Why Their Life Is Shortened
When They Are Power-Cycled
Miller-Larson Effect affects power tube life and why on/off time is an
Why we shorten the life of thoriated
tungsten filaments every time we turn them on and off
Basically, every time we turn
tube with a thoriated-tungsten filament on or off, and the filament
temperature therefore passes through a certain temperature range, we
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
By: Morgan Jones
Pub. Date: December 2, 2011
Print ISBN-13: 978-0-08-096640-3
Pages in Print Edition: 672
TUNGSTEN FILAMENT FRAGILITY
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
- 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).
Vacuum Tubes: Handbook
Author: Jerry C. Whitaker
Screen captures from this book:
I think that it therefore might follow
that if we held the filament temperature in this range, the tube would
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.
© 2013 Comtech Research LLC. All
Last Edited June