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

[Barry Scott]

Hi Jacques,

Thanks for the detailed explanation.  I've been monitoring the
elements of the third mixer and that's what I'm seeing as well.
Primarily, I'm asking because the manual states one should see between
-3V and -11V at the grids of any of the three mixers.  The third mixer
is only showing about -3.5v and it concerns me that it is so close to
the minimum.  The first and second mixers have slightly stronger
voltages (perhaps in the -4v to -5v range) the third mixer seems lower
than it should.

I've checked the output of the PTO by disconnecting P723 and using
that as the vertical input on my Tektronix 465B.  There I get about
80v p-p but at the cathode of V205, that's attenuated down to around
10v p-p so only around 0 to -5v for each half-cycle so that's
reasonably close to yield a -3.5v mean voltage.

If R219 is dropping 35v on each peak, that seems to be excessive but
I'm not sure what else would account for the difference in the voltage
at V205's cathode.  Granted my R219 is at the high end of 10% but that
doesn't seem like it would make enough difference to cause the grid
voltage to be that close to the bottom end.

If I remember correctly, I think I put the strongest 6C4 at V203, the
next strongest at V204, and the weakest tube at V205 so that might be
one reason but even the weakest tube is still well over the minimum
umho.

I'm just trying to squeeze as much out of these circuits as possible
and this seems like it could be a spot that could use some improvement
but I'm unsure what could be done about it.

Sorry for the long-ish rambling reply...

Thanks,
Barry - N4BUQ

On Wed, Dec 4, 2024 at 1:38 PM Jacques Fortin <jacques.f at videotron.ca> wrote:
>
> 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
>