>Rich says:
>
>>// I have never heard of a tetrode that is more linear when grid current
>>flows than when no grid current flows.
>
>Pappenfus, Bruene, Schoenike, Single Sideband Fundamentals and Circuits,
>McGraw Hill 1964. Sec 13-3, P191 et seq. 'A small amount of grid current can
>sometimes
// (N.B.)
>be used to cancel plate current distortion'. Also explains how biasing for
>AB2 can put the characteristics on a more linear part of the palte current
curve.
>Explains why the IMD characteristics don't follow the classic third order
>intercept graph, but show a dip. Happens with all tetrodes I've ever tried
>
// Why doesn't Eimac recommend AB2 for the plethora of tetrodes other
than the 4cx600 and 4cx1500B? Why did the tetrode amplifiers that I
built become less linear with grid current?
>>// 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?
>
>Which is why you need a low impedance RF source. Quoting Pappenfus again
>'Increased grid swamping in the form of lower values of swamping resistor
>reduces the nonlinear variation in r-f grid voltage'.
>
>A low impedance source can be produced in a number of ways: grid swamping is
>only one of them.
>
>>// A swinging choke filter produces constant V-out Only when the load
>>current is constant.
>
>In that case, you could use a resistor.
// 0.01-ohm should do it - provided it's in parallel with the choke.
>
>As the current increases, the value of critical inductance needed in a choke
>input filter decreases, which is why swinging chokes were introduced. If the
>value of inductance follows linearly the current drain such that the ratio of
>actual inductance to critical inductance stays constant, it's quite well
>regulated. What you do need is sufficient output capacity that the supply
>has a low impedance at the syllabic rate,
// A couple of Farads ought to do this.
>partly because of increased drops in
>rectifiers, choke and transformer (and maybe mains). The swinging choke
doesn't
>follow so well as frequencies go up because of the hysterisis curve.
// So Continental, Harris, Hughes, Henry Radio, and Collins made a
mistake when they utilized a resonant-chke filter in their anode supply?
>In the case
>under discussion of the Tbird,
// Thunderbolt
>the choke serves the purpose of dropping the
>volts without screwing up the regulation in the way a resistor would.
>
// ?¿?¿
>>// But it's enough for Class C.
>
>Sure - for 1kW input. In linear service, 1kW in is about 500 watts out,
>give or take. You hardly need a pair of 4-400s for that!
>
>>Hows about a G2DAF Class C linear?
>
>Fine, Rich, you build it!
>
// I am a tad reluctant to risk getting tarred and feathered, Peter.
>The G2DAF was not Class C.
// The three I was familar with were seemingly Class C. One of them was
pretty close to Class D. (the one that could be heard for 150kHz on 75m)
> Many of its implementations aren't linear either.
>I've never come across one that was, but it doesn't mean that it's totally
>impossible that there are any.
>
// As I understand it, as of 1999 there were two leprchaun club stations
in EIØ and EI9 that had semi-linear DAFs.
>I certainly found I could get better linearity in AB2 than AB1 with
>4-250As when
>I went to regulated screen, shunt regulated bias, and negative feedback. Rich
>has called me a liar
// Quote please
>before for saying that, but despite the fact that given
>half a chance, the HP141 spectrum analyser will lie like a personnel
>manager, it didn't get the chance.
>
// The acid test for linearity is on the air measurement of voice
emission feculence above and below the transmit passband. From my
experience with G2DAFers, such measurement Must be covert.
>
cheers, Peter
- 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
|