Re: [Amps] Re: matching network

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

R. Measures wrote:

> > you can safely operate *any* tetrode in class AB1,
> > cathode-driven. The requirement is that the control grid has the correct
> > negative bias, and the screen has the correct positive voltage. These
> > voltages are exactly as recommended by the manufacturer for class AB1,
> > grid-driven.
> >
> > For cathode drive, both grids must be grounded to RF. But in order to
> > apply different DC voltages, at least one grid cannot be connected
> > direct to chassis - it has to be RF-grounded through a bypass capacitor.
> > Because the recommended voltages are measured relative to the cathode,
> > all three power supplies (anode, screen and control-grid bias) generally
> > have to 'float' relative to the chassis potential.
> >
> > The correct DC bias voltages ensure that all the DC "grid" current goes
> > to the screen, and *none* to the control grid... in other words, correct
> > class-AB1 operation.
>
> **  Ian --  Is the power gain with cathode-driven and  no g1 current the
> same as with 'super-cathode' drive -- i.e., c. 5?

No.

In 'Single Sideband Principles & Circuits'  by Pappenfus, Bruene and 
Schoenike (aka the Collins company) they first describe GG triode 
operation, and quote typical power gains of 7 to 20.

They then continue: "Tetrode tubes can also be used in cathode-driven 
operation. The power gain is considerably higher, and is in the 20-to-50 
region."

They then consider semi-cathode-driven, super-cathode-driven and the RF 
feedback arrangement used on the 30S-1.

It's well worth reading.

> -  If g1 is never to be driven more positive than the cathode potential,
> why not connect it to the cathode?

It's all in that book.



--
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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