[AMRadio] Pi-Net vs Link Couple


John E. Coleman (ARS WA5BXO) wa5bxo2005 at pctechref.com
Tue Sep 27 10:32:17 EDT 2005


I'll beat Don to the punch here.  
	Basically, high level modulation is the process of varying the
supply voltage at an audio rate.  This process works properly as long as the
ratio of plate current to plate voltage remains constant in the final.  The
efficiency of the modulated final must remain constant over the range of
variation, so that the output RF voltage follows the supply voltage.  Class
C finals work best with this type of modulation and yes you can run a
grounded grid final in class C.  It is very complicated to modulate it
however.  The driver stage is part of the output in a GG circuit.  So it
will need to be modulated as well.  The efficiency of a class C final is
very dependent on the drive.  So modulating the driver affects the
efficiency of the final over the modulation cycle.  It would be an extremely
difficult task to get all the levels correct and compensate for the non
linear modulation characteristics caused by having to modulate the driver.  
	GG is best left for linear operations.   

John, WA5BXO 

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of bcarling at cfl.rr.com
Sent: Tuesday, September 27, 2005 7:03 AM
To: amradio at mailman.qth.net
Subject: RE: [AMRadio] Pi-Net vs Link Couple

On 22 Sep 2005 at 20:20, Donald Chester wrote:

> A tube
> with as much grid-plate capacitance as the 450TL or 250TH must be 
> neutralised for proper operation, unless it is grounded grid.  Grounded
> grid 
> is not recommended for plate modulated finals.

For all tubes? Please explain...




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