CRT driver design

[ian.rees]

A while back, I picked up a 1930s vintage electrostatic CRT (a Sylvania 3AP1 - https://frank.pocnet.net/sheets/121/3/3AP1.pdf ) for a vaguely defined IoT project (that's Internet of Tubes).  This weekend is a long one here, and I've started sketching up a circuit to drive the tube.  Before I get too far along, I wanted to check a few items about interfacing with it:

Is it correct to modulate the beam amplitude by varying only the grid potential, leaving the anode potentials constant?

This particular CRT has two of the deflection plates at the same potential as "anode 2" - is it safe to assume that with the deflection plates all at the same potential, that the beam will be [more-or-less] centred in the display?  My thinking is that the other deflection plates (the ones not tied to anode 2 potential) are meant to be driven at +/- V from anode 2.

Any estimates on the current I can expect the electron beam to require?  The only spec I've found is the max 10mW/cm^2 from the above data sheet, which works out to uA territory, so I'm assuming it's OK to use a somewhat wimpy supply for the anodes.

Thanks!  -Ian-

[T3sl4co1l]

I did a one of these, once:

http://seventransistorlabs.com/Images/Tubescope_Supply2.png

The +/-250V forward converter never quite worked well, and I cut it all off and built a flyback instead.  But that's for the other stuff, not the CRT, so that's okay.

The single transistor aux supply actually does a fine job.  I don't really like those circuits, because they can fail to start up, and just sit there cooking until they die.  And there's no control of operating state or current limiting or whatever, just a little throttle (hence the opto).  If something in the HV fails shorted, for example, it might not be pretty.

The regulation also sucks (hence the zener chain), but that's not usually a big deal, as voltage divider resistors tend to dominate over electron beam current, which usually averages out okay anyway.

Pretty good picture, once all the voltages are adjusted.  I forget what focus and cathode are set at, but they're close to the datasheet values anyway.



You'll probably want negative cathode, so that the anode and deflection plates are near ground (give or take a few 100s V).

Beam current is in the fractional mA range.  Not sure how much the above tube is using.  I think I designed the supply for about 1mA capacity, and most of that is burned in the zener stack and bias resistors.

This is a bigger tube, and I wanted to run it a bit higher (2kV is the limit I think), but YMMV.  Lower voltage gives higher deflection sensitivity, but a bigger and dimmer spot.

And yes, deflection is neutral when the plates are equal, give or take some tolerance in manufacturing.

Tim

[ian.rees]

Thanks for the response, that's a good point about negative cathode.  Did you build the rest of the oscilloscope too?

[T3sl4co1l]

At the moment, it's X and Y.  Need to do the trigger and sweep some time, but it's on a pretty far back burner..

Tim

[Circlotron]

Many years ago I measured the cathode current of a 17 inch b/w television tube. Was about 15uA *average* IIRC.

[tautech]

In this post there's a link to a mid 70's CRO manual with simple schematics:
https://www.eevblog.com/forum/beginners/another-beginners-oscilloscope-question!-)-sod-all-storage-space/msg956012/#msg956012

Well worth downloading and study IMO.