You will run across matches that require very little capacitance.
Ideally and if you can afford it:
A.) place all cap. switching as close to rear "panel" and connectors as possible and make open wire runs to it as short, direct and located away from adjacent ground as possible. ( your breadboard lash up takes care of the cabinet capacity.
)
Your dpdt switch may now have to have a shaft extension or equivalent to operate it from the "front" panel. Even if you go with a breadboard, make up a front panel, Bakelite or wood for convenient mounting and dial indexing of components. ..your turns counters, heh, heh. --May not be idiot lights but nice to know where you are.
B.) exchange your Johnson 50 to 500 cap. for a vacuum variable of much less minimum capacitance.
Ask Alpha. They're modifying their latest creation on the fly as they run into these problems, particularly on the higher freqs.
Edited, added.
http://archive.constantcontact.com/fs163/1102056358212/archive/1116792808481.htmlWill be one heck of a c/l/c net tuner.
