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Hi Romy,
I am very interested in what you have found out about the 6C33C tube lifetime because I am using a pair of Silvaweld OTL tube reference (their name for them not mine) amps that use 4 x 6C33C-b tubes per monoblock (so a total of 8).  These amps use a fixed bias scheme and so I check the bias more or less every other time I have them on after about 1 hour of warm up.  Also, as a push-pull OTL amp they are run in Class AB and therefore have significantly less current running through them than say for example the Class A Lamm ML2 (with 310mA).  With such conservative usage on the tube I would expect at least 2-3000 hours, wouldn't you?  Or are your comments only directed towards using the tube in Class A?

About the lifetime specification:  This is for their specified purpose inside a military aircraft radar power supply.  From what I understand they are driven quite hard in this purpose not to mention the constant subjugation to very high g forces most likely results in mechanical failure of some kind (also explains their extreme internal build quality).  My guess is that used in a more moderate environment that alone will extend their lifespan considerably (unless you have a habit of dropping them on the floor regularly).  Also, generally, when one runs a tube well below its rated max current or max. voltage the lifespan of the tube should increase accordingly as well.

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