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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
dgarretson
2,394 posts
 
The pair of 6C33C-B tubes providing 75 wpc in my BAT VK75SE are biased 200ma with 210V plate voltage. They begin to degrade sonically at 1200-1500 hrs. I have not needed to replace a tube socket in 10K hours of service.
rgurney
279 posts
 
Agree with Dave's statements above. I once asked Victor about needing to replace a 6C33C-B tube socket and he said it is quite rare, if ever, in his experience.
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.
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