[AMRadio] filament voltages

Bob Bruhns bbruhns at erols.com
Fri Aug 20 10:43:16 EDT 2004


I looked at the 250TH specs, but I really can't make a solid judgement on
acceptable filament voltage at reduced specs for long life.  Generally I
would be conservative; where you might reduce filament voltage a great deal,
I recommed that you only reduce it slightly.

Emission falls off very rapidly with reduction in filament voltage, and a
small reduction probably has a large effect on tube life.  I really can't
specify how much you can safely reduce filament voltage, but with
thoriated-tungsten filamentary cathode tubes, I would recommend that you
don't reduce operating filament voltage more than 10% from spec.  And make
sure your socket connections are clean, etc.
Ref: http://www.bext.com/filament.htm

Some recommend not reducing filament voltage on large oxide-cathode tubes
such as the 8877, etc.  Ref:
Others claim remarkable improvements from filament voltage reduction. Ref:
http://www.broadcast.harris.com/ support/kb/supportdocs.asp?fid=1050
(Note: I believe the 4CX20000 is an oxide-cathode tube.)

Other factors probably have as much effect on tube life.  Excess current, of
course, or operating before full emission temperature is reached, and
thermal shock at turn-on. ref:

Burn-in: cook long-unused tubes at rated filament power for 100-200 hours.
The idea is to allow the "getter" elements to absorb gas that has leaked
into the tube or out of the materials in the tube.  In tube operation, gas
becomes ions which attack the cathode.  Ref:

Filament and cathode structures do not like thermal shock.  The sudden
application of filament voltage to a cold filament is a significant shock,
because the resistance of a cold filament is much lower than its resistance
when it is hot.  A gradual application of filament power is best.  This can
be done with a resistor in series with the filament transformer primary, a
small filament transformer that overloads at turn-on, etc.

Filament voltage regulation is a good idea. Ref:
But, be careful not to make the turn-on shock situation worse.  Basically, I
suggest one or two big constant-voltage transformers and a few small
filament transformers, possibly with a series resistance of a few ohms in
their primary circuits.  The little transformers will overload on turn-on,
limiting thermal shock to the tubes, yet the system will maintain operating
filament voltage nicely, over a wide range of mains supply voltage.  Then
you can tweak the filament voltage to extend emission life, and the voltages
will stay put, and turn on will be gentle.  The gentler, the better.

  Bacon, WA3WDR

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