[AMRadio] Pi-Net vs Link Couple


John E. Coleman (ARS WA5BXO) wa5bxo2005 at pctechref.com
Wed Sep 28 18:12:57 EDT 2005


Let me correct my spelling and grammar before Jeff "OMR" does.
"Want" and "won't" are not the same word. 
I wish I had a proof reader.

Hi Don:
	At first it would seem that modulating the driver in phase with the
final might correct the condition you are speaking of with GG configuration.
But I was also concerned with the fact that the efficiency changes with
drive in a class C circuit.  So when the driver is modulated how will this
affect the modulation linearity?  When the final is modulated won't the
driver see a varying load and won't this be reflected back to the modulator
as a varying load and won't all this have to be compensated for while
maintaining phase. 
	Like you, I just don't think that GG is a thing to do with high
level modulation.

John

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Donald Chester
Sent: Wednesday, September 28, 2005 4:35 PM
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...
>

First of all, you would have to run the amplifier class C for plate 
modulation.  Class C triodes already require a lot of grid drive, usually 
about 10% of the expected power out.

In addition, some of the driving power in grounded grid service feeds 
through to the output, since the input and output circuits share a common 
impedance at the cathode.  So a grounded grid class C circuit would require 
still more than the nominal 10% driving power.

Additionally, that driving power is present in the output at all times, even

with the plate voltage turned off.  This is somewhat like a backwave on a cw

signal. Therefore, it would be impossible to achieve 100% modulation of the 
carrier, since even with the plate voltage of the tube driven negative on 
the negative modulation peak, there is still some rf output present.

Don k4kyv


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