[Boatanchors] AM linear question

[Rob Atkinson]

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