Re: [Amps] Re: matching network

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

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.

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).

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.

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.



--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                           Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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