7) Some things on the to-do list
So far I've run into some problems that probably need to be solved for me to be able to use this amp with my 7W BITX40 to get 20 - 50W SSB.
Get a more capable power supply. The one I ordered is on its way.
Output low pass filter. I have one built with T37-6 toroids, but I would like to have one with larger T50-6 toroids instead. I need to order suitable capacitors.
Lower feedback resistors. As usual, F5NPV provides great advice. The kit comes with 150 ohms, but I want to try with 100 ohms or 47 ohms to get better linearity (at the cost of lower output power).
Add source degeneration resistor and capacitor? Another way to add negative feedback to get better linearity, I think. Try what F5NPV and NA5Y did.
Add temperature compensation. I don't know exactly what to do here. I like a different bias circuit that I saw in one a video from Charlie Morris ZL2CTM. It had a current source and two diodes for each MOSFET.
Make a power attenuator for use on the input. This requires me to first figure out the suitable input power to drive the amp.
Wire up the fan. The heatsink comes with a fan. I just haven't used it yet.
Add temperature switches. I have 40 °C NO, 100 °C NC, and 120 °C NC temperature switched. The idea is to used them to turn on the fan (when above 40 °C) and kill the bias (when above 100 °C or 120 °C). I don't know how to fit them to the heatsink at a proper place yet, though.
Fix the BFO / crystal filter alignment on the BITX40. Currently, the high side of the passband seems to only be about 1.5 kHz instead of somewhere like 2.5 - 3.0 kHz. This is a know problem and I know of ways to fix it. (Add tuning capacitor to the BFO, or disable the BFO and use an output from the Si5351 instead.)
Rasmus SA5RJS /
npub133…hfazx
2025-01-19 22:56:48
in reply to nevent1q…m4nq
Author Public Key
npub133kx3vtsccvrn4uxjn85wacm45uqglre8qz7z7tr86p6lhy0whsqmhfazxPublished at
2025-01-19 22:56:48Event JSON
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"content": "7) Some things on the to-do list\n\nSo far I've run into some problems that probably need to be solved for me to be able to use this amp with my 7W BITX40 to get 20 - 50W SSB.\n\nGet a more capable power supply. The one I ordered is on its way.\n\nOutput low pass filter. I have one built with T37-6 toroids, but I would like to have one with larger T50-6 toroids instead. I need to order suitable capacitors.\n\nLower feedback resistors. As usual, F5NPV provides great advice. The kit comes with 150 ohms, but I want to try with 100 ohms or 47 ohms to get better linearity (at the cost of lower output power).\n\nAdd source degeneration resistor and capacitor? Another way to add negative feedback to get better linearity, I think. Try what F5NPV and NA5Y did.\n\nAdd temperature compensation. I don't know exactly what to do here. I like a different bias circuit that I saw in one a video from Charlie Morris ZL2CTM. It had a current source and two diodes for each MOSFET.\n\nMake a power attenuator for use on the input. This requires me to first figure out the suitable input power to drive the amp.\n\nWire up the fan. The heatsink comes with a fan. I just haven't used it yet.\n\nAdd temperature switches. I have 40 °C NO, 100 °C NC, and 120 °C NC temperature switched. The idea is to used them to turn on the fan (when above 40 °C) and kill the bias (when above 100 °C or 120 °C). I don't know how to fit them to the heatsink at a proper place yet, though.\n\nFix the BFO / crystal filter alignment on the BITX40. Currently, the high side of the passband seems to only be about 1.5 kHz instead of somewhere like 2.5 - 3.0 kHz. This is a know problem and I know of ways to fix it. (Add tuning capacitor to the BFO, or disable the BFO and use an output from the Si5351 instead.)",
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