>
>
>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.
? ... or, with a more capable exciter, one could use a bifilar (4:1 Z
ratio) (2:1 V ratio) step-down broadband transformer driving a 12.5 ohm
grid term. resistor. In this case, in order to develop the needed
60v-peak at the grid, 120v-peak.is needed at the input. At 50 ohms,
120v-peak is 144w pep. Surely, 12.5 ohms at the grid sounds weird.
However, it provides better stability and better input swr than 50 ohms.
. To enhance vhf stability, the grid term. R should connect directly to
the grid, the cold end being triple-bypassed to gnd. Grid bias is
applied to the cold end.
> 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.
>
? Curious indeed, Marv. With a typical 5 to 10 pF between the grid
input and the anode/plate -- without neutralization -- no excitation
whatsoever would be needed to achieve full RF out. When I first heard
about this cacamamie scheme, I thought it must be a misplaced April Fool
article. .
- cheers -
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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