"Fred Nachbaur" wrote in message
. ..


Marko wrote:
I purchased some tubes, NOS and a couple of them (dual triodes) had very
weak emissions in only half of the tube.

Nothing to lose, so I turned the heater voltage in the tester up to
anout
250% of rated voltage and let it glow for about 15 seconds.

Afterwards the emissions were much better in the weak half of the tube.
I
did let it cool down again before testing the emissions.

I had several new late date tubes from some foreign country (what most
would
call foreign junk) that had very weak emissions until I toasted them for
a
little while. Afterwards they checked as strong tubes. Job "well
done"?

Anyone know what actually happened to the filament? Mabey it had some
junk
on it.

This also worked on some NOS mid 50s octals (mentioned above). Best
Regards, Mark

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.

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