[Andrew2]

Hi T. No circuit I'm afraid, as it was one of those projects that 'took shape' as it went along.
Basically the main signal from the Wellbrook loop goes via a level control to the resistive combiner at the output.
The 'noise' signal goes to a set of switchable tuned ccts covering 300KHz to 3.8MHz and into a low-gain dual-gate preamp. This section of the circuit enables the phase of the noise signal to be swung relative to the main signal.

Then there comes a simple transistor phase splitter (output switchable from collector and emitter) to 'flip' the phase 180 degrees should it be necessary.
I think the 'noise level' pot is at this point.

Finally an emitter follower shoves the noise signal into the resistive combiner to add/subtract from the main signal.
The input tuned ccts limit this circuit to a max freq of 3.8 MHz, and probably the audio style of design of the phase splitter etc would not work well at HF either.
It certainly works well across the designed range.
Hope this helps!