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In order to allow my 3M1 to work with 40 V pulses, I added a second resistor in parallel to R104, increasing the quiescent bias point of the 6DJ8. The resistor I added was a 620 kΩ, and the resulting parallel combination measures ~185 kΩ. This now works perfectly in my 564B. It should work equally well in any 560 series scope.

You can see the extra resistor piggy-backed on R104 in the last two photos of the 3M1.

Tektronix revised the input circuit in the latest (067-0500-00) version in order to allow it to respond to pulses as low as 40 V, making it workable with the calibrators of all the 560-series scopes. This latest version has a 56 V Zener diode in the center of the input resistor chain. It's appears that replacing the middle resistor in the input divider of a 3M1 or 067-500 with a 56 V, 1 W Zener diode would be a good choice, but simply adding a 56 V Zener across that center resistor might also work if the static voltage drop across that resistor was more than 56 V.