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Re: [Amps] Pi L networks

To: "James Joyce" <jjjoyce@optonline.net>, <amps@contesting.com>
Subject: Re: [Amps] Pi L networks
From: "Tom W8JI" <w8ji@w8ji.com>
Date: Thu, 29 Mar 2007 22:32:21 -0400
List-post: <mailto:amps@contesting.com>
> Hi Joe, I started it when I asked the question about the 
> use of a 4to1
> torroid step down transformer in place of a coil and 
> switch assembly for
> the Pi-L. Many of the commercial ham amps use this method.

Jim,

These threads seem to take off into all kinds of tangents, 
but here is what I was saying before we got lost on talking 
about what we think happens in commercial amps.

1.) I've prototyped and measured harmonics to see that the 
amp meets TA for at least two dozen amps, probably more.

2.) There was a statement an amp couldn't or likely wouldn't 
meet TA because it takes a pi-L to get enough harmonic 
suppression.

3.) I know that is wrong, because I have measured dozens of 
amps that pass by a wide margin that don't use pi-L's. I 
have no doubt at all that Commander and QRO can exceed TA 
limits by a very comfortable margin without a pi-L.

Let me put some hard numbers on this people can look at, 
rather than all the wild speculation. We had a claim an HF 
AB linear could barely, if at all, pass FCC TA with a pi 
network.

Look at QST's review of the AL800H. It uses a conventional 
Pi network on 40 and up and a pi-L on 160 and 80. The ARRL 
measured 2nd harmonics and worse case harmonics of:

160m   -61 2nd and -56 worse case on the 8th harmonic (this 
is with pi-L)

80m   -50 2nd (which is also worse case)  and this is a pi-L

40m   -59 2nd and worse case also is the 2nd....and here it 
is a pi. No L section!

So we can all see by looking at published data we can run on 
and on about what happens but when we actually look at an 
actual amplifier we suddenly learn layout and stray 
impedances have much more to do with harmonics than throwing 
an extra coil in the circuit.

If we look at a number of amplifiers that do NOT use a pi-L 
we can find 60dB of harmonic attenuation or more. I have an 
AB2 class triode homebrew amp running next to me right now 
using vacuum caps that does much better than -60dB for any 
harmonic on any band. So the claim we need a pi-L is easily 
proven wrong, and not just by what I guess is happening.

4.) In at least a half dozen amps when I needed more 
suppression, adding a L section and using a reasonable 
center impedance (say 200 ohms) didn't help any significant 
amount and actually increased some higher order  harmonics. 
Now it is true I could have shielded the tube from the tank 
and the pi from the L and probably not increased the higher 
harmonics, but that wasn't a reasonable solution when a 
change of some lead lengths caused everything to fall into 
spec for no cost increase at all!


I can show data from a dozen amps or more where a pi easily 
meets all requirements, and I can probably show data (if I 
still have the engineering test files) where adding the L 
section made things worse on some harmonics.

On the other side of the coin we seem to have people who 
"believe" through some blind faith that a pi-L always makes 
things better and always is required.

So from actually having measured many different amps in all 
configurations.... my position is a pi-L might help. It 
also, if the layout doesn't fully isolate or shield the L 
section from sources of harmonics as most amateur layouts do 
not, it might make some harmonics significantly worse. It 
also is a fact that what we do on a dummy load is actually 
less repeatable with a real antenna when we use a pi-L. All 
it takes is the antenna system go high impedance and 
capacitive and by adding the L section, if center impedance 
is the traditional few hundred ohms, the second harmonic can 
go way up in level.

73 Tom 


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