>
>Rich says:
>
>>// Class AB2 is not linear with a grid driven 4-400. Class AB1 is
>>linear.
>
>This is not strictly true. AB2 can be more linear than AB1 - see the classic
>work by Pappenfus et al.
// I have never heard of a tetrode that is more linear when grid current
flows than when no grid current flows. Even a tetrode that is designed
to be linear with grid current present - such as the 4cx1500B and 4cx600A
- becomes less linear with grid current. Also, the amount of grid
current specified for them in Class AB2 service is only about 0.3mA. So,
technically speaking, it's AB2 bot only barely.
- The 4cx1500B and 4cx600A tolerate a small amount of grid current
because of a special design. The 4-400A/C is not of this design Eimac
lists only Class AB1 for RF linear amplifier service for the 4-400A/C .
> In order to get that performance, you need a well
>(shunt) regulated bias supply, a low impedance RF source so the grid volts
>don't get distorted when grid current flows, and well regulated screen grid
supply.
>Some negative feedback from unbypassed cathode resistors helps, too.
>
// What if the exciter isn't able to handle AB2's non-symmetrical flow
of grid current in parallel with the symmetrical flow of grid current in
the grid termintion-resistor?
>If you don't mind the Tbird becoming distinctly non original, I'd seriously
>consider going to grounded grid. Turn the old grid input circuitry into
>switched
>pi networks, go to a bridge rectifier with choke input
// A swinging choke filter produces constant V-out Only when the load
current is constant. Capacitor filters and resonant-choke filters
handle changing load current without anomalies.
>and a stack ofelectrolytics for the HV capacitor. 4800 volts AC will give
about 3800 DC
>with a choke input filter, although you'll need a bleeder resistor. You may
need
>some bias: probably something like a 10 volt 10watt zener, but you'll need to
>experiment there. I'd go for about 70mA per tube standing current. The
>4-250/4-400 family perform quite well at 3600 or 3800 volts. Although I use
>passive grid input AB2 on the LF bands with 4-250s or 4-400s (the basic
>tube is the same
>,
// Bigger isn't always more.
> the filament is the same so the peak cathode current is the same) I
>also use one on 10m in g-g as above with good results. Incidentally, in
>g-g with an 8.2v zener for bias, you could substitute 3-500Zs - same cathode
>structure.
>
>Passive grid with a 50 ohm resistor will give you a <1.5:1 SWR with up to
>40pF of stray C across the swamping resistor.
>
>If you want to stay with grounded cathode, the HV is far too low - for either
>AB1 or AB2.
>
// But it's enough for Class C. Hows about a G2DAF Class C linear?
>
>Peter G3RZP
>
>
>
>--
>FAQ on WWW: http://www.contesting.com/FAQ/amps
>Submissions: amps@contesting.com
>Administrative requests: amps-REQUEST@contesting.com
>Problems: owner-amps@contesting.com
>
>
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|