Thanks chaps. I've found the basic relaxation osc circuit and a few other bits online for PUT's, but nothing specific for the BRY39. The 2N6027 comes up in searches for PUT's but this has no cathode gate. In the example for the 2N6027 here -
http://www.circuitstoday.com/programmable-ujt there is an unused P region, does this mean the 39 has an extra N region which is the cathode gate?
They're programmable as mentioned by a couple of resistors a bit like setting variable zener's.
A few term's come up in relation to the operation of the 39 which I'm struggling with. I presume negative resistance isn't the same as current gain in that it's linear or almost linear. Still reading up and searching for info so hopefully things will become clearer.
Andy.