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Boater Sam · 16th May 2015, 10:32 pm

It is really a monster as you say. The anode of the 1sr audio amp has a high positive DC voltage on it.
The grid of the audio output valve needs a small negative control voltage to amplify.
Any leakage in THAT coupling capacitor puts a positive voltage on the control grid causing over dissipation in the valve, the excessive current ruins the valve if you are lucky. Or it burns out the output transformer and possibly the mains transformer if its your worst day.
If the control grid has a positive voltage relative to the chassis, it is usually the capacitor, but some output valves have a reputation for internal leakage causing the same problem. The UL41 is a case in point.
So even if the set works well, it is always a good idea to check the grid voltage.

16th May 2015, 10:32 pm	#3
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: "That Cap" It is really a monster as you say. The anode of the 1sr audio amp has a high positive DC voltage on it. The grid of the audio output valve needs a small negative control voltage to amplify. Any leakage in THAT coupling capacitor puts a positive voltage on the control grid causing over dissipation in the valve, the excessive current ruins the valve if you are lucky. Or it burns out the output transformer and possibly the mains transformer if its your worst day. If the control grid has a positive voltage relative to the chassis, it is usually the capacitor, but some output valves have a reputation for internal leakage causing the same problem. The UL41 is a case in point. So even if the set works well, it is always a good idea to check the grid voltage.