Does anyone have any impressions of this amplifier?
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Never buy a power amplifier without LPF.
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Yess,now I’m looking for information on a suitable LP filter.to make myself.
You will regularly find old ham radios broken down for sub-assemblies on the worlds favourite junk shop. Here’s a ready to use LPF setup Yaesu FT-840 LPF-Low Pass Filter-Used-Excellent | eBay. The wiring may look confusing at first but there’s RX/TX coaxs and the rest are normally connected one at a time to GND or +ve to switch the filters in for a single band.
Just keep your eyes peeled for the bargains that show up.
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Not exactly your amplifier model, but I once built one of those Chinese kits.
See
I hope it gives you some ideas.
Just one warning, these things have no SWR protection, one error (select wrong filter, forget to connect antenna, etc …) and one or both FET’s will be gone. Don’t ask me how I know 
I still need to make something for my amplifier that detects bad SWR or overcurrent, and cuts off the bias for the FETs.
Luc ON7DQ
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I just made a low pass filter. And with NanoVNA that’s no problem!
I did it for 10m and used 100pF for C1 + C4 and 2x 100pF for C2 + C3. L1, L2 and L3 are wound with 1mm enameled copper wire on an 8mm drill. (Outer diameter 10mm) - 9 turns. Adjusting with the NanoVNA by slightly pulling the coils apart was easy.
I think it will certainly be good for 200 watts. The transmission loss is about 0.4 dB and the SWR is always below 1.5… The material price was less than 15 euros.
73 Armin
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I built this single band LPF for 10m to fit inside a small CB amplifier. Good for 12m too.
(PCB on the right)
I have since changed the connectors to BNC 
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I tried… but I didn’t get enough harmonic attenuation with such a filter… I needed 1 L and 2 C more…
- But if the PA is running in AB mode, it could work.
73 Armin
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Yess, maybe but I don’t know if this will be a problem at 30 watts on the Field.
I will try with two LPF 20-17 and 15-10m
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If you only attenuate 20db on the 2nd harmonic at 14.1 MHz… then you are transmitting on 28.4 MHz with 300mW… On an EFHW for 40/20/15/10m that is currently enough for DX… 
73 Armin
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Yes… i’ll keep that in mind
Thanks
Vasil
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Since the second harmonic does not have the same power as the useful frequency, it is significantly lower than 300 mW after a filter with 20 dB attenuation.
73, Peter - HB9PJT
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You’re right of course… The attenuation of the PA has to be added to that of the harmonic filter…
I’ve seen pictures where the attenuation of the PA was only 10 - 15 dB… it could still be enough with the DX
By the way, here is my attempt at attenuating the harmonics for 10m Band:
Double PI Filter:
Single PI-Filter:
In DL the harmonic must have an attenuation of 60 dB up from 50 MHz… I wouldn’t have achieved that.
73 Armin
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With one of the usual push-pull power stages, the 2nd harmonic is generally strongly attenuated. And the 3rd harmonic by about EDIT 20-30db.
With my Chinese kit I “only” use a 5-pole filter and comply with the German regulations with -60db over 50MHz.
Of course, a 7-pole filter is the better choice if you cannot measure the harmonics directly.
But I have to mention that my kit included a high-quality MRF 9120 RF powertransistor pair apparently recycled from Chinese GSM stations.
And not the usual IRF5xx switching transistors.
73 Chris
Measurement of harmonics at 40Watt out at 28Mhz and 60db attenuator
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