Thoriated-Tungsten Filaments:
Why Their Life Is Shortened
When They Are Power-Cycled

How the Miller-Larson Effect affects power tube life and why on/off time is an important consideration

Why we shorten the life of thoriated tungsten filaments every time we turn them on and off

Basically, every time we turn a tube with a thoriated-tungsten filament on or off, and the filament temperature therefore passes through a certain temperature range, we shorten the life of the filament. Here's why that is so.

Thoriated tungsten filaments --the quick-heating type, as opposed to oxide-coated filaments which require a warm-up period--  are found in 811A, 572B, 3-500Z, 3-400Z, 3-1000Z, 4-1000A, 4-400A, 304TH, 304TL, 833A, 833C and many other PA tubes.

Two references follow; the first is text; the second is a screen capture (since I could not copy the text).

Valve Amplifiers, 4th Edition
By: Morgan Jones
Publisher: Newnes
Pub. Date: December 2, 2011
Print ISBN-13: 978-0-08-096640-3

Thoriated tungsten cathodes operate only slightly below the melting temperature of thorium (2,023K), and to reduce the evaporation of thorium from the surface, the tungsten filament is partly converted to tungsten carbide. Unfortunately, although hard tungsten carbide is brittle, so the degree of carbonization is a delicate compromise between reducing thorium evaporation and fragility. Because thoriated tungsten filaments are very brittle, so tubes such as the 211, 813 and 845 should be handled with extreme care and not be subjected to mechanical shock.
Unfortunately, thermal shock also kills thoriated tungsten filament tubes. A 1994 study of transmitter tube longevity found that each off/on cycle reduced filament life by 0.2% from its maximum life of 30,000 hours. This doesn’t sound too bad, but it implies that 500 off/on cycles will destroy the filament, so if you switched the tube off and on everyday, you could expect it to expire in less than 17 months. Understandably, the broadcasters took a dim view of this, and looked to see how life might be extended.
There are two reasons why the off/on cycle kills thoriated tungsten filaments:

  • As the filament temperature passes through 900 degrees K [either when they are turned on or switched off], the Miller-Larson effect causes the grains of the metal to reorient themselves, so that the wire becomes thinner and longer. Worse, if a given section of the filament is slightly thinner, the increased current density causes increased localized heating, which exacerbates the Miller-Larson effect and causes further necking of the filament. Eventually, this necking leads to such deep cracks that the remaining conductive material has sufficiently high current density and local heating to vaporize it, thus destroying the filament.

  • The resistance of a cold filament is far less than that of a hot one, and assuming an operating temperature of 1,975K, but an ambient temperature of 293K (20 deg. C), the initial cold current is 8.6 times higher than the operating current. The inrush current through the filament interacts with the Earth's magnetic field to produce a small kick. Combined with the Miller-Larson effect, this gradually deepens the surface cracks in the brittle filament. The damage done to the filament is proportional to the cube of inrush current, so a 'softstart' circuit can be worth while.

If you had bought a quartet of NOS 845s at considerable expense, you would have a vested interest in avoiding the Miller-Larson effect, might want to permanently operate the filaments in standby mode at 80% of full voltage, and only apply full voltage at full switch-on, but note that standby still expends the emissive life at a rate of 1% compared to full filament voltage (but no anode current).

Power Vacuum Tubes: Handbook
Author: Jerry C. Whitaker

Screen captures from this book:

Excerpt from Power Vacuum Tubes by Jerry Whitaker
Excerpt from the book Power Vacuum Tubes by Jerry Whitaker

I think that it therefore might follow that if we held the filament temperature in this range, the tube would rapidly fail.

Since I could only read certain portions of these books online, there may be other considerations, and  some of this information may not apply to some tubes. Comments are welcome.

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  Last Edited June 24, 2013


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