-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
Converting a fatboy
- Thread starter kdrury70
- Start date
Just sever all connections from vr to the input match of the amp reconnect the coax from the relay to the input of the amp and if the input match is close then you're fine and you can use the radio's vr for input power.
You might want to recheck the input match for the amp by installing an power meter between the amp and radio.
Here's a calculator to figure input swr.
You might want to recheck the input match for the amp by installing an power meter between the amp and radio.
Here's a calculator to figure input swr.
There is no input matching circuit on a Fatboy. It's just padded with the cap and resistors. A simple PI type input circuit could be added to this amp by wrapping 4 or 5 turns of #16 or #18 enameled wire around a Crazy glue bottle and installing a 464 trimmer capacitor from each end of the coil to ground. Feed RF into one end of the coil and drive the input transformer directly off the other side of the coil. That means removing the original 100 pf cap and 4.7 ohm resistor. Use an SWR meter between the radio and amp to set the two 464 trimmers for minimum SWR. Using 5 turns often allows us to spread the coil to help find the perfect match.
This circuit not only provides a better match between the radio and amplifier, it reduces 2nd and 3rd order harmonics too. A similar circuit could be used to clean the output up by using a larger gauge wire for the coil providing a higher Q factor and using caps able to handle the higher RF output voltage. Placing another coil in series with a cap to ground might add enough poles to the output filter to bring the amp within FCC specifications for second and third order harmonics. Other then the Q factor of the coil used and number of poles, there is very little difference in the design of this matching network and a harmonic filter. To cover the relatively narrow bandwidth of CB, these circuits can usually be high Q with excellent harmonic rejection.
This circuit not only provides a better match between the radio and amplifier, it reduces 2nd and 3rd order harmonics too. A similar circuit could be used to clean the output up by using a larger gauge wire for the coil providing a higher Q factor and using caps able to handle the higher RF output voltage. Placing another coil in series with a cap to ground might add enough poles to the output filter to bring the amp within FCC specifications for second and third order harmonics. Other then the Q factor of the coil used and number of poles, there is very little difference in the design of this matching network and a harmonic filter. To cover the relatively narrow bandwidth of CB, these circuits can usually be high Q with excellent harmonic rejection.
I used a 5/8 form and wrapped #12 gauge wire around it and found 5 turns with 1/8 inch spacing works good. I tried this with a 2 pill and seems to work great. Didn't have the smaller gauge, otherwise would have tried that.