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[Amps] Re: AB1 v. AB2

To: <amps@contesting.com>
Subject: [Amps] Re: AB1 v. AB2
From: 2@vc.net (Rich)
Date: Sat, 30 Nov 2002 15:40:04 -0800
>Rich wrote:
>
>> >
>> >----- Original Message -----
>> >From: Rich <2@vc.net>
>> >To: <bearlabs@netzero.net>; AMPS <amps@contesting.com>
>> >Sent: Friday, November 29, 2002 1:40 PM
>> >Subject: Re: [Amps] Re: AB1 v. AB2
>> >
>> >
>> >>
>> >>
>> >> >amps-request@contesting.com wrote:
>> >> >
>> >> >>
>> >> >>
>> >> >> Message: 8
>> >> >> To: amps@contesting.com
>> >> >> Reply-To: ad5gb@myway.com
>> >> >> From: "ad5gb" <ad5gb@myway.com>
>> >> >> Cc:
To: <amps@contesting.com>
>> >> >> Date: Thu, 28 Nov 2002 22:18:13 -0500 (EST)
>> >> >> Subject: [Amps] AB1 vs AB2 ???
>> >> >>
>> >> >> Greetings Amps gurus,
>> >> >>
>> >> >> Happy Holidays and special thanks to all who've been so helpful to me
>> >over
>> >> >the past year!  It means a great deal to me.
>> >> >>
>> >> >> I'm wondering if someone can explain why AB2 GG service is more linear
>> >than
>> >> >AB2 grid-driven service?
>> >> >
>> >> >I dunno about that one... check a handbook.
>> >> >
>> >> >> I've been working with my latest project ( still on paper ) which is a
>> >pair
>> >> >of 4-400As and it appears that all other things being equal, about
>> >200-250
>> >> >watts more is available in AB2 operation than is available in AB1.
>> >> >
>> >> >This is because in effect the load line is "longer" in AB2 than AB1. AB2
>> >> >permits you to draw grid current, whereas
>> >> >AB1 does not. That means you can drive the grid *positive* past "0"
>> >> >whereas in AB1 your drive stops at "0"
>> >> >since current can not be delivered to the grid in AB1.
>> >
>> >> >
>> >> **  AB2 doesn't permit anything.  AB2 neans the operator Chose to drive
>> >> the tube into its less-linear grid-current region.  Up until that point,
>> >> the tube is operating partly in Class A and then in Class AB1 -- as the
>> >> grid-V sinusoidally decreases as it approaches 0V .
>> >
>> >By *definition* AB2 means that the grid is driven positive with respect to
>> >the cathode - and regardless *is* in the grid current region. AB1 by
>> >definition is up to
>> >"0" volts with respect to the cathode, and NEVER draws any grid current.
>> >
>> >An amp set up for AB1 can NOT be driven into grid current - if it is able 
to
>> >be driven into grid current then it is an AB2 amplifier, regardless of how 
>you
>> >chose to run the input levels.
>> >
>> **  increase drive to the point that grid current flows and the AB1 amp
>> becomes an AB2 amp.
>>
>> >In the case of most circuits, the AB1 amps are AC coupled and the AB2 amps 
>are
>> >DC coupled to the grids - which permits current to flow. This is especially
>> >clear  in audio applications...
>> >
>> **  with AC coupling, grid-current flows into the grid bias supply.
>>
>> >You can *say* that an amp is "in" AB1 until it recieves a signal that 
drives
>> >it into grid current, but I don't know of anyone that can control peaks 
that
>> well.
>> >So, ultimately
>> >the difference in how an amp is classified is not where you run it, but
>> >*how* it is designed.
>> >
>> **  so increasing drive will not eventually cause grid current in a well
>> designed AB1 amp?
>
>it can't. If it can then it is an AB2 amp.
>
So an AB1 amp can be AB2 if it is overdriven, and an AB2 amp can be an 
AB1 amp if it underdriven?       

How would you design an AB1 amp that can not be overdriven?

-  Rich..., 805.386.3734, www.vcnet.com/measures.  


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