2 wrote:
>
>>
>>The Bill Orr Handbook, sixteenth edition,
>>describes a 10-20
>>meter cathode excited AB-1 amplifier using a pair
>>of 4cx300a's.
>>The twenty-first edition of the Bill Orr Handbook
>>describes
>>an 80-10 meter cathode exited AB-1 amplifier using
>>a pair of 4cx250b's
>
>/\ A grid dissipation rating of 2w max. is not good news for g-g
>operation.
"Grounded grid" gets a bad name because of the amateur practice of
connecting both grids to chassis ground. As Rich says, the control grids
of most modern tetrodes won't handle the excessive grid current (only
the old glass 4-xxx tetrodes will handle that).
>With Class AB1 grid-driven operation, grid dissipation is 0w.
You can have true class AB1, cathode-driven, if the grids are bypassed
to *RF* ground, and have the same bias voltages as for grid-driven class
AB1. There's a review of all the tetrode grounding/biasing options on my
website at
http://www.ifwtech.com/g3sek/boards/tetrode/an-4-1.0.pdf
Cathode-driven with correct bias voltages is popular for large tubes in
single-band VHF/UHF amplifiers, because the cathode impedance is easier
to match at those frequencies than the grid impedance would be. For a
multiband HF amp you'd need to provide switchable tuned input networks,
so grid-driven operation with untuned resistive input is a much better
option.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|