Concluding this deconstruction of the CRT grid-drive circuit...
Here's the final circuit much as built on the left, Harry's voltages are slightly higher so currents will also be slightly higher too, but the principal is the same.
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The above should be familiar...the resistors and pot for the focus has been simplified into a single resistor as before. (Right above).
Next we add +1400V to the voltages in red, this makes it easier to understand when dealing with positive voltages (Left below)...then get rid of the resistor(s) between the +80V point and +1400V. (Right below).
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....and really that's it! The right hand sketch should be very familiar to what has been covered before. The voltages and currents might be different, but the workings are just the same.
There will be more on this which goes into other aspects where this circuit could be improved, but let's take a break for a while.
In pulling this to bits what tools have we used? Basic arithmetic (I'm not going to even call it Math), add, subtract, multiply and divide, plus Ohms Law which uses so far just those four 'tools'. No long-winded algebra, no calculus, nothing 'exotic' at all. The biggest task is finding the bits that 'fit' and work together as you wish. The design was/is just the reverse of what has been done here.
Steve A.
Postscript: What's R905 for? Any guesses? It's not an easy nor obvious answer! Remember, the CRT grid should not go positive so there's no current in R905, therefore no voltage drop across it. So why is it there? Hint: and a rather obscure one...R905 should be mounted right on the CRT socket pin, really close to the CRT, preferably within 1cm, I should have mentioned this before, but it 'slipped through the cracks'.