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It's curious that few vacuum tube manufacturers directly mention the
benefits of soft starting filaments on power vacuum tubes. However, it
is definitely known by old timers, hams, commercial/research tube users,
and vacuum tube equipment designers.
One of the modern "bibles" for commercial power vacuum tube users is
Jerry Whitaker's 662pp tome, "Power Vacuum Tubes Handbook, 3rd edition"
(Taylor and Francis Group, LLC, 2012, ISBN-13: 978-1439850640). Mr.
Whitaker discusses reliability issues associated with filament inrush
current on page 513:
"When a cold filament is turned on, damage can be caused by two effects:
• Current inrush into the cold filament, which can be up to 10 times the
normal operating current if the filament supply is of low impedance.
• Grain reorientation, which occurs at about 600°C–700°C, known as the
Miller–Larson effect. This grain reorientation will result in a
momentary plastic state that can cause the filament wire to grow in
length and, therefore, become thinner.
The Miller–Larson effect is aggravated by variations in the
cross-sectional area of the filament wires along their length. This will
cause hot spots in the thinner sections because of the greater current
densities there. The higher temperatures at the hot spots cause
increased growth during warm-up through the 600°C temperature range,
when they have a higher current density than the rest of the wire and a
much higher power dissipation per unit length as a result of the higher
resistance. Each time the filament is turned on, the wire becomes
thinner until the hot spot temperature enters a runaway condition."
"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."
So, soft-starting filament power (via variac, NTC inrush limiter, etc.)
is an excellent approach to extend operating life, especially for used
brittle-filament Hamfest bargains.
Bert
dave pierson wrote:
On February 17, 2018 at 11:57 PM Steve White <steve.white1@xxxxxxxxx> wrote:
Does anyone know if limiting the in-rush current during a pre-heating period
for an 833A filament will extend its life? I looked in many 833A data sheets
and in only one did I find any mention of limiting filament start-up current.
The Machlett 833A data sheet states a maximum filament current of 20 amps.
If it will extend the life of the tube I can easily use a resistor to make
a pre-heating circuit. Right now for my Amperex 833A VTTC, I just switch the
filament on with 10 volts with no pre-heating. Any thoughts?
As a matter of physics, limiting the current will increase life. Without
more data can't say how much reduction would provide how much more life.
best
dwp
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