>
>Greetings Amps gurus,
>
>Happy Holidays and special thanks to all who've been so helpful to me over
>the past year! It means a great deal to me.
>
>I'm wondering if someone can explain why AB2 GG service is more linear
>than AB2 grid-driven service?
** AB2 grid-driven has c. 20db gain - but the configuration has no
inherient RF negative feedback to counteract the distortion products
brought about by operating the grid in the grid-current region.
- AB2 cathode-driven has RF negative feedback with the tradeoff of c.
8db less gain. As a result, cathode-driven AB2 is roughly 15x cleaner.
For c. 20db gain and low distortion, an AB1 (0 grid-current) grid-driven
tetrode or pentode is simpler to build than two cathode-driven/g-g stages
in series. AB1grid-driven also has the advantage of not requiring a
flock of tuned input circuits with a Q of 2 to 3 -- which naturally do
not cover much frequency range.
> I've been working with my latest project (
>still on paper ) which is a pair of 4-400As and it appears that all other
>things being equal, about 200-250 watts more is available in AB2 operation
>than is available in AB1.
** True enough, however, by increasing an AB1 stage's anode supply V by
the amount of the screen-V (800V in this case), the anode-screen V will
be the same as in AB2 cathode-driven/grounded-grid, so the output in AB1
will be quite similar to that in AB2. Thus, the output from a g-g/AB2
config operating at 4000V will be c. the same as grid-driven AB1 config
operating at 4800V. For 4-400As in AB1/SSB, 4800V is do-able. For AØ
or RTTY, forgetaboutit.
> I guess I understand why this is but don't
>quite understand why AB2 grid-driven service gives poor linearity yet AB2
>cathode-driven service is acceptable.
>
>I decided to try grid driven service because it simplifies bandswitching (
>given the parts in my junkbox ) and in passive configuration, makes for
>construction simplicity ( no tuned input needed ).
>
>Again.... many thanks to all and Happy Holidays
>
>RD
- Rich..., 805.386.3734, www.vcnet.com/measures.
|