[Amps] Re: matching network

[Ian White, G3SEK]

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