>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.
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