[Amps] Re: [Amps] Driving Impedance

[Richard]

>A tetrode can be grounded grid.
>
>Polarization requirements are not the same than dynamic circuit and 
>have nothing to do with the amplifier configuration.
>Stated it, and generally speaking, why to bother with a tetrode in a Gg 
>and why to be forced in building a second PS.
>Use a triode, instad.

The tuned input circuits for the soon to be 10 amateur bands below 30 MHz 
are not a piece of cake for a Q of 2-3, which is needed with solid state 
transceivers driving G-G amplifiers.  To fully cover the 3.5MHz and the 
28MHz bands, two additional tuned inputs are needed.  With a grid-driven 
amplifier, one roller-coil and one terminating R gives a good match 
everywhere.
-- A second power supply is not needed for the 4cx800 (30mA/350V screen) 
because the ideal setup is to obtain the screen V from a shunt zener 
string powered by the current from a resistor connected to the HV-PS.  
The neg. grid bias potential can come from a half-wave rectifier, 
C-filter connected to the mains. 

>All the matter is a bit obscure, it becomes expecially annoying when 
>using tubes where screen current has to be zero or anyway very low.
>
There is normally screen current under signal conditions. 

>In my experience I never found a tetrode with more gain than a triode 
>when used as a Gg amplifier at full power.
>
Agreed, and there's usually less gain than with a high-Mu triode.  
However, a grid-driven 8281 tetrode will do c. 23db gain with 170w drive. 
 

ciao, Mauri
>
>
>
>Grids be dynami
>
>
>
>> Richard wrote:
>> >
>> >
>> >>Greetings,
>> >>
>> >>What's the input impedance for a GG 4CX800A?
>> >>
>> >With a 2w? grid, running it in g-
>g might not be a wonderful idea.  The
>> >4cx800 is designed for Class AB1, which means 0 grid current.
>> >
>> With the correct DC voltages applied to the control grid and screen, a
>ny 
>> tetrode will work fine with both grids at RF ground. This is very comm
>on 
>> at VHF and above, and occasionally seen at HF.
>> 
>> According to the way the power supply is wired, you can chassis-
>ground 
>> either grid, and RF-bypass the other one; or you can DC-ground the 
>> cathode and RF-bypass both grids.
>> 
>> As Rich says, what you can *not* do is run a modern tetrode with both 
>> grids directly grounded to the chassis. The control grid takes too lar
>ge 
>> a share of the combined grid current.
>> 
>> Unfortunately - and wrongly -
> this method of operation seems to be what 
>> most hams understand as "Grounded Grid". It only works with the old 
>> glass tubes because they were primarily designed for Class C, and have
> 
>> very hefty control grids that can stand the extra current. But even 
>> these tetrodes were never designed to be operated with both grids 
>> chassis-grounded - it's a ham invention.
>> 
>> 
>> -- 
>> 73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
>>                            'In Practice' columnist for RadCom (RSGB)
>> New e-mail: g3sek@ifwtech.co.uk
>> New website: http://www.ifwtech.co.uk/g3sek
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-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
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