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Couple of Questions About a CRT Tube

RayRay1132

New Member
This might seem like a stupid question, but is it possible to make a simple dot on an old CRT tube using just the Filament Heater, Cathode, And Final Anode. EX. (No focusing or accelerating anodes.) It is a very small sony 02 JM mono flat picture tube. I have a very small flyback transformer (maybe 5kv ?). All I want to do is just make a simple dot on the screen, dont really care about the deflection coils right now. Thanks in advance, Ray.
 

unclejed613

Well-Known Member
Most Helpful Member
you need the focus and acceleration voltages as well as the other grids. you need to be able to control the beam current, or the beam will burn phosphors off the screen... G1 is usually grounded and negative voltages applied to the cathodes control the beam current.
 

dr pepper

Well-Known Member
Most Helpful Member
Depends on the tube, might work without focus but you'll get a blurred splodge of a dot, without driving the brightness drid you might not get anything.
That said if you have a power supply for the Eht, power for the grids shouldnt be so hard to arrange.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
you need the focus and acceleration voltages as well as the other grids. you need to be able to control the beam current, or the beam will burn phosphors off the screen... G1 is usually grounded and negative voltages applied to the cathodes control the beam current.
I think you may be a little confused?.

The control grid (G1) needs to be negative of the cathode - and this is often (but not always) done by grounding the grid, and having the cathode about 100-120V positive (on a colour CRT). A common technique in early colour TV's was to feed the three cathodes with the chroma signal, and the single grid with the luminance signal, effectively 'mixing' them back together in the CRT. Another use for the grid was also flyback blanking.

Obviously the advent of IC's changed all that, and everything was done via the cathodes.

So connecting the control grid is essential, and without focus and first anode voltages any resulting electron beam may be so dispersed as not to be visible at all. The first anode is usually connected to a big pot, and used for setting the DC conditions of the CRT (varies up to about1000v) - and if you turn it down too far you get no picture whatsoever.
 

unclejed613

Well-Known Member
Most Helpful Member
The control grid (G1) needs to be negative of the cathode -
yup... you're right.... last time i worked on anything with a CRT had to have been about 15 years ago... i must be getting rusty.
 

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