Tubes Asylum

RE: Sums it up nicely. nt

222.152.248.177

This Post Has Been Edited by the Author

In Reply to: RE: Sums it up nicely. nt posted by Victor Khomenko on August 04, 2023 at 14:58:54

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). 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 swicthed off at night and on in the morning. 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. Power supply was stabilised - I've got a compter 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%.

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.

Follow Ups: