02-06-2022, 07:27 AM
Hi all,
I thought I'd post a quick summary of the changes to the Chinese 2m amp board I built...
My white coax measured as 25 ohms, lengths worked out to 4 inches
Blue output coax also got quite warm, replaced that with 6 inches of RG400
Input match is completely wrong, you need to use 3 turns instead of 4 turns on the input transformer.
Also all those series caps don't help either. Replaced those with 16pF ATC100b for better match (11pf+5pf in parallel). It doesn't need to be an ATC but I had spare.
The wire supplied for the LPF is 1.2mm and a tad thin for this amp. They get quite hot so I rewound those coils with 2.5mm wire to better handle the current.
The 100pF parallel caps on the output are not optimal. Replaced with 2x30pF caps in parallel in each leg.
Also added a 10pF cap between drains which increased gain and efficiency, actually, its not directly across the drains, its at the input of RG400 - see photos.
Added temperature compensation to the bias circuit be installing a 10kNTC in series with an 80k resistor and putting that in parallel with the 10k resistor in the bias voltage divider.
End result 575W out for 4W in. 15.1A@50v so about 76% efficient at 144.200 MHz
High resolution photos of my build can be found here...
https://photos.app.goo.gl/xgV5319t5iQ9F4iV9
cheers
Tim
I thought I'd post a quick summary of the changes to the Chinese 2m amp board I built...
My white coax measured as 25 ohms, lengths worked out to 4 inches
Blue output coax also got quite warm, replaced that with 6 inches of RG400
Input match is completely wrong, you need to use 3 turns instead of 4 turns on the input transformer.
Also all those series caps don't help either. Replaced those with 16pF ATC100b for better match (11pf+5pf in parallel). It doesn't need to be an ATC but I had spare.
The wire supplied for the LPF is 1.2mm and a tad thin for this amp. They get quite hot so I rewound those coils with 2.5mm wire to better handle the current.
The 100pF parallel caps on the output are not optimal. Replaced with 2x30pF caps in parallel in each leg.
Also added a 10pF cap between drains which increased gain and efficiency, actually, its not directly across the drains, its at the input of RG400 - see photos.
Added temperature compensation to the bias circuit be installing a 10kNTC in series with an 80k resistor and putting that in parallel with the 10k resistor in the bias voltage divider.
End result 575W out for 4W in. 15.1A@50v so about 76% efficient at 144.200 MHz
High resolution photos of my build can be found here...
https://photos.app.goo.gl/xgV5319t5iQ9F4iV9
cheers
Tim