Hi Paul,

>I already have plans to build the big T type LPF. 

Out of curiosity I've done some spice simulations of your PA.
They indicate that a T type LPF won't help at all.
This was to be expected because your PA is not a class D voltage mode
push pull PA like W1VD's or IK1HSS's or 2E0ILY 's.
Indeed the T type LPF will cause considerably higher voltage spikes at the 
FET'S drain.
In the attached screenshot you can see three simulations,each with a different 
output filter.
A shunt C LPF,a series L LPF and even a diplexer filter.
The red traces represent voltage at the drain,green is DS current.
With the diplexer voltage spikes are lowest.
But I obviously the filter is not your problem.
It seems that the near field of the antenna is causing the trouble.
I would suggest to try a second isolation transformer or a good 
common mode choke *at the shack* end of your coax run instead of building a 
T-type LPF.

P.S. Has your Ultimate 3S a LPF installed before you feed it into the PA?

73
Clemens
DL4RAJ 

>-----Original Message-----
>From: [email protected] 
>[mailto:[email protected]] On Behalf Of N1BUG
>Sent: Monday, December 25, 2017 1:15 PM
>To: [email protected]
>Subject: Re: R: Re: LF: TXing 2200m WSPR
>
>Hi Marco,
>
>I don't detect any heat in the LPF but at this low power level it 
>could be difficult to detect it.
>
>I already have plans to build the big T type LPF. I have the 
>T-225A-2 cores and metal to make a compartmented box. Capacitors are 
>on the way, could arrive around 29 December. I still need to order 
>the wire and some kind of material for mounting the big inductors.
>
>I'm making that LPF because eventually I will also build the big 
>amplifier. I'm not sure when that will happen, probably a bit late 
>for this LF season.
>
>The most curious thing is what seems to be some measurement error or 
>impedance transformation but the cause remains elusive. I hope to 
>find the reason so I can be more confident about measurements when I 
>go go high power.
>
>73,
>Paul N1BUG
>
>
>On 12/25/2017 04:35 AM, [email protected] wrote:
>> Hello everybody,
>> 
>> it's ever interesting to read that somebody else has some troubles as
>> experienced from myself ;-)
>> Me too am grateful to Andy for the support I received
>> while struggling with odd experiences..
>> Let me join the discussion and share a possible solution: I see that
>> LPF is a PI version, similar to what I used in the past. It 
>was nice in
>> RL, armonic suppression and losses when tested with low power
>> instruments, than I was increasing the wire diameter of coils because
>> it was a... warming thing! The current was indeed very high 
>in the FETs
>> (2+2 FETs in push pull) and the RIP area was growing...
>> After I tried the T LPF (see attachment) no more problems, no more
>> warming boxes..
>> You could re-arrange your LPF components for a quick trial and save
>> some caps ;-)
>> In my version I had to make some change because the cores were not
>> available and used T200-A2 cores calculating the turns to get 59 and
>> 109 uH coils.
>> 
>> Maybe it's worth to try ;-)
>> 
>> CU on 137kHz
>> 
>> Marco, IK1HSS
>
>
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1237kHz-PA.png
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