Re: [Amps] Re: matching network

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

R. Measures wrote:
> > 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?

Which means: "Keep on reading, before typing".

> > 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.
Except when it's 13, for a 4CX350A.


> > 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.

The crossover point is actually around 144MHz. Above that frequency, 
matching into the grid becomes quite tricky, as anyone who has built a 
K2RIW grid circuit for 432MHz can testify.

> > 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.
Apparently I failed to make my point.


A more detailed Application Note entitled 'Tetrode Grounding Options' is 
on my website at:
www.ifwtech.co.uk/g3sek/boards/tetrode/an-4-1.0.pdf  (6 pages, 35KB)
It fully explains the options for grid or cathode drive, and for DC 
grounding and RF bypassing.




--
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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