I've seen many tubes used in grounded grid, but 6146Bs are usually
shown in class AB2. Can 6146B tubes be used in grounded grid with
good results?

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

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.

"Ken Scharf" wrote

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).


Ken:

I think you meant to say that cathode driven tubes have LOWER power gain.
Grid driven amps can have as much as 10 dB or more gain than cathode driven
(GG) amps, all other factors being equal. My Johnson Thunderbolt running a
pair of grid driven 4-400's can be driven to full output with about 20 watts
of RF drive (representating close to 20 dB of power gain). The same tubes
in GG configuration would need close to 80 watts to drive them to full
output.

A grid driven 4CX250 can produce over 200 watts of output with 2 watts of
drive (per the RCA manual). Because the tube isn't commonly used in GG
configuration, that manual doesn't provide power gain figures for cathode
driven service. My guess is you would need more like 8 to 10 watts of
cathode drive to get the same 200+ watts out.

The widespread use of transceivers in the 100 watt class has made GG amps
popular since there is no need to attenuate the transceiver's output. With
my grid driven Thunderbolt, I use a 6 dB attenuator at the amp's input to
keep from overdriving it.

With the right choice of tube, a GG amp can have a much simpler power
supply. And since there is no need for neutralization, homebrewing a GG amp
is quite appealing.

Roger K6XQ

On Sat, 13 Aug 2005 20:45:08 -0400, Ken Scharf
wrote:

[snip]
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).

You should take this up with the guys at Eimac. (Well the used to be
Eimac anyway).

Section 5.4 of "Care and Feeding of Power Grid Tubes" is titled,
"Neutralizing Ground-Grid Amplifiers."

On Mon, 15 Aug 2005, Roger Leone wrote:


"Ken Scharf" wrote

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).


Ken:

I think you meant to say that cathode driven tubes have LOWER power gain.
Grid driven amps can have as much as 10 dB or more gain than cathode driven
(GG) amps, all other factors being equal. My Johnson Thunderbolt running a
pair of grid driven 4-400's can be driven to full output with about 20 watts
of RF drive (representating close to 20 dB of power gain). The same tubes
in GG configuration would need close to 80 watts to drive them to full
output.

A grid driven 4CX250 can produce over 200 watts of output with 2 watts of
drive (per the RCA manual). Because the tube isn't commonly used in GG
configuration, that manual doesn't provide power gain figures for cathode
driven service. My guess is you would need more like 8 to 10 watts of
cathode drive to get the same 200+ watts out.

The widespread use of transceivers in the 100 watt class has made GG amps
popular since there is no need to attenuate the transceiver's output. With
my grid driven Thunderbolt, I use a 6 dB attenuator at the amp's input to
keep from overdriving it.

With the right choice of tube, a GG amp can have a much simpler power
supply. And since there is no need for neutralization,

Often there is no need for neutralization, but depending on frequency and
wiring layout, yes there is a need for neutralization. Usually its worse
at higher frequencies. And, it can get very very complicated, too.

I have an Ameritron 811-H (four 811s in grounded grid) and it comes from
the factory with one kind of neutralization!!!! It would not be there if
it were not needed.

I had a thunderbolt, too. And it also had neutralization built in.

I also had a Heathkit DX-40 back in my novice days and it would go into
self-oscilation (big time) on ten meters if I removed the crystal or shut
off the VFO. I made my own neutralization wire and fixed that. Its funny that
Rangers had no built in neutralization, either. Same tube, similar (but
not identical) circuit in final (6146). You don't know if you need
neutralization until you do the tests (read the manuals on this).

Art, W4PON

homebrewing a GG amp
is quite appealing.

I've built several (including a single 4-400) and I'm happy when I don't
need neutralization.

Roger K6XQ

Roger Leone wrote:
"Ken Scharf" wrote

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).



Ken:

I think you meant to say that cathode driven tubes have LOWER power gain.
Grid driven amps can have as much as 10 dB or more gain than cathode driven
(GG) amps, all other factors being equal. My Johnson Thunderbolt running a
pair of grid driven 4-400's can be driven to full output with about 20 watts
of RF drive (representating close to 20 dB of power gain). The same tubes
in GG configuration would need close to 80 watts to drive them to full
output.

Actually I meant to say that the "cathode drive tetrode" has HIGHER gain
than a TRUE grounded grid amp. The only difference in gain between a
grid driven amp and a cathode driven tetrode amp would be due to the
driving power lost in the input circuit. But don't forget that in a
cathode driven circuit, some of the input power actually feeds through
and appears as part of the output, so it really isn't 'lost'.

Your statement about needing 80 watts to get full output in gg would
NOT be true if you ran the 4-400's in "cathode driven tetrode" service.
The BIG difference is applying normal bias and screen voltage rather
than grounding both grids and running in zero bias class B. I suspect
you'd need less than 25-30 watts for full output in "cathode driven
tetrode". BTW you'd need quite a bit LESS than 20 watts drive in
grid driven service, if you ran the tubes in class AB1 where only
voltage (no grid current) is required. (But it might take THREE tubes
to get the same output as TWO in class B or AB2).

A grid driven 4CX250 can produce over 200 watts of output with 2 watts of
drive (per the RCA manual). Because the tube isn't commonly used in GG
configuration, that manual doesn't provide power gain figures for cathode
driven service. My guess is you would need more like 8 to 10 watts of
cathode drive to get the same 200+ watts out.

Sounds about right. Depends on how good your input matching network is.
Those figures must be for class AB2 though. In class AB1 your IMD goes
way down and the driving power might be only 1-2 watts in cathode drive.
The widespread use of transceivers in the 100 watt class has made GG amps
popular since there is no need to attenuate the transceiver's output. With
my grid driven Thunderbolt, I use a 6 dB attenuator at the amp's input to
keep from overdriving it.

With the right choice of tube, a GG amp can have a much simpler power
supply. And since there is no need for neutralization, homebrewing a GG amp
is quite appealing.

A newer way is to run tetrodes in grid driven service with a 50 to 200
ohm high power non inductive swamping resistor from grid to ground. NO
matching network needed, same drive requirement as grounded grid, no
neutralization. Waste of power though. Many commerical amps using
Sevtlana tetrodes using this circuit.