So last issue in the Laney has been solved. It needed the use of an oscilloscope but it got fixed in the end. Cheers to JPF Amps in London for help fault finding this last bit. The problem was that when the Middle pot was turned to 0 (wiper to ground) a rather noisy hum came through the speakers. Quite loud. This turned out to be caused by the impedance of the wire to the Master Volume wiper forming a resonant circuit with the grid capacitance of the gain stage it was fed into.
The solution was a grid stopper. Grid stoppers are awesome. They protect against various nastiness in amplifiers. The SLO uses large value ones around 470Kohm, but they can be as little as 10Kohm. I went for 15K here at the suggestion of Frank at JPF. Problem solved.
Here is an artists impression of a grid stopper preventing bad things happening to the grid of a gain stage.
The original pots I used turned out to be shockingly poor. I used the Bourns pots available from Tubetown.de. TubeTown sell a lot of great stuff and are super reliable and helpful. The Bourns pots are also kind of expensive, but the Alphas are more robust. I prefer CTS in general but they seem hard to locate in certain values. In the end I went right over the top and got hold of some Clarostat mil-spec conductive plastic 2Watt pots. This meant I had to redo the tone stack board... here is version 2!
You can see an extra hole I popped in before I realised I could get away doing it neatly with just 2 turrets and some bus wire. The wiring needs to be tidied up, and some grid stoppers added, and then hopefully this will come gigging with me! Fingers crossed!