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6C33C-B tubes

I have 35w p/p Class A monoblocks which use two 6C33C-B tubes per unit. The manufacturer recommends a bias setting of 150mv. Before biasing (once a month), the tubes' measurements rarely deviate more than 10mv over/under the 150mv. Over the last year, I’m guessing that they’ve been in use for about 1000 hours. I have no idea what their expected lifespan might be. So far, biasing is still relatively easy. The manufacturer is no longer around to answer any questions. So, here’s a few for the Audiogon community.

1) For those of you using 6C33C-B tubes, are you able to guesstimate their lifespan in your amp – in Class A operation?
2) Generally speaking, how necessary is it to bias exactly to the manufacturer’s recommendation – in this case 150mv? Is there an acceptable tolerance +/- %?

Thanks! P.S. I’ve been replacing the sockets once a year.
steakster
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crossfire
13 posts
 
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".
crossfire
13 posts
 
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.
plangco's avatar
plangco
158 posts
 
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.
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