Help!! Mu-Doubler debug?

[R.G.]

if the resulting even harmonics are low & therefore need a lot of gain to get up to a useable level.....won't the SN ratio be poor? (I'm thinking hiss here?)

Maybe. Requires careful design and attention to noise. It wasn't too bad on my proto.

Presumably you get similar even harmonic distortion happening through a JFET type opapmp?

No. Well, maybe it's inside there somewhere, but there isn't any practical way I can think of to get it out. A JFET opamp has a JFET diffamp at the front all right, but the followup gain is bipolar and has its own distortions. On top of that, the following gain is so high that the opamp must be used with feedback for all practical purposes, so it covers up the distortion you were trying to isolate.  It's what opamps were designed to do - hide the internal imperfections.