Marko wrote:
"Fred Nachbaur" wrote in message
. ..

[...]

Yes, this was once a common technique for "rejuvenating" tubes. What
apparently happens is that cathode contamination due to ion bombardment
is literally burned off, exposing a fresh layer of cathode material.

Ion bombardment is, in turn, caused by residual gas inside the tube. So
if contamination happened once, it will probably happen again. In other
words, don't expect your "fix" to last indefinitely.

On the other hand, I've seen a more lasting improvement on occasion,
usually on tubes that had run at lower-than-rated filament voltages for
long periods of time (such as can occur in older gear, due to failing
selenium rectifiers in DC filament supplies).

Cheers,
Fred
--
Hi Fred.


I recall reading that rejuvenating tubes only works on old tubes made in the
30s or so. They had different alloys and the heater and filament were the
same. I have the article around here some where.

Anyhow, these were NOS tubes, straight out of the box.

Oops.. somehow managed to miss your obvious statements that these were NOS.

New tubes will frequently have lower-than-normal emission until they've
"burned in" for awhile. I believe this is due to residual gas after
manufacture taking awhile to be absorbed by the getter, though I'm not
certain about this. It would also make sense that this might be
aggravated by long periods of inactive storage.

Operating them normally for awhile should cure this, no need for
excessive filament voltage and hence cathode temperature.

Also, if this superheating of the filament did work on new tubes (where
filament and cathode are separate) ,why would getting the filament very hot
burn the contaminants off of the cathode?


Any thoughts on this Fred? MH

Some directly heated tubes (the filament is the cathode) are of
thoriated tungsten construction; these will typically glow bright yellow
rather than dull orange. As I understand it, these will be the most
responsive to such "rejuvenation," bringing new uncontaminated thorium
to the surface of the filament (cathode). This may be what your
reference to "tubes made in the 30's" is about.

Other directly heated types (e.g. 80, 5U4 etc.) have very similar oxide
coating to the usual indirectly-heated types, The rejuvenation technique
will still be effective on either variant, though probably less so than
the thoriated tungsten types.

Another form of rejuvenation which is actually more effective in the
long run is to *permanently* increase the filament voltage by about 30%.
This is still a commonly-used technique to squeeze some extra life out
of oscilloscope CRTs.

There used to be a good business in "Picture Tube Brighteners" for
tube-based television sets also. These were either small step-up
transformers, or (later) a simple solid-state voltage doubler, that
increased the filament voltage (and hence the cathode temperature) of
the picture tube.

Cheers,
Fred
--
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