Just a note on using the BITX40 with an external RF amp.
This is a reminder that the 2nd Harmonic output on a stock BITX40v3 is generally pretty high (I believe that someone measured at only around 20dB down from the fundamental signal).
If you are feeding this into a 100w or greater amp you probably should consider fixing this issue at the source (i.e. the Bitx) and not just rely on the output filtering of the amp.
The simple fix, as documented in message #22366 of this group, suggests putting a 100pF or 110 pF NP0 (C0G) capacitor across L7. A 100 v capacitor is fine if you are running your PA off
of 12V. More voltage in to the PA would require that you use a capacitor rated at 500v.