Questions on hooking up an old cathode ray tube (CRT)

Question

I bought an 8LO29I Soviet era CRT (intended for use in oscilloscope/instrumentation):

Now I'd like to get it to work. After almost endless reading up (I'm not an electrical engineer) I think I now know what to do. The CRT also came with extensive original documentation (in Russian, dare I mention it!), so that helped a lot.

The function of the pins is explained as:

Electrode leads:

• 1, 14 - heater
• 2 - cathode
• 3 - modulator
• 5 - the first anode
• 7 - signal deflecting plate Y 1
• 8 - signal deflecting plate Y 2
• 9 - second anode
• 10 - temporary deflecting plate X 1
• 11 - time deflecting plate X 2
• 4, 12 - connected.

Pin schematic:

For my use the deflection plates will not be needed.

What I still don't get is what the modulator (pin 3) is supposed to do and whether I need one. Some searching came up with this:

An electronic system is described for modulating the beam intensity of a cathode‐ray oscilloscope in such a manner as to reduce variations in trace brightness due to differences in writing speed. A voltage derived by a push‐pull differentiating amplifier from the vertical deflection signal is added to the instrument's intensity‐control voltage. By use of this system, improved photographs of wave forms canbe obtained.

So is that what it's supposed to be, an intensity modulator?

Second question:

All these smallish CRTs seem to have 2 anodes, why is that? In one publication I read it described the second anode as the focusing anode. Is that correct?

Answer 1

Intensity Modulator

The brightness of the dot or beam on the face of the CRT depends on how many electrons impinge on that spot. The more electrons, the brighter the spot.

Assume that the number of electrons coming from the cathode & making their way to the face of the CRT is constant. Then if the beam were stationary, you would see a very bright spot at one location of the face. This may actually cause damage to the CRT by "burning" the phosphor coating.

So then if you move the beam, the number of electrons hitting the face of the CRT at any point in time is reduced, which means a lower brightness. The faster the beam moves, the fewer electrons hitting any point on the face and so the dimmer the spot/trace.

The purpose of the intensity modulator is to prevent this from happening, by reducing the number of electrons coming off the cathode as the beam moves slower. And this is done by controlling the voltage on the cathode.

Answer 2

Looks similar to US tubes like 3JP1 that I have fiddled with ages ago. There is a control grid to change the brightness of the spot. It might have to be -50V relative to the cathode to extinguish the spot (the higher the anodes and focus electrode voltages the more negative the grid voltage will have to be).

Normally you'd used it to blank the retrace on an oscilloscope, for example. If you were trying to display a picture you'd also modulate the brightness during each scan line.

If you power it up at nominal voltages and currents you'll burn the phosphor if you don't deflect the spot or dim it. It's intended to be scanned.

On the illustrated type of CRT, there's a high voltage 3rd anode (maybe there is a connector on the side of the tube) that is fed a constant voltage in the kV range, a relatively high voltage 2nd anode (also fed a constant voltage), and a lower voltage first anode (hundreds of V) that you adjust in order to focus the spot. It should be 20-35% of the 2nd anode voltage in this particular case.

See the linked datasheet.

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Yours does not seem to have a separate 3rd anode so more designed for slower scan speeds and probably more deflection sensitivity. 1500V constant on A2, adjustable 280-516V on A1 for focus. -67.5 to -22.5V to blank the spot. Heater is 6.3V.