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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
semi
646 posts
 
why do you need to replace 6C33C socket? in the case of BAT, it's a difficult job so not likely owner is expected to replace socket once a year.
steakster OP
720 posts
 
Re: sockets - When a few years old, my amps had great difficulty maintaining proper bias. It wandered all over the place. When I peeked inside, the spring clips were charred and had lost all tension on the tubes' pins. I tried cleaning them up but they were too far gone. Replacing the sockets wasn't so difficult - just patience and a steady soldering iron. Ever since replacing them, the bias has been steady and the amps have been very reliable.

I'm still interested in hearing about tube life expectancy.
Thanks!
steakster OP
720 posts
 
A bump.

Besides replacing and comparing the sound of old 6C33C tubes to new ones, is there a way to electronically test them - to see if their values still measure well? I've got a pile of used ones. They still bias okay but have over 1200 hours on them. I don't know whether any of them are still good or to toss them all.
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.
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