wrote:
Doug, a suggestion about "Longevity" for 4-400's (Actually, any
thoriated-tungsten filament power tubes):

These tubes are very sensitive to filament voltage. Make sure the tube
pins and socket are nice and clean and shiny (don't use any
abrasives!).. Measure filament voltage at the socket. This is all to
make sure the tubes REALLY get the correct filament voltage to the
filament... It's 28 Amps for two tubes!

Actually in amateur service filament voltage is not that big a worry. I
think the excessive worry comes as a carryover from BC service where
tubes are operated far below ratings and can be operated for almost
9000 hours per year.

In BC service, excessive voltage shows up in a noticable period of time
since the tubes are operated 24/7. The voltages and currents of other
elements are a much lower percentage of maximum compared to amateur
service, and filaments are not cycled. As such, emission failures are a
common failure mechanism.

In amateur service, it takes many years and thousands of fialment
cycles to accumalate the hours a BC tube sees in one year. Tubes in
amateur service much more often fail from seal leakage, element
structure defects, and occasional abuse.

Out of hundreds of tubes I've seen fail in amateur service over the
past 30 years, I can count the number of emission failures on my two
hands.

I'm not saying filaments should be run extra hot, but I am trying to
point out what worries BC stations is often not a concern in amateur
use.

By the way, two of the emission failures I saw were in 8877 tubes that
were operated too cold. The owners had reduced filament voltage below
4.75 volts. NEVER run a MOX cathode tube at reduced voltage unless you
also plan to limit peak emission current.

My advice is to run all tubes in amateur service at RATED filament
voltage or in the allowable tolerance range for voltage.

As for cooling, seal cooling is critical in glass tubes. So is periodic
operation to keep the tubes gettered.

http://www.w8ji.com/vacuum_tubes_and...e_failures.htm

73 Tom

wrote:
wrote:

Doug, a suggestion about "Longevity" for 4-400's (Actually, any
thoriated-tungsten filament power tubes):

These tubes are very sensitive to filament voltage. Make sure the tube
pins and socket are nice and clean and shiny (don't use any
abrasives!).. Measure filament voltage at the socket. This is all to
make sure the tubes REALLY get the correct filament voltage to the
filament... It's 28 Amps for two tubes!


Actually in amateur service filament voltage is not that big a worry. I
think the excessive worry comes as a carryover from BC service where
tubes are operated far below ratings and can be operated for almost
9000 hours per year.

In BC service, excessive voltage shows up in a noticable period of time
since the tubes are operated 24/7. The voltages and currents of other
elements are a much lower percentage of maximum compared to amateur
service, and filaments are not cycled. As such, emission failures are a
common failure mechanism.

In amateur service, it takes many years and thousands of fialment
cycles to accumalate the hours a BC tube sees in one year. Tubes in
amateur service much more often fail from seal leakage, element
structure defects, and occasional abuse.

Out of hundreds of tubes I've seen fail in amateur service over the
past 30 years, I can count the number of emission failures on my two
hands.

I'm not saying filaments should be run extra hot, but I am trying to
point out what worries BC stations is often not a concern in amateur
use.

By the way, two of the emission failures I saw were in 8877 tubes that
were operated too cold. The owners had reduced filament voltage below
4.75 volts. NEVER run a MOX cathode tube at reduced voltage unless you
also plan to limit peak emission current.

My advice is to run all tubes in amateur service at RATED filament
voltage or in the allowable tolerance range for voltage.

As for cooling, seal cooling is critical in glass tubes. So is periodic
operation to keep the tubes gettered.

http://www.w8ji.com/vacuum_tubes_and...e_failures.htm

73 Tom

If oversize filament transformers are used (never know what kind of
surplus junk hams use!) there is a danger of filament failure
due to starting in-rush current. My home brew linear started these
tubes up with a power resistor in series with the primary of the
filament transformer for a few seconds, which was then shorted out
by a relay. A transistor r/c delay circuit did the dirty work.
This lets the tube filaments preheat a bit before applying full
voltage.

Hi, Tom

Thanks for the hint about gas in large glass tubes.
Also thanks for the link to your web site, plenty of wonderful info!

One question please - I have 8 type 3-500Z which
have not been used for 20 years. You mentioned operating
them at low plate voltage and positive grid voltage,
so that the plates show color (for gettering action).

What range of plate voltage/ grid voltage do you recommend,
and what current limiting resistor would you suggest
in the plate circuit?
Do you use a current limiting resistor in the grid circuit also?
I plan a test fixture to recussitate
the 3-500Z tubes before risking them in an amplifier.

73,
Ed Knobloch

wrote:
snip
As for cooling, seal cooling is critical in glass tubes. So is periodic
operation to keep the tubes gettered.

http://www.w8ji.com/vacuum_tubes_and...e_failures.htm

73 Tom

Edward Knobloch wrote:

What range of plate voltage/ grid voltage do you recommend,
and what current limiting resistor would you suggest
in the plate circuit?
Do you use a current limiting resistor in the grid circuit also?

Ed,

I use lots of airflow and a variable voltage grid supply. Since the
grid voltage is dc, you can use a simple Ig times Vg (to cathode) to
calculate dissipation. In a operating amplifier you would have to use a
Fourier (harmonic) analysis to calculate grid dissipation, but not at
dc.

Set the grid just below rated dissipation. When Eimac initially
gettered the tubes, they heated the grids well into color. I'm not that
bold, although they told me they could get almost white hot without
problems! So I stay at or below rated dissipation.

I use about 500-1000 volts of anode voltage, whatever it takes to show
good anode color. I use an old AL80B transformer for grid bias and HV,
except I don't voltage double the HV winding. A variac would be better,
I switch taps.

73 Tom