I just finished a 5F1 built from my scrap pile. It has four significant mods: 1. choke replacing the resistor between B+1 and B+2 2. old brown base Tungsol 5881 with 300K cathode resistor rather than stock 470K 3. full bridge rectifier to get a bit more wallop from the tiny PT 4. 10" eminence Lil Buddy Patriot speaker (but I may go with a Jensen alnico I got lying around). Results are good volume and lots of headroom, up to about 10 on the dial. Above that it breaks up nicely. I am now playing around with NFB resistor. Stock 22K is too quiet, and no NFB, while the tone and volume are good, limits range. Probably 56K will be the sweet spot. I will put a semi-closed back on it to boost the bass a bit, and likely a 100pf bright cap across volume pot to keep the highs sharp. While no longer truly a 'Champ', indeed, sounds a bit like a Vox, it has lots of singled-ended class A chime. I recommend these mods. And now I convert the 5E3 to 5F10 with choke, fixed bias, NFB, bigger PT, paralleled 12AX7, and 5881s. Wish me luck as I dive into the aroma of solder smoke.