The trace over at the FSB forum was made be me. I found that there are different versions of the Erupter. Some have an additional Volume-Trimmer to set the output volume and also some seem to have 2 bias trimmers additional to the Bias pot. So you can include an additional volume control if you like. It should be a 100k.
Edit: I'm not sure what the notes about the transistor hfe really means. Does it mean, they should both have about the same hfe in the range of 60-120 with Q3 being the higher? Or does it mean Q2 should have around 60 and Q3 should have around 160 hfe?
Built this yesterday and it's quite picky abaout transistors. A 2N2369 (hfe 62) for Q2 and a 2N1711 (hfe 141) for Q3 worked best for me. Did an equivalent search at alltransistors on 2N497 and ended up with a lot of odd part numbers. LINK
I guess the motto "socket and see" should be used here.