CRT High Voltage Power Supplies

[Y2KEDDIE]

With CRT supplies, why do some circuits ground the positive and others ground the negative. Does making the cathode several thousand volts negative make the circuit physically safer, or are there other reasons. It seems most oscilloscopes use negative HV referenced to ground, and Television CRT's use positive HV referenced to ground.

 

[Harald Kapp]

With CRT supplies, why do some circuits ground the positive and others ground the negative

That depends on whether you require a negative or a positive high voltage. When you ground teh positive end, the other end will have a high negative voltage and vice versa.

Does making the cathode several thousand volts negative make the circuit physically safer,

No. A dangerous high voltage is dangerous regardless of polarity.

It seems most oscilloscopes use negative HV referenced to ground, and Television CRT's use positive HV referenced to ground.

I'm at a loss for an answer to this point.

 

[duke37]

Televisions use a high positive voltage to energise the screen. The beam deflection is magnetic with scan coils which are insulated from the high voltage. The cathode needs to be close to earth to allow variable brightness.

Oscilloscopes use electrostatic deflection and the deflection plates need to be close to earth to connect to the amplifiers. Thus the cathode has to be at high negative voltage.

 

[Y2KEDDIE]

I've taken a few days to reflect on what you are saying, I am still somewhat confused. I can grasp the need for Positive HV on a television CRT, but why the do the deflection electrodes in an electrostatic CRT need to be close to ground in order to work ( how are they affected) . Z axis modulation also varies the cathode voltage/current.
I seen some scopes with positive scope CRT (DG-7) don't have a 2nd anode, is it because there is enough beam current that it doesn't need acceleration?

 

[Y2KEDDIE]

Do old antique electrostatic deflection television sets all use Negative HV on their CRT Anodes?

 

[duke37]

It would be very inconvenient to have the deflection electrodes at a few thousand volts. The drive amplifiers need to be directly connected to go down to DC.
Z axis modulation on my scope is AC only and fed through a capacitor.

I have not made a TV but perhaps you could investivate the circuit for a TV using a VCR97.

 

[Minder]

The Tv High voltage tube final anode is the inside wall of the tube consisting of an aquadag coating for the positive HV supply in order to accelerate the beam toward the phosphor coating , the outside wall (crt) is coated with aquadag connected to chassis GND.
This forms a large capacitor, as seen when the set has been off for a considerable time, the inner coating can retain its charge.
M.
,

 

[Y2KEDDIE]

So, do the deflection plates have to be referenced to the 2nd anode supply polarity? I thought they are independent . ( 0V spot center, + to right, - to left). I realize the wider the deflection angle, the higher voltage needed, but how does the 2nd Anode polarity come into play? Are the deflection plates aiding in beam acceleration, possibly?