Hello all,

While talking to Doug at RF Parts today (about NOS 4CX1500B power
tubes) he mentioned that he recommends a 'burn in' period for older
NOS units, filament powered up only, of 10 to 20 hours before giving
it plate voltage.

This might be good advice when dealing with (certain? ceramic?) power
tubes?

If this has merit, I'm curious if any of the reason(s) to do this
would have any validity when dealing with larger glass power tubes
(803's, 813's, etc) that are X years old... (something I've never
heard of / encountered before today, but it can be stated accurately
that I don't get out enough ;-)

TIA for any comments!
-Robert
QTS
http://www.Braught.com
Real email addy: (remove NoSpam to reply:
Duh!)

If you do that, remember to enable the cooling fans as the filaments in
those big sucka' t00bz will melt their own pins.

--
Gregg
*It's probably useful, even if it can't be SPICE'd*
http://geek.scorpiorising.ca

Robert M. Braught wrote:

Hello all,

While talking to Doug at RF Parts today (about NOS 4CX1500B power
tubes) he mentioned that he recommends a 'burn in' period for older
NOS units, filament powered up only, of 10 to 20 hours before giving
it plate voltage.

This might be good advice when dealing with (certain? ceramic?) power
tubes?

If this has merit, I'm curious if any of the reason(s) to do this
would have any validity when dealing with larger glass power tubes
(803's, 813's, etc) that are X years old... (something I've never
heard of / encountered before today, but it can be stated accurately
that I don't get out enough ;-)

TIA for any comments!
-Robert

Yes, a burn in is highly recommended for any directly-heated
thoriated-tungsten type of vacuum tube. It brings fresh thorium to the
surface, where it can do its intended work of improving emission.

Whether it's helpful in the case of rare-earth coated cathodes (directly
or indirectly heated) seems to be debatable. My view on it - burn them
in. It can't hurt. I know that personally I've seen sometimes
significant emission improvements when doing this, but the jury's out on
whether it's caused by an actual cathode improvement, systemic
degassing, or other phenomenon/a.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

Yes, a burn in is highly recommended for any directly-heated
thoriated-tungsten type of vacuum tube. It brings fresh thorium to the
surface, where it can do its intended work of improving emission.

When I worked in Broadcast, it was not a specific recommendation from the
tube manufacturers, but we often did that for an hour at least, but
cycled the filaments on for about 10 minutes and off for 5. The theory
that I heard from somebody at NBC was that shipping and vibration and
storage might affect the interelectrode alignment a bit, and a few
temperature cycles might settle them down in their operating position.

We were able to almost triple the working life of thoriated-tungsten
power tubes like 4-400-A's, 5762's and 4CX5000 - 10,000's by running them
at rated filament volatge for a day, and then lowering the filament
voltage until the output started to droop (or, for modulators, the
distortion started to increase), and setting the voltage 1 or 2 percent
above that point. You HAVE to have a voltage regulator in this case, or
line voltage fluctuations will move you into another region. A new Eimac
4-400A with a 5.0VAC rated filament would run fine in a 1 KW transmitter
at 4.7 volts or so when new. 10,000 hours later you'd be up to 5.0. If
you were a Las Vegas station you might just run the voltage UP as far as
5.2 before replacing the tube, especially an expensive 4CX5000 in a
tightwad-operated 5KW FM station...

--
Regards, Terry King ...In The Woods In Vermont

Capturing Live Music in Sound and Images
http://www.terryking.us

Hi Fred,

Yes, a burn in is highly recommended for any
directly-heated thoriated-tungsten type of vacuum
tube. It brings fresh thorium to the
surface, where it can do its intended work of
improving emission.

I am somewhat reluctant to believe that thorium
"boils up" to the surface of a heated solid tungsten
wire doted with some thorium atoms in it - why
should it?

I am no expert in solids physics, but I think you
mixed this up with the case of barium/bariumoxide coated
cathodes, where "boiling up" fresh barium from within the
coating (not from the wire itself) to the surface by a
short period of over-heating sometimes "refreshes" emission
least for a limited time.

For coated cathodes of current production tubes this
procedure shouldn't be too helpful anyway, since cathode
coating mixtures have been formulated (and as I remember
also patended) in, say, the 50s or 60s, which don't tend
to build up concentrated barium "Zwischenschichten"
(intermediate layers?) of concentrated barium in the
coating itself, blocking good emission unless they are
at the surface of the coating.

It can't hurt.

Yeah, so why not try to "regenerate" valuable thoriated
tungsten filament tubes gone bad emissionwise, too, before
dumping them ... if it helps to improve emission, even
only for a limited period, it is certainly okay to
do so.

Tom

--
The opposite of a correct statement is a false statement. But the
opposite of a profound truth may well be another profound truth.
- Niels Bohr

Tom Schlangen wrote:
Hi Fred,


Yes, a burn in is highly recommended for any
directly-heated thoriated-tungsten type of vacuum
tube. It brings fresh thorium to the
surface, where it can do its intended work of
improving emission.


I am somewhat reluctant to believe that thorium
"boils up" to the surface of a heated solid tungsten
wire doted with some thorium atoms in it - why
should it?

I don't know either. There appear to be two mechanisms at work: 1)
cathode contamination, cured by moderate heating, and 2) loss of thorium
from the surface, cured by intense heating. At those intense heats, is
"solid" tungsten truly still solid? Perhaps the term "boiling" is
inappropriate, maybe something like "atomic migration" would be more
accurate.

Have a look here (there are lots of other references, this one just
seems the most concise):

http://www.antiquewireless.org/otb/rejuve.htm

Note the section on thoriated filaments, where the two apparently
separate phenomena are discussed.

I am no expert in solids physics, but I think you
mixed this up with the case of barium/bariumoxide coated
cathodes, where "boiling up" fresh barium from within the
coating (not from the wire itself) to the surface by a
short period of over-heating sometimes "refreshes" emission
least for a limited time.

I don't think so. The mechanisms behind rare-earth cathode coatings
appear to be a lot more complex than the "simple" tungsten or thoriated
tungsten filaments. Anyone's guess as to what actually happens here,
though I'd tend to agree with the "cathode poisoning" due to residual
gas theory.

For coated cathodes of current production tubes this
procedure shouldn't be too helpful anyway, since cathode
coating mixtures have been formulated (and as I remember
also patended) in, say, the 50s or 60s, which don't tend
to build up concentrated barium "Zwischenschichten"
(intermediate layers?) of concentrated barium in the
coating itself, blocking good emission unless they are
at the surface of the coating.

.... I wouldn't know, and defer to your expertise on this.

It can't hurt.


Yeah, so why not try to "regenerate" valuable thoriated
tungsten filament tubes gone bad emissionwise, too, before
dumping them ... if it helps to improve emission, even
only for a limited period, it is certainly okay to
do so.

They do! That's the whole point!

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

Hi Fred,

I am somewhat reluctant to believe that thorium
"boils up" to the surface of a heated solid tungsten
wire doted with some thorium atoms in it - why
should it?

I read up in an old german book about tube technology
and I obviously made a wrong assumption in the paragraph
quoted above:

"Thoriated tungsten" as a cathode material does _not_ mean
that the tungsten wire material itself is doted with thorium,
but that the tungsten wire is _coated_ with some material
containing thorium.

So, when "rejuvenating" them the same principles as with
oxid coated cathodes apply.

Sorry for my misunderstanding.

Tom

--
To err is human - to purr feline.
- R. Byrne

Hi,

I am somewhat reluctant to believe that thorium
"boils up" to the surface of a heated solid tungsten
wire doted with some thorium atoms in it - why
should it?

I read up in an old german book about tube technology
and I obviously made a wrong assumption in the paragraph
quoted above:

"Thoriated tungsten" as a cathode material does _not_ mean
that the tungsten wire material itself is doted with thorium,
but that the tungsten wire is _coated_ with some material
containing thorium.

??? It is my understanding that tungsten powder is mixed with
about 1% of thoria (thorium oxide), then sintered and drawn out to a
wire. So the filament is indeed doped (I believe that's the word you
intended to use) with thorium.

The thoria was originally used in lamp filaments to reduce
"offsetting" and breakage. Some thoriated wire was accidentally used
for radio tubes around 1920, resulting in greatly increased emission.
GE physicists investigated the effect, and the thoriated filament went
into production in 1923 with the 201A and the 199.

This is from memory, and there may be different processes used for
transmitting tubes.

73, Alan

This agrees with what Roseberry says (Handbook of
Electron Tube and Vacuum Techniques, pp89-92).
To quote, "The filament is made of pure tungsten
wire to which a small amount of thorium oxide has
been added as an alloy during manufacture. ... The
thoriated filament is momentarily heated to ...
drive some of the thorium oxide to the surface.
It is then aged ... so that some of the thorium
oxide is reduced to metallic thorium."

John

"Alan Douglas" adouglasatgis.net wrote in message
...
Hi,

I am somewhat reluctant to believe that thorium
"boils up" to the surface of a heated solid tungsten
wire doted with some thorium atoms in it - why
should it?

I read up in an old german book about tube technology
and I obviously made a wrong assumption in the paragraph
quoted above:

"Thoriated tungsten" as a cathode material does _not_ mean
that the tungsten wire material itself is doted with thorium,
but that the tungsten wire is _coated_ with some material
containing thorium.

??? It is my understanding that tungsten powder is mixed with
about 1% of thoria (thorium oxide), then sintered and drawn out to a
wire. So the filament is indeed doped (I believe that's the word you
intended to use) with thorium.

The thoria was originally used in lamp filaments to reduce
"offsetting" and breakage. Some thoriated wire was accidentally used
for radio tubes around 1920, resulting in greatly increased emission.
GE physicists investigated the effect, and the thoriated filament went
into production in 1923 with the 201A and the 199.

This is from memory, and there may be different processes used for
transmitting tubes.

73, Alan