Philip Pemberton wrote:
> OK, what I want to do is use a voltage between 0V and 5V to vary the
> brightness of the spot on the screen. Note that the brightness control allows
> V(G1) to vary between -600 and about -500V (just above the cathode voltage).
> -600 is full blank, -500 is full brightness.
> 
> I want the external control to work with the brightness control. If the
> brightness control is set so that the beam is just off, then 0V will leave
> the beam in that state, and 5V will make the beam visible. In other words,
> the 0-5V variation is added to the brightness setting.
> 

Some random comments...
If you are designing a one-off circuit, then adding some parts might 
make life easier.  Your circuit with the long divider makes everything 
you do interactive.  If you break out the jobs and wire the brightness 
as a parallel path from ground to -600 it *might* allow for more 
possibilities.

If it's for production in a cost sensitive area then maybe that's not 
appropriate.

To deal with the marriage of external control along with an internal 
pot, I would make an amplifier with the brightness pot working from 0 to 
5 volts.  Then a simple opamp summer can add the external signal in and 
you can easily scale them to do what you want.  Take the output from 
that and a high voltage transistor or two and the rest should be 
straight forward.

Good luck!

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