|[AMRadio] Pi-Net vs Link Couple|
k4kyv at hotmail.com
Wed Sep 28 17:35:10 EDT 2005
>On 22 Sep 2005 at 20:20, Donald Chester wrote:
> > A tube
> > with as much grid-plate capacitance as the 450TL or 250TH must be
> > neutralised for proper operation, unless it is grounded grid. Grounded
> > grid
> > is not recommended for plate modulated finals.
>For all tubes? Please explain...
First of all, you would have to run the amplifier class C for plate
modulation. Class C triodes already require a lot of grid drive, usually
about 10% of the expected power out.
In addition, some of the driving power in grounded grid service feeds
through to the output, since the input and output circuits share a common
impedance at the cathode. So a grounded grid class C circuit would require
still more than the nominal 10% driving power.
Additionally, that driving power is present in the output at all times, even
with the plate voltage turned off. This is somewhat like a backwave on a cw
signal. Therefore, it would be impossible to achieve 100% modulation of the
carrier, since even with the plate voltage of the tube driven negative on
the negative modulation peak, there is still some rf output present.
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