[Amps] 4CX1500 class AB1 vs. AB2

[Gary Schafer]

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Tom W8JI
> Sent: Thursday, November 02, 2006 10:04 AM
> To: Gary Schafer; 'Ian White GM3SEK'; amps at contesting.com
> Subject: Re: [Amps] 4CX1500 class AB1 vs. AB2
> 
> >> Contrary to the ham folklore, edging over the border into
> >> AB2 doesn't
> >> have to be an IMD disaster. The two possible causes of
> >> IMD when crossing
> >> from AB1 into AB2 are a sudden change load impedance on
> >> the exciter, and
> >> a sudden change in DC grid bias. The change in load
> >> impedance is
> >> minimized in modern amps that use a grid swamping
> >> resistor, and a well
> >> regulated bias supply will keep the voltage constant.
> Ian GM3SEK
> >
> > A regulated bias supply with AB1 operation?? That sounds
> > like a recipe for
> > disaster.
> > Gary  K4FMX
> 
> Why would that be a disaster Gary? It is a disaster when
> people have a loose grid supply and use them in what they
> think is AB1. Virtually every exciter on market today has
> severe overshoot at leading edges. If the amplifier changes
> grid supply change voltage during that transient it's a
> disaster for anyone else on a nearby frequency.
> 
> 73 Tom

If there is severe overshoot coming out of the exciter any amplifier is
going to have problems with it causing distortion products.

With an AB1 amplifier with a stiff bias supply how do you keep from
destroying the grid unless you NEVER bump into grid current? With a higher
impedance bias circuit it is self limiting in grid protection and alc is
easily developed as is done in the Collins 32S1 etc.

73
Gary  K4FMX