Re: VTTC Operation and Filament Warm-up

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <teslalist-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
 >
 > Original poster: "David Sharpe by way of Terry Fritz 
<teslalist-at-qwest-dot-net>" <sccr4us-at-erols-dot-com>
 >
 > Dan
 >
 > According to  my reference (Power Vacuum Tube Handbook 1999), it
 > is recommended that for thorated tungsten filament tubes of unknown 
background
 > should be powered for 24hrs on filaments prior to applying HV.  This
 > insures that
 > the getter(s) have removed all residual gas molecules/ions prior to 
applying HV
 > to tubes.  HV flashovers in tubes are often caused by energetic radiation
 > (stray
 > gamma radiation), excessive B+ voltage (>1.5 Vbb rating), or residual 
ions/gas
 > molecules in tube.  Heat up time from there is open for conjecture, but 
I will
 > point out... What is the wear element is a "hollow state" device (YEP 
it's the
 > filaments).  Thermal stress of heating plus dielectric stress from 
applying HV
 > prematurely I'm sure have taken many tubes over the years to an early 
grave...
 >
 > Also watch filament voltage, wear is a function of normalized voltage ^ 4th
 > power.
 > A 10% over voltage on filaments will cut normal service life by almost 1/3.
 > On my large VTTC when I build it, I will allow 2 minute warmup prior to
 > applying HV.  Also limit inrush current on cold filaments either by using a
 > "step-start" current limiting resistor with bridging relay, or
 > thermistor.  Magnetic
 > shock on cold filaments can also lead to permanent damage (open).
 >
 > Also recommend you check Econco's website
 >
 > www.econco-dot-com
 >
 > They rebuild high power external anode power tubes for induction heating,
 > radar, industrial customers.  There is an extensive reference section of 
proper
 > protection, filament management, application of high power tubes, etc.
 > Excellent website...
 >
 > FWIW
 >
 > Regards
 > Dave Sharpe, TCBOR
 > Chesterfield, VA. USA

	Since tungsten acts as a getter at temperatures above about 1000° C,
the 24 hour period sounds like a good precaution, and shouldn't have any
effect on the life of the tube.  A warmup period before operation is
also a good idea, but the time will depend on the size of the filament.
Surge current limiting is also a good idea, but really important only
for very large tubes.  I couldn't find it just now, but as I remember it
the RCA data sheet for one of their very large VHF triodes recommends a
warmup time of 7 HOURS, with a scheduled application of filament
voltage.

	My Drake L-4B 1.5 kW linear amplifier uses a pair of 3-500Z's with
thoriated tungsten filaments.  The filament power is "instant on".

Ed

Ed