Re: Type 519 -- Why 125 ohms?

Tom Lee

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,

Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 5/10/2021 18:54, Sean Turner wrote:
Pondering the beast that is sitting in my lab got me thinking, why 125 ohm? Does anyone know of the history of why this was chosen? I haven't been able to find much about this, so insights are appreciated!


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