Sébastien Bourdeauducq of /tmp/lab visited PWL, a “one-man vacuum tube laboratory” headed up by Aleksander Zawada.
He starts the triode by assembling the grid. To do this, he takes a piece of nickel wire, and soldered a small spiral of molybdenum wire on it – one turn and one solder at a time. He uses molybdenum because of its low emission of free electrons when heated (which causes unwanted grid current in tubes) and its high melting point. Soldering is done with a spot welding machine, which passes high current through the parts to be soldered (nickel and molybdenum wires). The current is so high that the metals heat and melt locally and form a small solder spot. How does one obtain such a high current? Aleksander simply took the transformer of a microwave oven, removed the high voltage secondary, and wound instead a few turns of a thick aluminum bar whose ends are connected to the copper electrodes of the welding machine. The solder current can be controlled by a triac-based dimmer connected in series with the transformer’s primary.