[Amps] Triode vs Tetrode Amp

[TexasRF at aol.com]

Hi Keith, many, if not most,  engineering decisions are $$ based. A  source 
of cheap 4CX800 tubes from Russia could cause a company to use them  instead of 
expensive 3CX800 tubes in an amplifier design for example.
 
The tetrode tube has to be cheap enough that the cost of the  screen supply 
and tube socket has to be more than covered by the cost  savings.
 
Performance wise, the cathode driven triode will typically  have lower imd 
than the grid driven tetrode (due to the heavy negative  feedback). 
 
Where the tetrode has an advantage is in the rare case that  one wants to use 
very low drive power, such as a few watts from a QRP  transmitter. The input 
grid circuit can be designed to allow full output power  with very low drive 
power levels. The FCC will not bless a commercial amplifier  like this as the 
rules don't allow such a design.
 
73,
Gerald K5GW
 
 
 
 
In a message dated 4/26/2007 11:16:22 A.M. Central Daylight Time,  
kdutson at sbcglobal.net writes:

Something that has been bugging me...

Why do some designers use  a triode versus a tetrode for linear amplifier
finals?  A perfect  example is the use of a pair of 3CX800's compared to a
pair of  4CX800's.

Thanks and 73,
Keith  NM5G

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