tweak fusing valves.

Posted on Feb 6, 2014

One of the EL34 output valves had blown, the normally silver coloured getter had turned white completely. When I opened the amp, I noticed the remains of a trimpot, including some melted solder, lying on the bottom cover of the amp. Its value was only just readable: 1k. The official schematic diagram is rather rudimentary, so it gave me no clue as to where this trimpot had come from. Inspection of

tweak fusing valves.
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the circuitry showed some burnt remains near the heater wiring to the EL34s. In the Jadis Orchestra the heaters of the 4 output valves are wired in series and fed with about 24 V, so comparison between the affected channel and the channel that seemed still intact wouldn`t be of any help here. Further inspection revealed that it the 1k trimpot must have been soldered across the heater wires, with the wiper contact of the trimpot connected to ground: the classical way to minimize hum. But I had never seen such a trimpot melt before, so I was greatly puzzled. At 24 V the dissipation into 1k is about 0. 5 W, which a standard plastic trimpot should be able to handle (if only just). There is no conceivable reason to suppose that the heater voltage had gone up spontaneously for a while. So what on earth had happened It took me quite a while before I could reconstruct the chain of events which led to this result. Mr. Jadis decided to protect the power valves with a fuse in series with the combined cathodes of each output channel. This seems sensible but it now appears it is not. Think of what happens if a valve looses its vacuum. The valve will develop full conductivity. Now the fuse blows. The cathode will be lifted to a voltage near the +500 Volt of the HV supply present at the anode. The insulation between cathode and filament will not survive this attack for very long because the maximum Ufk of an EL34 is just 100 Volt. A...

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