Quote:
Originally Posted by PJL
Leaving the grid open circuit is not a good idea as the anode current will run wild. The fix is likely to be short lived as the cause is typically deposits on the inside of the glass. I have had some success using a gas hob spark igniter which is likely to clear a larger area of the conductive deposits, however, a replacement valve would be safer.
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Thanks PJL. I will bear this in mind in the future. I will run the set and see if the hum returns and obtain a replacement UL84. Oddly I did not notice a prominent hum when I tested the UL84 in another set (a Ferranti U1032). Unfortunately that set is on the other side of the Irish sea at the moment so I can't swap valves again.
The valve does have that high hour baked look with a brownish tint and the getter darkened. Generally if the valve seem to work fine I disregard the appearance. I do remember the UL84 in the Ferranti looked newer and there was a slight drop in volume only when the valve from this set was tried in the Ferranti.