Re: Type 519 -- Why 125 ohms?

Sean Turner


That explanation makes a lot of sense. I did note that the manual directs you to be a bit careful about how much power you dissipate in the termination of the deflection plate network (I think it's a 1.8 or 2W terminator)



On Mon, May 10, 2021 at 07:57 PM, Tom Lee wrote:

The 519 has no vertical amp (as I'm sure you know), so the input signal has to
drive the CRT's distributed deflection plates directly. The deflection
structure looks like a transmission line, basically, and has a characteristic
impedance of 125 ohms.

So, your next question would be, "Why is the deflection structure 125 ohms?"
The answer to that is that you'd actually like it to be fairly high so that
you can deliver the deflection voltage (about 10V per cm) at reasonably low
power. At 125 ohms, you're already taking quite a bit of power to deflect it.
50 ohms would be painful.

Why not much higher than 125 ohms? Because it's hard to construct -- and fit
into the CRT -- a deflection structure with a much higher impedance, since Zo
= sqrt(L/C). And even if you could pull off that trick, plumbing for, say, a
300 ohm impedance is no easy task by itself. The geometric mean of 50 and 300
just happens to be about 125 ohms (for small values of 125). That's somewhat
of a coincidence, but not entirely.

-- Cheers,

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