Tubes Asylum

Sums it up nicely. nt

68.81.142.21

Victor Khomenko

In Reply to: RE: for 1 hour, leave on; for 2+ hours, turn off YMMV (nt) posted by Bill Way on July 21, 2023 at 09:36:46

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Follow Ups:

RE: Sums it up nicely. nt - 7868 18:41:29 08/18/23 (1)
So much misinformation - doesn't anyone study the tube datasheets anymore or apply some thinking?
What caueses a filament to fail? Thermal expansion and contraction. Some datasheets will tell you that the filament life expectancy is e.g. 2000 hours when turned on not more than x times / day.
Some driver tubes will light up like a flash bulb when initially switched on.
Similarly tubes do not take too kindly to being cold and getting more than B+ on them - it does damage to the cathode coating.
The 300B was developped for telephone purposes and would seldom be replaced 100 000 hours of continous duty was normal.
When I build my tube amplifiers I would
a) Put a number of NTC's in series with the primary of the mains transformer ensuring gladual heat up of filaments. NTC's to be switched out with a thermal delay relay (keeping heat under chassis down).
b) I used hexfed recitifiers but that causes aproblem: until the tubes are warmed up the B+ is above the operating B+ voltage which was solved by using a damper diode to the CT of the HT winding.
Initially I switched off at night and on again in the morning (14 ~ 16 hour running/day). Later I swicthed on on Monday morning and off on Friday night.
All tubes mesureed like new after 20 000 hours. Output tubes were run in class A at design centre rating, not at absolute maximum rating.
c) Power supply was stabilised - I've got a computer server room UPS that first converts to DC and then reconstructs pure sinus mains. US military noticed imcreased life expectancy when mains voltage was kept stable +/- 3%.
d) And if you are running tubes hard (output) than a Pearl HiFi tube cooler can extend the life of a tube by 50% ~ 100%. It may not look sexy but it works and your wallet will thank you with current tube prices and some newer tubes no longer available.

Edits: 08/18/23
- RE: Sums it up nicely. nt - immatthewj 12:09:54 08/19/23 (0)
  So what is your answer to OP?
  To never turn his tube gear off?
  
  "Two Corinthians, right? Two Corinthians 3:17, that's the whole ballgame. Where the spirit of the Lord-right?-is, there is liberty!"

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