On Fri, 21 Jan 2005 07:23:53 +0000, Ian White, G3SEK
<G3SEK@ifwtech.co.uk> wrote:
David Pruett wrote:
I'd like to point out another tetrode configuration which has some
useful characteristics, and that is cathode driven,
[...]
but not in a zero bias, strapped as a triode configuration. The grid
has negative voltage, and the screen has medium high voltage, very
similar to that of a grid driven configuration.
That is almost exactly the 30-S1 configuration that Colin K7FM and Iwere
talking about.
The correct DC operating conditions are *exactly* the same as for grid-
driven, because the grid, screen and anode voltages are all measuredfrom
the cathode. The tube only knows about the G1-cathode, G2-cathodeand
anode-cathode voltages that it sees.
So, DC-wise, the tube doesn't care which terminal of the power supply
you choose to connect to chassis. DC grounding of the cathode, G1 or G2
terminal have all been used successfully in tetrode amps. The other
terminals of the power supply then have to be above or below chassis
accordingly.
One would think that each extra grid connected to ground acts to impede
the electron flow that much more. In other words, that's an extra fence
for those electrons to have to jump before they get to the finish line.
Being that the screen grid is in line with the control grid, and the
electrons have to pass by its wires the same as the control grid, both
at ground potential, would cut off the flow times two. The only way to
over come this would be to crank the plate voltage up way more than a
normal value. The distance from the cathode to the anode is another
factor and is greater in most tetrodes than in a triode. The designers
knowing this, takes this into consideration because they know a screen
grid is there to boost the flow. This farther spacing causes the
impedance of flow to be more because the electrostatic pull from the
anode is not as great the farther it is away from the cathode. This
would be that it would need that much more plate voltage to achieve the
same electron flow too. For me, I would want to run the tetrode with a
proper screen and bias voltage so it would run at a lower and proper
plate voltage. As long as the bias and screen voltages are clean, the
IMD will be low as it should be.
Pictures are much better than words to describe this. There arediagrams
of the various configurations in the Tetrode Boards manualwhich is
downloadable from my website:
www.ifwtech.co.uk/g3sek/boards
Another common fallacy is that you can't have cathode-driven class AB1.
Sure you can, with a cathode-driven tetrode. It requires exactly thesame
DC grid and screen voltages - relative to the cathode, of course -
as it does for grid-driven. The difference is that the cathode-drivenamp
requires significant RF drive power, even in class AB1.
Exactly, which was my point above. Where I mentioned the differences of
terms between a cathode driven tube and a true G-G come in here. To me,
naming a tetrode running this way, should only be termed cathode driven,
and not G-G as some term it.
Will
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