Electronics > Metrology

Cesium beam tubes

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I acquired an HP-5061A with a dead high performance tube a few weeks ago.  The high performance cesium tubes run the cesium heater at a higher temp which increased the beam current and the resulting signal for the cesium hyperfine energy gap excitation frequency lock has less noise as a result.

I get zero Ion Pump current,  but it does flutter around a little.  I've checked the Ion Pump power supply and it read ~2700V.  So, I think the tube has good vacuum and the meter is showing an accurate result.  The heater for the cesium does turn on and eventually does enter a steady state where the cesium heater meter reading matches the recorded value from the service card. 

The timenuts mailing list has a few threads regarding reviving a seemingly dead tube.  This generally involves connecting 4000 to 5000V to the Ion Pump and waiting a few weeks to make sure the tube really does have a good vacuum.  I haven't tried that yet, but do have a couple variable HV supplies that I'm going to try once I get the proper HV connectors. 

My real question is if anyone knows of a source for used/nearly but not quite dead cesium beam tubes?  These tubes don't generally show up on auction sites and if they do they are most likely dead.  But, if someone has a tube that is on its last legs I would like to make an offer and try it out and see if I can get my HP5061A to lock.   It would be a moral victory more than anything else.   I don't plan on using a cesium beam standard for metrology, but it is certainly something interesting to talk about.  I should be the only guy with an old school atomic clock on my block. 



If you have a manual I'd recommend performing the low "low frequency coil" test on the tube.

Before the test make sure that the EM voltage at the A19 test point reads around .250V which equates to 2500V. If not reset it to that voltage.

If no signal visible during the LF test then the tube is most likely bad.



Agreed -- there's no need to mess with the ion pump supply, it doesn't sound like.

If you do have a gassy tube, stick with the same rated ion pump voltage, but use a supply capable of a couple mA (at most).  I don't know what would happen if you ran 5000 volts into the ion pump but it probably wouldn't be good.

I probed A19 testpoint 3 as described when replacing a cesium tube.  The Electron Multiplier voltage on the tube I have is listed as 1530.  The value read at pin 3 of A19 was .175 or -1750V. 

http://www.leapsecond.com/museum/hp5061a/5061a.pdf Page 90  Procedure 5-186

This procedure references A15R33  and A15R34.   Assembly A15 is the Power Regulator Board which fits into a slot. Several boards use board edge connectors to create a backplane of sorts.

http://www.leapsecond.com/museum/hp5061a/5061a.pdf  Page 300

This shows the Power Regulator Board which sets the voltages for various power supplies.   I seem to have a later model 5061A and the A19 -2500V power supply output is definitely controlled by resistors R33 and R34 on the Power Regulator Board.   An earlier revision of the manual from 1968 references different resistors, so it's important to know which revision you are looking at.

Procedure 5-186 says to decrease the value of R33 to increase the EM voltage.   R33 on the Power Regulator board in my 5061A is a 1M Ohm resistor.    There is no formula that I've found for setting the Electron Multiplier voltage, so I put a 480K and a 300K in series and replaced R33. 

The A19 pin 3 voltage is now -2700 volts.  This may be at it's limit, but I'm going to leave it as is for now.

The beam current now read 10 on the meter.  This is with the cesium control board setting set to oven high (high temp / high precision) mode.   So, the old tube does have some cesium left, but needed more encouragement to get the beam current going.   

Thanks for the advice about adjusting the Electron Multiplier voltage.  I may have got there eventually, but not before screwing something up along the way.

Of course, even with a reasonable amount of beam current, I wasn't getting lock.  I had played with the oscillator controls and never checked to see where I left them.  The oscillator was way off.   The procedure is to set the fine oscillator control in the middle and use the coarse oscillator control to peak the readings.  I cheated and used a HP5334A universal counter to set the HP5061A OCXO right at 5MHz.

Then I switched the mode from free running to operations and hit the logic reset, and BINGO.  Atomic frequency lock.  The next thing I'm going to try is comparing the phase drift between a cheap Chinese GPSDO, a Symmetricom rubidium standard, and the HP5061A.    I'll take some screen shots from the scope and post them.

Thanks again for the suggestion of bumping up the EM voltage.  I'm pleased as punch.

Well,  I think I must be getting a false lock.  While the beam current shows something like 10 on the meter vs close to zero prior to increasing the electron multiplier voltage, the control voltage is not stable. 

The control reading swings from -10 to +10 and the output frequency is being phase shifted relative to a Chinese GPSDO and Symmetricom rubidium as soon as I place the HP5061A into operational mode.  In open loop mode the HP5606A OCXO is pretty darn stable, but when controlled by the cesium beam tube, things start moving around.

The GPSDO and rubidium clocks are short term stable and don't show any phase drift when observed over 10 minutes or more.

Then manual has a procedure for routine adjustment.

http://www.leapsecond.com/museum/hp5061a/5061a.pdf  Page 68 Procedure 5-19

This procedure has you check the 2nd harmonic value.  If below 20 or above 50 the HP5061A Loop Gain control is adjusted so that the 2nd harmonic value is 20 and then the beam current meter scale is adjusted so that the beam current reads 20. 

I can't adjust the loop gain any further and the 2nd harmonic reads 10.    Since I'm at the end of the loop gain control I'm not sure if the control loop is really locked or just overdriven. 

I went back and tried to manually sweep the excitation frequency using the coarse oscillator control  to peak the beam current, and when I do that the beam current doesn't change.  I gave he coarse control several turns in each direction and the beam current didn't change.  The manual says that there should be smaller peaks to each side of the main peak.  I don't see any of that.  This was after setting the beam current reading to 20 per the procedure from 5-19.

I'm guessing that the beam current reading might not be valid and the EM voltage needs to be reduced.  The electron multiplier appears to be physically close to the mass spectrometer which generates the beam current measurement based upon a drawing in the manual. 

I'll drop the EM voltage to 2500 and see what happens.


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