[R-390] Seeking to understand grid voltage at mixers

[Jacques Fortin]

Hello Barry,

When the R-390 is in STRANDBY, the +180V line is disconnected from the main
supply, so the mixers and IF + RF amp stages are OFF (no plate voltage).
However, the three oscillators (1st crystal, 2nd crystal and PTO) are
directly powered by the PS output, so both three are running all the time.
Because those oscillators are running in class C, it is normal to have
pulsating grid current in the related tubes that develop a negative voltage
across the feedback network capacitors (like across C704 in the PTO) and
that translate as a mean negative grid bias for the three oscillators.
That mean negative grid bias can be used to check the correct working of the
oscillator stages.
But as the three oscillators are running, their RF output signal is also
applied to the three mixer cathodes at all times.
Despite that there is no plate voltage present on the 6C4 tubes in STANDBY
mode, the cathodes goes periodically negative enough to draw current between
the grid circuits and the cathodes of those mixer tubes and this is why a
mean negative voltage can be measured between grids and ground also.

73, Jacques, VE2JFE in Montreal

Note that this is in reference to the R-390/URR but is applicable to the
R-390A/URR as well.
The manual states that when the function switch is in STAND BY, the grids of
the three mixers should have a negative voltage within a certain range and
I'm trying to understand where that voltage is coming from.
In STAND BY, B+ is removed from the plate circuits of the mixers so there's
nothing but the 1M grid (leak?) resistors and the cathode circuit.
Considering the case of just the third (PTO) mixer, its cathode is tied to
ground through various elements of Z702.
Given all this, for the third mixer in STAND BY, is the negative voltage at
the grid a product of electrons that are drawn to the more positive grid due
to the much higher resistance to ground than the cathode's circuit?

Thanks,
Barry - N4BUQ