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Tube life over bias rule of thumb?

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Posted on November 9, 2016 at 16:50:24
used-hifi
Audiophile

Posts: 508
Location: Surprise AZ
Joined: March 18, 2003
Looking to calculate tube life from different bias points

Is there a rule of Thumb? for instance, if the book says tube life of 5000 hours at rated current, What would double current be? half of 5000?


Thank you

Lawrence

 

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RE: Tube life over bias rule of thumb?, posted on November 10, 2016 at 09:36:18
6bq5
Manufacturer

Posts: 2201
Location: SF Bay
Joined: August 16, 2001
Contributor
  Since:
September 14, 2012
The RCA receiving tube manual is a great place to start-
As you over bias the tube you will cause the plate to off-gas and add to the shortening of the tube's life- in addition to the 'wear' that occurs from running the tube hot-
There may not be a real study for tube life at 110% bias, 125% bias, 200% ...etc.
And I suspect that the degradation of tube life is a bit more asymptotic than linear...

Why do you want to run the tube hot?
Happy Listening

 

RE: Tube life over bias rule of thumb?, posted on November 10, 2016 at 12:06:57
used-hifi
Audiophile

Posts: 508
Location: Surprise AZ
Joined: March 18, 2003
Linearity!

 

RE: Tube life over bias rule of thumb?, posted on November 10, 2016 at 13:05:39
Paul Joppa
Industry Professional

Posts: 6687
Location: Seattle, WA
Joined: April 23, 2001
I have a 1939 data sheet for the 300B, which lists a wide variety of operating conditions, with all of those at 450v plate voltage being "Maximum Operating Conditions" and notes that such operation will give a shorter lifetime; the conditions at 400v or less are called "Recommended Operating Conditions." I also have a 1950 data sheet which lists the maximum plate voltage as 400v; it also says 36 watts max dissipation (was 40 watts in the 1939 sheet.) No mention in 1950 of lifetime effects.

Because that is only a voltage difference (not current), it seems that ion bombardment (stripping) of the cathode is the primary life-shortening cause. This is in contrast to popular opinion which says the bulb temperature is the main effect.

Filament voltage errors are another limit - too much voltage raises the filament temperature which has a strong effect on filament lifetime - much like incandescent light bulbs. Reducing the filament voltage below the spec will extend its mechanical lifetime, but it will reduce emission and thus the protective effect of the electron cloud. You can do it (I have in fact done it) safely if you reduce the plate current, but it must be a substantial reduction to match the emission reduction.

 

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