Looking for pointers on Deflection Amplifier Output Stage Design

[AlanInSitges]

This is such an awesome project! I have often thought it would be cool to do something similar but my level of knowledge is way below what's required to even start. Here's hoping this really takes off.

Speaking of which, and following on with the general theme of deflection and yokes, and not intending to hijack this excellent discussion, I wonder if someone can tell me if all of these huge pulses and inductors, etc., are strictly necessary. I mean, if one were to start over from scratch, in a vacuum, and set out to build a circuit whose requirements are to move an electron beam from A to B over a precise period of time, and given that we are already in possession of a waveform that exactly graphs that movement, would it be possible or feasible to simply drive a yoke winding from a simple power amplifier circuit? Both V and H?

I don't know how much juice is really needed to deflect that beam five inches on a 34CM CRT, but I do remember as a kid back in the 80s building a lissajous project from Radio-Electronics out of a 19" tube B&W set and a second yoke, and powering it from a 6L6 amplifier, and that it could very nearly move the beam off the screen. I've often wondered why, if that's the case, they didn't just put an STK chip or similar in these sets along with a SMPS for the HV.

[old_tv_nut]

Originally Posted by AlanInSitges

This is such an awesome project! I have often thought it would be cool to do something similar...Here's hoping this really takes off.
...

I totally agree!

Regarding output amplifier type:

The horizontal sweep is operating a load that is mostly inductive at horizontal frequency. This is because of the number of windings needed to create a magnetic field to deflect the beam. This dictates that the output circuit apply a constant voltage to the yoke during sweep, so the output ends up just switching on and off. This is convenient because it reduces the power handling requirements (wasted energy) tremendously. The horizontal output device has to conduct for a bit more than the right half of the sweep (to make up for resistive losses) but most of the energy is stored in the magnetic field and transferred to the resonant flyback capacitor and then to the damper diode to make the left half of the sweep.

In both tube and transistor horizontal outputs, you want the switch-off to be as fast as possible, to prevent having both current and rising retrace pulse voltage present simultaneously. In tubes, it's a matter of total power dissipation, and in transistors it's much more critical because the safe operating area (limit of simultaneous voltage and current) is very small. A bad design will kill an output transistor on the first retrace, before you can even figure out what happened.

The vertical output amplifier is much closer to linear, modified somewhat, because at vertical frequency the yoke is mainly resistive but partly inductive. So, it already operates mainly as you proposed.

Electrostatically deflected tubes use linear drive waveforms with incidental power losses in the amplifier but none in the deflection plates.

[LukeSimon]

Originally Posted by AlanInSitges

Speaking of which, and following on with the general theme of deflection and yokes, and not intending to hijack this excellent discussion, I wonder if someone can tell me if all of these huge pulses and inductors, etc., are strictly necessary. I mean, if one were to start over from scratch, in a vacuum, and set out to build a circuit whose requirements are to move an electron beam from A to B over a precise period of time, and given that we are already in possession of a waveform that exactly graphs that movement, would it be possible or feasible to simply drive a yoke winding from a simple power amplifier circuit? Both V and H?

I don't know how much juice is really needed to deflect that beam five inches on a 34CM CRT, but I do remember as a kid back in the 80s building a lissajous project from Radio-Electronics out of a 19" tube B&W set and a second yoke, and powering it from a 6L6 amplifier, and that it could very nearly move the beam off the screen. I've often wondered why, if that's the case, they didn't just put an STK chip or similar in these sets along with a SMPS for the HV.

The output stage of both vertical and horizontal deflection circuits is a power amplifier. However, the horizontal deflection output stage has to deal with the high voltage created by the shock excitation caused by the rapid change in current flowing through the yoke during horizontal retrace or flyback time. In fact, this is one reason why the horizontal yoke inductance is lower than the vertical yoke inductance. Inductive impedance is higher, the faster the current is changed. The vertical deflection current's most rapid change is during vertical retrace, which is still significantly slower than horizontal retrace.