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.