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Re: VTTC Operation and Filament Warm-up



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


Tesla list wrote:

 > Tesla list wrote:
 >  >
 >  > Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>"
 > <dhmccauley-at-spacecatlighting-dot-com>
 >  >
 >  > Question regarding VTTC operation.  Is it necessary to allow adequate 
time
 >  > for the filament to warm up before powering up the rest of the hv 
circuit?
 >  > I know with TWT's, we typically
 >  > allow 10-20 minutes filament warm-up time before pulsing these tubes.
 >  > Didn't know if these vacuum tubes (833As) were just as sensitive.
 >  >
 >  > Thanks
 >  >
 >  > The Captain
 >