The VLF up-converter on my website (used to receive SAQ last year) has a new box and had a tidy-up! It works well and is sensitive from around 2kHz up to 200kHz. It should be useful on 137kHz receive. The actual circuit is built ugly style on a small piece of copper clad PCB material and this is about half the size of the 9V battery used to power the converter. The circuit uses a "back to front" SBL1 mixer with a 2N3904 oscillator and post mixer buffer.
The original lash-up received SAQ at RS58 and a QSL card was received for this historic 17.2kHz CW transmission from Grimeton in Sweden.
The various time signals (on 50, 60, 75 and 77.5kHz) are audible as are the Russian Alpha beacons below 15kHz. There are various unidentified data transmissions from 17kHz to around 80kHz audible but not identified.