[AMRadio] Pi-Net vs Link Couple


Jim candela jcandela at prodigy.net
Wed Sep 28 07:51:41 EDT 2005



John has said:

"GG is best left for linear operations"

Why is that John? With G-G Class C plate modulated we get around the
neutralization issue, and can use a pi-net instead of link coupled, and
because of the feed through power from the driver, these cannot be over
modulated causing "buck-shot" from chopping up the carrier with too much
audio. Are these negatives? Sure, in order to modulate the output 100%
negative, you need to modulate the driver as well at a reduced percentage
and in phase with the final. This extra complication isn't much different
than plate and screen modulating a big tetrode RF amp. I think the idea is
worth a second look to at least ponder the pros and cons.

I still like my GG linear with 2 X 304TL idea. We don't need a big modulator
here, and it acts as a legal limit brick between a modulated driver such as
a DX-100, Viking I or II, 32V, etc. and the antenna. Sure the efficiency is
low, but no modulator is required, or attenuator between the driver and
linear amp. Nobody commented on this so I assume the idea was viewed with
skepticism. Here is that post again, in case you want a second look:


 Hi all,

   I have a different twist on this wonderful topic. Years ago I was going
over the very issue, and I had a Viking I transmitter, and I wanted to run
the legal limit of about 375 watts carrier 100% modulated am. So that would
be 6 db to go from say 100 watts to 400 watts. I went through a lot of
charts, curves, and calculator work, and I worked out a design. Of course I
lost my notes, so this description is from memory.

   What I came up with was a parallel grounded grid linear using low mu
304TL's with a tuned filament choke input, and pi network output. Using
grounded grid should reduce, or eliminate the need to neutralize, well at
least from 160-40 meters that is. With no neutralizing circuitry, things get
a lot simpler. A 3 band pi network on the output is doable, as is resonating
the high current filament choke on multiple bands.

   So I was guessing a little on the gain, but 6 db was the target, and a
low mu 304 TL is going to have low gain in G-G mode. Since class B RF linear
amplifiers on AM get about 33% efficiency (no modulation), I figured 900
watts DC input would provide 300 watts RF output. A rough guess is the drive
would be 100 watts from the Viking I, 25 of which would be lost to drive the
304TL's, and the remaining 75 would feed through to the output (normal
characteristic for a G-G amplifier). So the output would be 300 + 75 = 375
watts.

So in summary:

exciter output 100 w

304TL drive power 25 w

final dc input 900 w

final amp % eff (un-mod) 33%

feed through power from exciter 75 w

rf output = (900 * .33) + 75 = 375 watts

304 TL Pd each tube = 300 watts

So with 100 watts at 100% driver modulation, 400 watts PEP drive provides
1500 watts PEP RF output.

  So fellows, here is a big rig running AM legal power with big triodes,
pi-net output, NO neutralizing circuit, with NO modulator, band switched,
and driven by a Viking I or other 100 watt class AM rig. I never built the
beast.

Do you folks think this would work?


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of John E. Coleman (ARS
WA5BXO)
Sent: Tuesday, September 27, 2005 9:32 AM
To: 'Discussion of AM Radio'
Subject: RE: [AMRadio] Pi-Net vs Link Couple


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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