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Well, it is not necessary a fully qualified urban legend as the problem does take please with fixed based 6C33C but..... only under certain conditions. I have written about it a lot on my site but will very briefly recall it again.

Dima might be correct pointing the reasons for anode overhearing but there is one interesting aspect in it. It is irrelevant if the tube is used up and old one or a brand new right from a box. The 6C33C “go red” ONLY if it is insufficiently heated before it begin to care high grid currant. Trust me guys; I burned a lot of 6C33C. Sine 1996 I used BAT VK60, then ML2, then Melquiades… all of them around the 6C33C (Although BAT used automated bias – it is a different story). I always was very liberal with them – $6 per tube you can do any stupid things for them. So the conclusion that I made is that if do not dissipate on full 6C33C more then 50-55W and let it to burn for 45 minutes before loading the tube with a strong string then the 6C33C does not have any temperate instability and any secondary emission. Sure, with low sensitively speakers when people pump a lot of current across the grid and running the tube “cold” (15-20 minutes of heard make the tube still “cold”) do create the avalanche heating effect. Interestingly is that if the tube was “avalanched one than it will have tendency to do it in future….

Jim, it would be very interesting if you do the following test. Take a half of the 6C33C that you did not overhear yet (currant goes up and the plate becomes visibly red). Heat it for a few minutes (most like as you did) and confirm the "hysteresis". Then reset the plate current and kept running the tubes it for another 60 minutes. I presume that the temperature-inspired "hysteresis" will not manifest itself then. If it is will should up then it will be at WAY higher grid currant (close to full power) and at MUSH lower frequency.  In Melquiades, during the testing I forgot to turn my generator off and a well heated 6C33C (60 minutes) did care for half hour 10H at full power with 55W on plate. Actually it could care longer but it burned the 12R loading Miles 12W resistor….

Rgs, Romy

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