When I was building my Nelson Pass F5 power amplifier, a Friend (Dave H) was building his F3 at the same time. This is the result, using the same Italian Made case as the F5. The F3 is again a class A design like the F5 but that is where the similarity ends, the F3 being 15W single ended were-as the F5 is 25W push pull. The F3 output is Capacitor coupled since it uses a single rail power supply were-as the F5 needs a split rail supply. Also due to the F3 lower power output it better suits an easier load were-as the F5 is a little more forgiving.
Dave managed to blag some very nice Aerovox caps for nothing were-as I originally used snap-in Panasonic caps in the standard power supply PCB with some additional low ESR Philips. The AeroVox caps should be better if you can afford them new or find some cheap on eBay. You may also notice the F3 has a B1 buffer PCB fitted on the rear panel. The B1 has unity gain and acts as an impedance matcher making driving the F3 much easier for any external pre-amp. I used a B1 buffer in a separate case with a volume control so it becomes the pre-amp itself and not just the buffer stage.
The F3 and F5 have a similar signature sound both being class A. Like the F5, the F3 is smooth and free from grain but the F3 sounds a little more 'rounded' were-as the F5 is on the warmer side of neutral and a little faster sounding.