>R. Measures wrote:
>>** In a grounded-grids, tetrode amplifier, c. 75% of the total grid
>>current flows in the control grid and c. 25% of the current flows in
>>the screen grid.
>
>We need to be very careful about what we mean by "grounded grids".
>
>Both grids need to be grounded to RF... but they don't *have* to be both
>connected directly to chassis.
** which means?
>
>What Rich says is only true if both grids are connected directly to
>chassis (and even then, the current division between g1 and g2 will
>depend on the mu-g1g2 value for that particular tube).
>
** True, but g1-g2 amplification is typically 4 to 5.
>If one grid is directly grounded, and the other is bypassed to ground,
>and the g1 and g2 voltages - measured relative to the cathode voltage -
>are the same as for grounded cathode, then the tube's DC operating
>conditions will still be OK. Many VHF/UHF tetrode amplifiers use this
>configuration, because at those frequencies the cathode driving
>impedance is much easier to match than the grid driving impedance.
>
** Especially above 1GHz.
>In other words, cathode drive is strictly an RF thing. It doesn't *have*
>to involve incorrect DC operating conditions.
>
>However, for this particular 4CX350 amp, it makes no sense to convert it
>to cathode drive, because the amp will then require a band-switched
>input network. As many have said, resistor-swamped 'passive grid' is the
>way to go.
** The 2mA max grid current rating would seem to preclude grounded grid
operation.
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