[AMRadio] Pi-Net vs Link Couple


ronnie.hull ronnie.hull at glowbugs.com
Thu Sep 22 07:53:13 EDT 2005


Mike I'm the fellow that is going to build up a Class C final using a 450TL. 
I would love to get a copy of that schematic for the 450TH rig in the Editors 
and Engineers handbook. I have a few of those, but apparently, not that one.

I have a very nice B&W 850A or 852 that I can use in this rig. Either should 
handle that tube fine, considering I'm not going to run much more than 2500 
volts on it anyway. Maybe 3000.

I just got the worled biggest honker daddy plate transformer from W5OMR. 
8200V center tapped ( 4100 - 0 - 4100 ) at 4.5KVA !!!  Yeah suh, takes 3 men 
and a dolly to move it!!

This will be a fun project.

73's & batten down the hatches, here comes Rita..

W5SUM


---------- Original Message -----------
From: "Mike Dorworth,K4XM" <k4xm at arczip.com>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Wed, 21 Sep 2005 23:10:59 -0400
Subject: Re: [AMRadio] Pi-Net vs Link Couple

> The Pi-Net will give a total of 50 db suppresion of harmonics. about 
> 20 in one spot and 30 in the other. Link couple can pass harmonics 
> to VHF by capacity coupling, hence the Faraday Shield Links used for 
> same later on. There is a nice single 450th Pi-Net rig shown in the 
> Editor and Engineers handbook. Also a couple of single ended ones in 
> the 1950 ARRL Handbook (for triodes).  Some triodes that require 
> lots and lots or drive can unbalance the grid tank, which is 
> required for triodes using Pi-Net. Lo capacity tubes like the 450th 
> is OK. The old timers mostly used tuners (antenna) and open wire 
> feeders to keep the harmonics down. Hazletine link neutralization 
> can also be used and no split tanks are needed in or out. Remember 
> Class C , which is required for Hi level AM, is a extreme distortion 
>  and harmonic generator so that some plan need to be in place to 
> handle the soup. Also a single band dipole is very frequency 
> selective and cuts way down on harmonics by itself. Multiband 
> dipoles, beams and multi dipole on one feeder and traps etc (G5RV) 
> are an open invitation to spread gook with only link output. Also 
> the guys that use CB lin years with no half wave filters get away in 
> mobile service without too many problems  due to the narrow 
> frequency discrimination of mobile antennas. Hope this helps, 73 Mike
> 
> ----- Original Message ----- 
> From: "Geoff" <w5omr at satx.rr.com>
> To: "Discussion of AM Radio" <amradio at mailman.qth.net>
> Sent: Wednesday, September 21, 2005 10:45 PM
> Subject: [AMRadio] Pi-Net vs Link Couple
> 
> > SO, here we are, well past 1991 and the 'law' that went into effect
> > saying that 1,500w PEP output is the maximum RF Power output that we
> > hams can run, regarldless of mode.  That doesn't deter the homebrewing
> > spirit, but it does suggest that acheiving 1,500w PEP output is much
> > easier than producing 1kW DC input to the final.  With the mindset of
> > still wanting to use the classic high-level plate modulation scheme,
> > engineering a rig to use only one tube in the final (a 4-250, 250TH,
> > 304TH/TL, 4-400, etc), modulated by a pair seems to make more common
> > sense.  That, and it's a bit more economic in filament requirements.
> >
> > I've heard recently that matching the output of the Class C
> > pate-modulated final to the antenna is better, and more efficiently
> > achieved by link coupling, vs Pi-Net.  On the other hand, it's argued
> > that Pi-Net coupling produces less RFI than link coupling does.
> >
> > So, which is better?
> >
> > Why?
> >
> > What are the effects of nuetralizing a single tube in a balanced tank
> > circuit?  If Pi-Net is to be used, does the final tube still need to be
> > nuetralized?
> >
> > I know of a guy who wants to build a rig using a single 450TL in the
> > final, modulated by a pair.  He wants to pi-net the output, but I've
> > heard that's a bad idea.
> >
> > I want to build a rig using a medium powered tride, perhaps a 250TH,
> > modulated by a pair of 811's.  Pi-Net, or Link Couple?
> >
> > I like seeing this kind of technical discussion on the list.  I'm
> > looking forward to all inputs.
> >
> > --
> > 73 = Best Regards,
> > -Geoff/W5OMR
> >
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> 
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------- End of Original Message -------




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