[Amps] RE: 4CX250B Screen Supplies

[R.Measures]

On Jul 22, 2004, at 7:30 AM, DEHAYS Dominique wrote:

> The zener chain or the OA2-OB2 VR tubes must be considered as "antique"
> technology. We hams have one duty : make our transmitted signal as 
> clean
> as possible , not only for the harmonic content , but also to enable
> others to work on the band at the same time we are on it. Just have a 
> look
> at:
> http://www.ifwtech.co.uk/g3sek/cleansig/imd1.htm
> There are some IMD curves drawn for a 2*4CX250R amp , with modern 
> screen
> regulator , and zener chain regulator , you 'll see it simply makes 
> 10db
> degradation on IMD3 . You will notice too , that the amplifier tested 
> was
> running 500w for a pair of 4CX250R , imagine what is the problem when 
> some
> guys are running the same kind of amp at 1Kw out....

Dom --  I also noticed that a two-tone test was used to measure IMD.  A 
voice-test is a better method of measuring IMD since it causes the 
screen-I to vary and the two-tone test produces constant screen-I.   
Other problems with the comparative tests at
> http://www.ifwtech.co.uk/g3sek/cleansig/imd1.htm
was that no comparative dynamic load regulation test was performed on 
any of the screen regulators, and we have no way of knowing how the 
loading was adjusted regarding screen-I before each measurement.  (i. 
e., AB1 tetrode or pentode loading is a major factor here since more 
screen-I = more IMD).
   Also, with a 2-tone test, it would seem not to matter whether the 
screen-V is regulated or unregulated since screen-I is constant and 
therefore screen-V would not vary unless the electric-mains V 
fluctuated.
end

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org