6C33C-B tubes

Testing 6C33C at home isn't a trivial task. They are power hungry and could be very unstable due to heat fluctuations. The only market tester capable of is the expensive Amplitrex (but the current will be still limited).

My advice is to get NOS early 60xx Svetlana - there some sources here in Russia that sell them for about $20/piece. Svetlana-made oldies were made using quality materials at St-Petersburg and less prone to 6C33C "unstability".

As for the life expectancy - it depends on stress for 6C33C in amplifier. According to your amp it's similar to my BAT so I would guess it'll be about 2000-2500 hours. I'm pretty sure that $40 for a NOS pair isn't a big issue...

There is also 6C18C - a prototype for 33 tube - but it has more mellow sounding and not recommended although it's older than 6C33C.

You can easily take off about 10% of the manufacturer's recommended bias without a problem. Your tubes will last longer and sound ever so slightly warmer. If you raise the bias, you risk putting too much power through the circuit than it is prepared to handle. The tubes themselves shouldn't mind a higher bias as the bias in the Almarro 318B is 200 with a suggestion in the manual to lower the bias to 180 if you wish to extend the tube life.

BTW, I compared some 6C18C tubes to the 6C33C and they sound very close. I think I may prefer the 6C18C as they seem to have a touch more bass and midrange weight although equally extended. I would not call them mellow sounding compared to the 6C33C. In fact, the 6C33C is not a real warm tube, and a slight increase in warmth is probably a good thing, unless you are trying to avoid that. Again, to keep things in perspective, the 6C33C and 6C18C sound almost identical.

If you do not have access to a tester for 6C33C tubes like most people, one way to test the 6C33C tubes is to put a new pair in your amp and see where the bias is. New tubes will usually not vary much more than 10% or so between each other. As the tubes age, the bias goes up, eventually reaching a point where you cannot bias the tubes down to the manufacturer's recommendations, and they should be tossed. So, you can adjust the bias of the new tubes in your amp, then to test the old, put them in without changing the bias and look at the readings, using a little interpolation to get an idea of how old your tubes are.

Before the tubes go bad, they should start to loose the highs and the tubes may start sounding a little dull or may start distorting before that. I had some older 6C33C tubes which did not require much of a bias change over the last few years, yet now sound a little dull compared to new tubes. No comparison of the sound between old and new tubes is fair until the newer tubes have broken in ideally at least 200 hours or so, although the bulk of break in will occur before that. Then the tubes gradually start to mellow and warm over time as they age. From experience with my system, the tubes sound better at first as they age, then when they get too old, as mentioned, they start to sound a little dull.