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[AMPS] More t'bolt questions

To: <amps@contesting.com>
Subject: [AMPS] More t'bolt questions
From: 2@vc.net (2)
Date: Thu, 8 Nov 2001 11:35:48 -0800
>At 06:56 AM 11/7/01 -0800, 2 wrote:
>
>> >Rich says:
>> >
>> >>//  I have never heard of a tetrode that is more linear when grid current
>> >>flows than when  no grid current flows.
>> >
>> >Pappenfus, Bruene, Schoenike, Single Sideband Fundamentals and Circuits,
>> >McGraw Hill 1964. Sec 13-3, P191 et seq. 'A small amount of grid current 
>> can
>> >sometimes
>>
>>//     (N.B.)
>>
>> >be used to cancel plate current distortion'. Also explains how biasing for
>> >AB2 can put the characteristics on a more linear part of the palte current
>>curve.
>> >Explains why the IMD characteristics don't follow the classic third order
>> >intercept graph, but show a dip. Happens with all tetrodes I've ever tried
>> >
>>//  Why doesn't Eimac recommend AB2 for the plethora of tetrodes other
>>than the 4cx600 and 4cx1500B?  Why did the tetrode amplifiers that I
>>built become less linear with grid current?
>
>    They are designed for best linearity with  no grid current. And you 
perhaps did
>not build an amp with tubes that are designed for grid current 

Eimac makes no tetrodes with handles for AB2.

>or you drove them past the point of non-linearity.

Which, from looking at the characteristic curves. is rather obviously 
grid currents much above 1mA for the 75w-rated grid.

>    It is a lot easier to design linear systems that require no grid 
>current because you do have a simple means of determining the drive limit. 
When grid 
>current is drawn reduce drive.

It's usually easier to set grid bias voltage on the verge of grid 
current, and then set ZSAC by tweaking the screen V adjustment.  

>Also, by use of large grid resistor (in DC path) you can easily limit grid 
>power thus preventing
>damage to the tube.

True, however, tube damage is very rarely a problem in grid driven config.

>         There are a number of tetrodes that are linear while driving the 
>grid positive.  Look at a classic... the 6166A. It is clearly designed to be 
used in a 
>linear amplifier  and the grid driven positive.
>         But today it is much easier to design an amp that requires zero 
>grid power and that is why tubes like this are not used now.
>         
cheers, Mr. W.  

-  R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.  
end


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