Somewhere in 1997-88 I bought a hundred 6C33C. I believe during those years the where more expensive then now and I paid around $7-8 per tube. Over the years burning the 6C33C in pair of VK-60 (8 tubes), two pair of Lamm ML2 (ML2), ML2 with Melquiades (6 tubes), in the Super Melquiades (6 tubes), conservatively giving the tubes away to some my friends (manse lending them but they never return) and scraping the objectively faulty 6C33C tubes (4 tubes among 100) I realized that I have a less then dozen tubes left. Yes, I admit that among those tubes that I have burned I did burn perhaps 10 of them purely intentionally. Still, how long the 6C33C lives? The last week I bought another 100 of 6C33C, so how long it will last in the Super Melquiades.
First of all, and many people might degree with me, but the life-span of the 6C33C dos not depends from the basing method. They use conservatively and sparingly in automated and semi-automated bias so what? If you use the 6C33C with fixed bias and overheat the plate then it is not problem with your amplifier but with the efficiency of your loudspeakers.
Also, if you stressed the “cold” 6C33C and overheated it one then it will be tend to be overheated in future, however it still will be perfectly functional and will sound with reasonable grid current perfectly normal.
So, how long the 6C33C lives. People out there claim that 6C33C will live for 5 year and more. If an amp works averagely 3 hour per day (an average number in average listing room) then it would make 5.500 hours. Good luck with those expectations!
Approximately 10-12 months of using 6C33C makes the tube loosing the getter’s gloss. This is a sign, as I undestend, that the tube’s getter worked up against the gas that built inside the tube. Sure, when it happens the tube should be replaces, but I feel that the tubes should be replaced sooner. When I do replace the 6C33C with another pre-burned 6C33C after 10 months I do hear the sonic differences. Dose it mean that the last 2-3 month I used an inferior, worked up tube. Yes, it is more likely is the case.
The manufactures suggests that the tube is good for 750-1000 hours. We in audio do not burn the tube at 400-500mA (actually Lamm does at 310mA) and therefore the life-span we could get from the 6C33C might be much longer. With our burning rate of 3 hours per day 1000 hours is approximately one year but there is a catch. What would manifest the manufacture recommendation? Interestingly that the manufacture promises the 30% redaction of cathode emission in the end of 1000 hour. How does it reflect in Sound? I do not know how it reflects in Sound but I know that when I measure the mutual conductance of the 6C33C on my tube tester then a redaction of conductance for 15% is quite auditable.
So, I personally feel that 6C33C should be use very accurately after 7-8 moths. Of course it will sound 10 times longer but it will not have the necessary virility and color saturation. So, the 100 tubes would make 16 sets. If each set would live for ~8 month then would end up with 10-11 years? Hm… not a lot….
Romy the Cat
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche