[sdiy] Voltage controlled gain from 4 transistor differential amp?

[Justin Owen]

http://www.sdiy.org/juz/diff_amp_schems_03_03.pdf

I've been looking at lots of differential-amp schematics. My aim is to learn about discrete differential and OTA VCAs. 

My understanding is that by adjusting the current (Iabc) across R1 and into the Q1/Q2 Emitters then I can control the gain out of the Q2/Q4 Collectors - is this correct?

I've tried a few different current source schematics to supply Iabc and eventually, I've got a good clean wave and unity gain out of all of them -  but changing the current always just produces a change in Bias.

Am I missing something or do I need to look at another differential structure like the Gilbert Cell? I would like to try and nail this - if it is possible to do it...

Thanks much,

Justin