On Wed, 27 Jan 1999 22:32:49 EST Skip S Isaham <nospam4me@juno.com>
writes:
>So how is the grid termination resistance value selected? I've seen
>amplifiers with 50 ohm termination values going higher. With
>higher resistance values, I've seen toroid matching transformer
networks.
>Is there a guideline for the selection of a grid termination resistor,
>or is it done by method ?
>
>thanks
>skipp
Hi Skipp,
I guess the best answer is that it is done for convenience...
considering the available drive power from the exciter, the required RF
grid voltage to drive the PA tube and how much power you wish to
dissipate in the termination resistor.
In a typical AB1 (no grid current) swamped grid driven tetrode
application with for example -60 volts of grid bias, your would want 60
volts peak RF which would be around 40 watts RMS into 50 ohms. This
would be an easy level, allowing some headroom (have to let the ALC do
something, just to see if it is still working), for most exciters but, a
lot of heat in the termination resistor.
Now, for instance, if you place a 1:2 turns ratio step-up transformer
on the input, then the termination resistor becomes 200 ohms (to maintain
a 50 ohm match to the exciter) and the required drive power drops to 10
watts... then, you could use one of those low power, rarely exported,
Japanese radios to drive the amplifier. The inherent stability of the
swamped grid configuration will be slightly degraded due to the lowered
grid loading.
Of course, your bias (voltage) may vary, depending on the tube employed
and class of operation. The start of this theme was a... curious...
class B triode amplifier.
73,
Marv WC6W
*
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