[ARC5] Suppressor connection in ARC-5 receivers.

[Neil]

> I have been very curious as to why the engineers at ARC connected the
> suppressor grids in the IF stages of their receivers to the cathodes externally,
> yet used variable cathode bias for gain control in those stages.


I'll paraphrase what the RCA Transmitting Tubes Manual (1956)
says about suppressor grids.

In the specific category of tetrodes, whenever the negative excursion
of the signal swings the plate to a voltage less positive than that of the
*screen* grid, electrons moving to the plate tend to reverse direction
and flow to the screen instead.

The abrupt rise in the plate voltage caused by this reversal tends to
draw electrons back to the plate, whereupon the cycle repeats.

"This interchange of electrons between plate and screen grid, called
dynatron action, may continue for several cycles, and is equivalent to
an oscillatory current."

It goes on to say that this dynatron action has been overcome in the
pentode by the fitting of the suppressor grid which is operated at a
fixed potential that is negative w.r.t both plate and screen "and thus
establishes a negative electrostatic shield between them."

It goes on to say that the suppressor grid is usually connected to the
cathode, but when brought out as a separate terminal may be used
for modulation or other gain control purposes, and is generally more
effective than the screen grid since it requires much less signal power.

In the ARC-5 receivers, raising the cathodes above ground with a
bias resistor makes the *plates and screens* less positive with
respect to the cathodes. Since the suppressor grids are connected
to the cathodes, this makes the suppressor grids more positive with
respect to both and gives greater gain variation than the control grid
variation that would otherwise result, and with virtually no power
consumption.

73 de Neil ZL1ANM