A few Valve Facts

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Ampdog

R.I.P. 23 June 2022
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This may not be a very energetic thread, but I thought of sharing not so well-known things that come to light.

Do valves age on-the-shelf? YES!

I spoke to an erstwhile colleugue, Wouter van den Bergh tonight, who is one of a very few remaining valve manufacturers. Wouter worked until the late 60s at the Philips Eindhoven valve factory in Holland before emigrating to South Africa.

The quartz glass valve envelope is apparently not inert to ingress/diffusing of gas from the atmosphere into the vacuum over years. They had particular difficulty with that in certain industrial valves. In that sense the "getter" can get 'saturated' over time and cease to absorb gas after a while.

That does not automatically render NOS valves useless, but they may have a shorter life than well-made new valves.

Maximum electrode voltages:

I have never been certain exactly what these mean. When a valve has a specified maximum anode voltage of 500V, that does not mean that it will start sparking internally at 520V, nor 620V nor 720V. One notices that the same internal valve construction is good for a considerably higher voltage under e.g. pulsed conditions. The small EL84 (and smaller valves) can be used at up to 1KV in sweeper sevice.

The problem is that at high anode voltages more ionisasion takes place as electrons hit the anode and these 'migrate' back to the cathode, destroying the cathode coating. Apparently the rating is based on a reasonable valve life under normal applications, but can be exceeded depending on conditions as long as there is a duration limit. (It is common knowledge that the electrode dissipation is the primary factor influencing valve life.)  
 
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