Amps
[Top] [All Lists]

Re: [Amps] 4CX1500 class AB1 vs. AB2

To: "'Ian White GM3SEK'" <gm3sek@ifwtech.co.uk>,<amps@contesting.com>
Subject: Re: [Amps] 4CX1500 class AB1 vs. AB2
From: "Gary Schafer" <garyschafer@comcast.net>
Date: Thu, 2 Nov 2006 09:44:48 -0600
List-post: <mailto:amps@contesting.com>

> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Ian White GM3SEK
> Sent: Thursday, November 02, 2006 2:02 AM
> To: amps@contesting.com
> Subject: Re: [Amps] 4CX1500 class AB1 vs. AB2
> 
> Tom W8JI wrote:
> >> I have decided to build a 4CX1500 amp.  I have a couple of
> >> tubes and sockets
> >> already. I know that it is designed for class AB2 service
> >> for very low
> >> distortion.  This seems to limit the tube to 1100W output.
> >> I am considering
> >> adding a switch to change the screen and grid bias to
> >> allow me to run it in
> >> AB1 service as well for 1500W output.
> >
> >What particular 4CX1500 do you have? The A  or the B? There is a huge
> >difference.
> >
> >The 1500B generally should be used at zero or very near zero grid
> >current.
> >
> The manufacturers recommend these tubes are used with an ALC system to
> keep them in class AB1. However, the most practical way to derive the
> ALC signal is to detect the grid current that starts to flow when it
> crosses the boundary into class AB2.
> 
> The current needed to activate the ALC on peaks of modulation only needs
> to be about a milliamp, or even less... but it isn't zero, so strictly
> speaking we have crossed the line into class AB2. This creates a
> confusing situation where the data sheet has to say "AB2" to be correct,
> but this hides the more important fact that the tube spends almost all
> of the time in class AB1.
> 
> 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.
> 
> 
> 
> --
> 73 from Ian GM3SEK

A regulated bias supply with AB1 operation?? That sounds like a recipe for
disaster.

73
Gary  K4FMX


_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps

<Prev in Thread] Current Thread [Next in Thread>