[Boatanchors] AM linear question

[Dennis DuVall]

Rob,

Interesting…  I’ve always just adjusted the linear tuning and loading for max output at the AM peak value with excellent results (very conservatively built HB 3-1000Z amp).  Nice flat sides on your trapezoid pattern makes sense.  Would be curious to know, though, the difference, if any, in linear tuning and loading control settings arrived at by that technique vs. those found by adjusting for max output at the peak value only.  Put another way, would adjustment for max output at the peak value mess up the shape of your trapezoid?

Dennis D.  W7QHO
Glendale, CA

*************
> On Dec 10, 2015, at 3:12 AM, Rob Atkinson <ranchorobbo at gmail.com> wrote:
> 
> On Wed, Dec 9, 2015 at 9:28 PM, Dennis DuVall <duvallddennis at gmail.com> wrote:
>> The process of setting up a linear amplifier for AM is complicated by the requirement that the amp be first adjusted for operation at the PEAK power level expected to be encountered which, in turn requires an RF drive source capable of providing this.   This creates a problem if a small rig like a Ranger or an Elmac AF-68 is used, neither of which is capable of providing sufficient CW power to properly tune up the amplifier (presuming GG 3-1000Z, pair of 3-500Zs or equivalent here).
>> 
> 
> I operate a modified (Ten Tec fanatics will say butchered) Ten Tec
> 3-500 amplifier in AM.  I have opened the cabinet up, added an
> external 1.5 A CCS plate transformer and doubled the air volume moving
> over the tubes, and put in higher RF current load padding caps and
> modified the in-rush circuit.  The output pi tank resonance point with
> the tune capacitor is the same regardless of drive.  The trick is
> loading the amp for linear AM amplification.  I have found a monitor
> scope, in my case a SB610, set up in 'fake' trapezoid mode with the
> exciter RF and amplifier output RF to be invaluable, modulating the
> exciter and adjusting the amplifier load to get a nice flat sided
> trapezoid on the monitor scope.  I would be uncomfortable operating AM
> with a RF amplifier without it.  Actually they should be used by all
> hams running amplifiers regardless of mode.
> 
> 73
> Rob
> K5UJ