[Test-Equipment] tube input impedance

[Allan Henry Kaplan]

You cannot let the grid float.   If the cathodes have some un-bypassed
resistance, as I believe they probably do you might be able to tie each
10-megohm grid resistor back to its cathode.  This should provide a
bootstrapping connection wherein the current that would be drawn by
voltage on the grid is bucked by voltage on the cathode.   This makes
the 10 meg resistor look much higher in impedance.   Hope this helps!
73, Allan H. Kaplan, W1AEL