Roger Leone wrote:
There are several considerations when choosing a tube for grounded grid
(GG) operation. One of the most important is whether all of the grid
connections are available on the base of the tube. In the case of the 6146
(including the A, B, etc variations), some are not. The control and screen
grids come out to pins on the base of the tube, so they could be connected
to ground. But the beam forming structure (identified as grid #3 on most
diagrams) is internally connected to the cathode. You cannot ground it
without grounding the cathode (and vice versa). In GG configuration, the
cathode gets the RF drive so it cannot also be gounded. Because of this
internal connection, the 6146 is not a good candidate for GG operation.

This factor also explains why you may see GG amplifiers using certain sweep
tubes like the 6JB6, 6KD6, and 6LF6. They have all their grid connections
available at the tube base and none are internally connected to the cathode.
Other sweep tubes like the 6HF5, which have internal connections to the
cathode, are not suitable for GG operations.

Having said this, I should also point out that not all tubes with access to
their grids at the base are good candidates for GG operation. Among other
factors are the necessity for biasing. The tubes usually chosen for
grounded grid operation require little or no DC bias voltage to limit plate
current and set the class of operation. An example of a tube not usually
seen in grounded grid configuration is the 4CX250 series. None of its grids
is internally connected to the cathode, but in order to run it in grounded
grid configuration, you must still supply bias and screen voltages. Which
means it is no easier to make into a GG amp than a grid driven (grounded
cathode) amp. One big advantage of GG amplifiers is they usually avoid the
need for bias and screen supplies.

Also, the class of operation, such as AB2 which you mentioned, is not
limited to grounded cathode operation. You can have a GG amp running class
AB2. The class of operation is determined by the portion of the RF cycle
that causes the plate current of the amplifier tube to vary. For example,
in a class A amplifier, plate current is always flowing; even during the
most negative voltage peak of the RF cycle. The tube is conducting for the
full 360 degrees of the RF cycle and is varying in accordance with the drive
signal. In class C operation (used mostly for FM, CW and plate modulated
AM modes), plate current flows during only the most positive peaks of the RF
drive signal and is cut off during most of the RF cycle. The output of the
tube is a series of pulses which are not a representation of the drive
signal, so Class C is not a "linear" mode of operation.

There is more to this, obviously. You should read though the "Transmitting"
chapters of any copy of the Radio Amateurs Handbook.

Good luck,

Roger K6XQ


One advantage of "cathode driven tetrode" (where normal grid bias and
screen voltage are applied) is that you get higher power gain and
usually don't need to neutralize the amplifier (technically it is
IMPOSSIBLE to neutralize a ground grid amplifier anyway).

Some brands of 807 and 1625 tubes had the beam deflection plates
brought out via a separate wire through the glass which was connected
to the cathode outside the bulb. It was possible to remove the base
and then connect this lead to it's own pin (on the 1625, or on the 807
by changing the base to one with more pins), or connect it to the screen
or control grid so the tube would work in GG. Thus modified the
807 / 1625 made a good gg tube. It's also possible this was true with
some brands of 6L6 tubes as well.

The 814 and 813 transmitting tubes also can be used in GG operation,
they have the beam deflection plates brought out to it's own pin on
the base.

4CX150 and 250 series tubes work VERY well in "cathode drive tetrode"
operation. Too bad the price of these tubes is now so high, even in
surplus.