Chuck Sherwood wrote:
Frankly I don't think 10W will drive any linear to 1kW.
You will get the most gain from a Tetrode but I still
don't think it will go 20db.
Most modern tetrodes will easily provide 20dB of gain, and with good
stability.
The favored technique is to use an untuned grid circuit which is
resistively loaded (often called 'passive grid').
In class AB1, grid-driven, a tetrode requires very little RF input power
(in simple theory, none at all). All the grid requires is a voltage
swing. For good linearity, the tube should never be driven into grid
current - in other words, even at peak RF input voltage the grid should
never become positive. That means the peak RF voltage should only be
equal to the grid's negative DC bias. These basic requirements are
totally independent of the output power capability of the tube.
For tube data, see:
http://www.g8wrb.org/tetrodes.shtml
The driver stage will need to see a 50 ohm load impedance, so the most
common input circuit in commercial tetrode amplifiers is simply a 50 ohm
resistor. However, 10W into 50 ohms is only 31V peak, and a typical
value of grid bias is probably about -50V (depending on the specific
type of tube and its recommended DC bias conditions). It is quite true
that 10W could not drive such an amp to full output.
But you're not forced to use a 50 ohm input resistor. Think about using
a broadband step-up transformer to get a larger voltage swing.
For example, a unun transformer with a 4:1 impedance step-up, terminated
in a 200 ohm resistor. This will provide a 50 ohm load to the driver
stage, and 10W will deliver about 62V peak to the grid of the tube. So
any tube that will operate in class AB1 with a grid bias of 62V or less
can be driven to full output by 10W.
10W into an input circuit consisting of a 9:1 unun terminated in 450
ohms will deliver about 93V peak. This is more than enough to drive any
modern ceramic tetrode to full output, so the driver stage can actually
be operated at significantly less than its 10W maximum, for improved
intermodulation distortion.
It should be quite easy to find or develop a suitable broadband
transformer design, especially since the original poster only wanted to
operate 160-40m.
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek