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The Rejuvenation Process
Since there is a shortage of high quality power triodes just like in the 1940s although, touch wood, for different reasons -- it is necessary for the tube amp user to consider some old recipes which were common in the 1940s and earlier on. This one was passed on to me by an old tube lover from Germany.
I will use as an example the European type AD-1. This tube is considered by some to be one of the best output triodes ever built and it has become very rare.
The AD-1 was invented by Philips in Holland and built there under Philips and Valvo and by Tungsram and Telefunken in Germany. Later, Tesla made a copy which was not on par with the Western European production. A professional version with different basing called the Ed was produced by Valvo under contract for Siemens for use in telecommunication systems. Also, RFT from East Germany built a tubular version, later replaced by the even more powerful, indirectly heated Ec.
A regenerated tube does not look like a new tube. The shiny getter deposit will still look
"burnt" or "bleached" like it was - but the tube will perform for its full life one more
time, usually another 2000 hours. If regenerated yet another time, it might last another
We start with the heating process: Instead of 4 volts we use about 7 volts and overheat our triode for about 10 minutes! Then go down to 5 volts on the filament and add plate voltage. Apply so much that the plate starts to glow red. That's right! Most important is a milliampere meter to read how much current the triode is drawing. With an AD-l, the max current is around 250 mA and this occurs around 600 V on the plate.
When new tubes are produced, a small amount of barium compound is vaporized inside the sealed tube in order to bind gases and impurities inside the glass. What we are doing is using getter material that wasn't used during the initial production process. Up to 60% of the barium is left over from the first process and we can use this potential now to rejuvenate the tube! As you heat the tube, you will see the barium glowing green, evaporating, and doing its proper job all over again.
The critical thing in this process is that you keep your eyes on the meter. After about ten minutes (and this time will vary), the current drops. You must switch off IMMEDIATELY!!! One second too late and the tube is dead! Forever. You have not lost anything since you couldn't use it anyway. I leave all you tube lovers to experiment with this idea and to use your common sense about which settings are correct for each triode.
2) Drop down to around 20% overrated filament voltage, then,
3) Add plate voltage until your milliamp meter does not rise,
4) Keep your eyes on the meter until current falls and immediately switch off!
In the old days there were special tube rejuvenators on the market, but nowadays you will have to build it yourself!