[time-nuts] HP5061B High Ion Current/Tubes Out of Cesium

paul swed paulswedb at gmail.com
Thu Apr 6 20:09:25 EDT 2017

Donald running higher temperatures on a a normal tube may indeed give you a
bit more life. Thats exactly how Frankenstein works. Its a hand me down
tube that normally showed as dead. Believe me it doesn't even move the
current meter and its working. My firm belief is that option 004 tubes do
not have anything left to give.

But yet with the higher temp (10-15 higher as I recall) it locks all on its
own after a good warmup period. Serious fumes.
Its been operating this way for some 4 years now. I don't run it all of the
time and I actually recently found what was wrong that always gave it a
slight offset.

So a very good conversation running here with everyone sharing really good
insights and pictures of detail I had only read about and generally without
any pictures.
The entire thread should be gathered up, cleaned up, and presented as the
dummys guide to the care and feeding of old 5061s.


On Thu, Apr 6, 2017 at 5:57 PM, Donald E. Pauly <trojancowboy at gmail.com>

> https://www.febo.com/pipermail/time-nuts/2017-April/thread.html
> I spoke to Corby on the phone a few days ago about our first HP5061B
> that locks fine but has ion current on the order of 76 μA.  It has not
> gone down significantly in two months of pump operation.  He mentioned
> that the electrodes in the pump are made from titanium and that sharp
> whiskers can form on the electrodes from metal migration.  They
> apparently cause corona and keep the ion current high indefinitely.
> According to Corby, the vacuum in the tube may be fine and the leakage
> current make it appear otherwise.
> We performed an experiment using our second HP5061B that we suspect of
> being out of cesium.  When we first got it, we had about 10 μA of ion
> current and within a day it went down to nearly zero.  Today we
> jumpered R12 on the cesium oven board to raise the oven temperature.
> We previously had checked all waveforms for normal operation on the
> board.  This includes measuring cold resistance of the cesium heater
> at 2.6 Ω and hot wire ionizer of 0.1 Ω.  Power to each was close to
> the nominal 2.6 Watts and 4 Watts respectively.
> We bumped oven heater voltage up to 11 Volts with the short on R12.
> This could have put up to 48 Watts into the oven heater unless its
> resistance went up significantly.  After a couple of minutes the oven
> 150° C  overtemp circuit shut down the switching regulator.  We saw no
> increase of beam current even though normal oven temperature is 85°
> according to the tube data plate.  We let the tube cool down and
> repeated the experiment several times.  We had turned the beam current
> adjust all the way up to -2,880 V.  On our good instrument we can get
> 20 μA beam current with only -1,700 V or so out of the -2,500 V
> supply.  We therefore concluded that the beam tube was hopeless and
> decided on the risky experiment.
> We removed the +3,500 V supply on the suspected bad tube from the ion
> pump and connected the -2,500 V supply to the pump.  We left the
> -2,500 supply on the electron multiplier as well.  We saw no drop in
> its -2,880 Voltage.  We would easily have seen 200 M Ω worth of
> leakage on the ion pump.  Therefore the ion pump will work with either
> polarity of voltage.  We have  decided to take the risk of reversing
> the diodes on the +3,500 V supply on our good instrument and watch the
> ion current.  We hope that the reverse polarity will burn out the
> whiskers or other leakage caused by long application of positive
> voltage.  We have devised a test that will show up to 1,000 M Ω of any
> resistive leakage on the tube before we apply reverse voltage to it.
> πθ°μΩω±√·Γλ
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