[Amps] Re: AB1 v. AB2

[Rich]

>amps-request@contesting.com wrote:
>
>>
>>
>> Message: 8
>> To: amps@contesting.com
>> Reply-To: ad5gb@myway.com
>> From: "ad5gb" <ad5gb@myway.com>
>> Cc:
>> 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 .  

>> I guess I understand why this is but don't quite understand why AB2 
>>grid-driven service gives poor linearity yet AB2 cathode-driven service is 
>>acceptable.
>
>Not sure that it does.
>
>>
>>
>> I decided to try grid driven service because it simplifies bandswitching ( 
>given the parts in my junkbox ) and in passive configuration, makes for 
>construction simplicity ( no tuned input needed ).
>
>Actually, it's not any different. Plenty of amps have been built without a 
>tuned input. Of course, a tuned
>input is a good idea for best operation. Assuming something like a xcvr 
>with a tuner built in it might not
>make all that much difference, since the built in tuner acts quite a bit 
>like the tuned input stage in the amp,
>maybe even better since it is likely higher Q. The amp input is lower Q in 
>order to get enough bandwidth.
>
>I guess you can use a smaller switch at the input - but that's not a 
>terribly good reason to base an
>amp design upon.
>
>>
>>
>> Again.... many thanks to all and Happy Holidays
>>
>> RD
>>
>
>Of course, I could be wrong...  :- )
>
>        _-_-bear


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