Upgrade-45-W-SSB-linear-Power-Amplifier-Set-for-Transceiver-Radio-HF-FM-CW-HAM

On ebay I found a 45 watt amplifier kit. It was really cheap so I bought it.
I will describe my findings here

The specifications looked good. 45 watt or more on the lower HF bands and still 25Watt at 10 meter.
I was skeptical about this. It uses 2 IRF-530 Fets. On internet I did not find an IRF-530 base amplifier that was able produce that output on 10 meter.

It was hard to get the manual. It should be supplied by email but I did not get it. The seller tries to resend but I did not receive it. I could find it on the internet on a Chinese site. It was an older manual.
It did not match the PCB. Still the schematic was simple enough so I was able to build it.
The manual is written in Chinese English. Probably translated by google translate or something similar. It is hard to read

After building it worked poorly. It worked on 80 meter and power was 5 watt. All tests were executed using a dummy load and a signal generator.

The schematic comes from a more recent version of the manual I found later.

R12 and R7 should be 5.6 Ohm. I had 100 Ohm mounted. No R6 and no R20 on my board.
After fixing this the results were better. But it did not match the specs.

Band    Power out
80        50 watt
60        50 watt
40        50 watt
30        35 watt
20        25 watt
17        15 watt
15        12 watt
12        5  watt
10        1  watt
DC was 13.8 V measured at the amplifier.

These levels are for CW only. For SSB levels should be 20 watt on the low bands. Then the 3th order intermodulation signals are 24 db down to the wanted signals. That is not really good but probably acceptable. Adjusting bias current may improve it. I intend to use it for CW and RTTY so For me it is OK.

I did some experiments with T2 to get more output. I used an additional secondary turn. That did work. I could get 25 Watt at 15 meter and 5 watt on 10 meter. Which is still not according to spec. Unfortunately it made the amplifier instable. So I went back to the single secondary turn.
The manual seems to contradict itself about the turns on the transformer. I use 4 turns primary and 1 secondary. Somewhere the manual mentioned 6  turns primary and 2 secondary. I did not try this.

On the dummyload the amplifier was stable. I needed to test what it would do when connected to an antenna. So I used a antenna tuner and a dummy load. I tuned the dummyload. This enables me to simulate several mismatches. It was a disaster. at SWR > 3 it was instable at all bands.

I was able to cure it by using an 36 Resistor at the input and a 500 Ohm resistor at the output.
Now it is quite stable. At very high SWR  at some bands some high frequency signals are seen. The lowpass filter should be able to remove this. I have to try.

So far the conclusion is that the amplifier is usable for CW  up to 15 meter. But don't expect the specified output at higher bands.

A warning. The power supply needs a current limitation as the bias current adjustment is very difficult. RV1 and RV2 are touchy  a little bit extra can cause an increase ofthe idle current of several Amperes. Better replace the trimmers by 10 turns trimmers.