tubes have a post-deflection accelerator element after the deflection plates. This is at a high positive potential with respect to ground while the cathode is at a high negative potential with respect to ground, the final anode and deflection plates being at ground potential.

Modern tubes include an electrostatic screen to prevent trace deflection from grounded objects being brought near the screen. This requires careful design in a tube with a post-deflection accelerator as the electrostatic screen can cause deflection of the electron beam!



Getting 6.3V for a CRT heater has always been a problem. A rectifier supply on an old valve transformer, as Dr Jefyll suggests, is one possible solution. I've always used a transformer salvaged from an old oscilloscope which has a heater winding insulated to 3000V.

There are two ways to couple the signal to the grid. One is to use a high-voltage rated capacitor. This is easy but it may result in the image brightness changing as the image changes. This could probably be cured with a dc restoring diode circuit but I can't help you there because I have never really worked out how they work! The other is to use some form of opto-coupled device. I've used an opto-coupler in an oscilloscope to get flyback blanking pulses to the grid, however, these are logic signals, not the analog signals needed for a monitor.

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I expect the info is available online, but to be honest I never bothered to learn how dc restoring circuits work either. Probably various techniques have been applied.

In case anyone's wondering what this is about, imagine for example a scene of a light-colored object on a comparatively dark background. Imagine how the scene looks in full daylight as opposed to how it looks at dusk. In full daylight the entire scene appears brighter. But, rendered via capacitively-coupled video, the daylight version and the dusk version appear on the CRT screen as being identical. That's because when no dc restorer is employed the average cathode-to-control-grid voltage is the same for both versions. The overall average brightness of the CRT screen remains approximately constant.

The opto-coupler idea is interesting. And, Dajgoro, what is the origin of your video signal? If it begins as a few digital signals feeding a DAC, then you could use a few opto-couplers to bridge the voltage barrier and then convert to analog -- ie, tie the DAC output directly to the CRT, and don't worry about the fact the common is floating at -1500V. A simple power supply for the DAC could run off of the 6.3VAC heater supply (which likewise floats at -1500V).

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I am trying to build an universal monitor that will accept composite video input PAL/NTSC and also VGA form a PC. I already built the circuit for that, although it works for VGA only for now, but that is not an issue.
So I have one PCB that generates the vertical and horizontal ramps, and I have a RGB mixer and amp for getting luma out of RGB.
So in other words this PCB generates X, Y, and luma for the crt.
I wanted to test it out with my scope in XY mode, but it turns out there is no Z input! So I was only able to get a nice square figure, and a just a bit of picture if I mixed luma with the horizontal ramp.
Now the second part should be a device made of vacuum tubes only which only task would be to drive the CRT. I got lots of various tubes form old TV-s and radios. Mostly I got EF80 pentodes which seems to be suited for higher frequencies, but I got also regular ECC like triodes and other weird tubes that were popular in European PAL TV sets.
For now I built two inverters, one is a 555 + BD441 transistor connected to a regular 230/12V transofrmer and that is able to produce 200-250V 10W and there is a rectifier so I get smooth DC levels, and that is ok for powering the pentodes/triodes.
The HV inverter is a laptop backlight inverter that I bought for few dollars form ebay, and I added a HV rectifier using old TV ceramic HV capacitors, and it works just fine, the DC is smooth.
So the initial plan was to power the heaters form 12 or 5V and to connect both inverters to the same supply. Once I got a green dot on the screen, and I was able to move it, the plan was to order a custom made transformer that would suit the propose from a small factory in Zagreb which would cost me 40$ max. Such transformers are rated to only 500V.
But now the problem is the heating must be at -1500V and that would completely ruin the concept, because looking at that schematic of the toy oscilloscope: http://g4oep.atspace.com/toycro/toycro.htm
I figured that the cathode is at ground potential, and the rest is at HV.

So what could I do now? How do I drive that CRT with what do I have available?

Once completed I planned to use it as a second monitor and as a auxiliary monitor for my 65xx and other related projects. Also since it has XY inputs I was thinking about toying a bit with vector graphics.

Btw. The rectifier tubes that I have EZ80 run on 6.3V, so that is not an issue.

You still could do it that way. But, assuming what PaulF said applies to this particular CRT, then grounded objects near the screen may result in image distortion. (To me that sounds more like an interesting feature than a problem! But if you intend to enclose the thing in a case it would be best if it were non-metallic.)
How much voltage are you getting, roughly? Anywhere near 1500V?
To be clear, does the 500V figure refer to the insulation rating, or the actual output? If they were to make you a custom transformer with a special 6.3VAC winding for the heater of your CRT, could it have insulation rated for 1500V?

Speaking of transformers, another option occurred to me and that is to use the transformer from a microwave oven! IIRC they produce about 1500VAC, and they even have a specially-insulated low-voltage winding intended to power the heater of the magnetron, but which might serve to power the heater of your CRT. Worth looking into!

Needless to say, these are dangerous voltages, especially if you're using a transformer like that (which is capable of substantial current on its high-voltage winding).

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That sounds like a great project! Keep in mind however that you'll probably get a maximum of about 256 dots across, because of the beam being so fat, as I found in my raster graphics on analog oscilloscope project shown at viewtopic.php?p=15348#p15348 . You might also want to take measures to make it brighter, because CRTs made for oscilloscopes aren't generally intended to put out a large total amount of light, and the more lines I tried to make, the smaller percentage of the time it would spend on each one, and the dimmer it got.

Here's a schematic for using a CRT, one I copied from a book 33 years ago:

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What's an additional VIA among friends, anyhow?

It is more than 1000V, since when I measure the voltage the pointer hits the end of the scale.
Looking at the speed of the pointer while moving towards the end I guess that it produces around 1500V.
Just in case I bought two of them so I could add them in parallel or series if needed.

The transformers allow max 500 secondaries, but I guess that is a limit for my case as well.

But if the 6.3V is at -1500V how do I heat then the tubes?
This heating is becoming a real pain.


I know, I can see that on my oscilloscope as well, but I don't mind.

If deflection plates are on HV as well, how then do you control them if the tubes will go 500V max?

Tube testers is one of range of subjects revealingly discussed in Robert B. Tomer's 1960 publication for Sams Photofacts, Getting The Most Out Of Vacuum Tubes. This is material unlike any I've come across before. Knowledgably written, very retro -- and highly recommended for anyone who has more than a passing interest in tubes.

It's fairly easy to extend the range of an analog meter, simply by placing a resistance in series. For example if the meter has a resistance of 20 Megohm, then placing a 20 Megohm resistance in series will alter the scale by 2:1 -- ie, 2000 actual volts will read on the meter as 1000 volts.The resistance of your meter may actually be marked on the scale, usually in terms of "ohms per volt." 20,000 ohms per volt is a typical figure for analog meters, meaning that on its 1000V setting the resistance is 20,000 times 1000 = 20 Megohm. Alternatively you could simply use another meter to measure your meter's resistance.

Yes, 20 Megohms is an inconvenient value. Probably you'll need to use several resistors in series. But that's actually a good idea anyway, since a 1000V drop across a single resistor would probably exceed the resistor's voltage rating.

Dajgoro, there's some ambiguity here still. I hope it's clear why a 6.3V winding might require 1500V insulation. When you mention 500V, does that figure refer to the insulation rating, or the actual output? If they were to make you a custom transformer with a special 6.3VAC winding for the heater of your CRT, could they provide it with insulation rated for 1500V?

cheers,
Jeff

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
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