Re: [Amps] Re: matching network

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

R. Measures wrote:
> **   Indeed.   ...   getting back to g-g operation for a tube with a max.
> g1 rating of 2mA, I would not try it.  Even with a stripe-cathode and
> aligned grid, typical grid currents are c. 100mA in g-g triodes.  .

The problem seems to lie in an over-casual use of the term "g-g".

Once again: 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.

Once again, this is common practice in VHF/UHF amplifiers, where 
grid-driven operation is difficult for RF-related reasons. Collins used 
a very similar system in the 30S-1, which has a 4CX1000A in 
cathode-driven class AB1.


--
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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