[time-nuts] HP5061B Ion Current

[cdelect at juno.com]

Things to be careful about!

"I think the final vacuum improvement can be achieved more 
quickly if the cesium oven is on."

The outgassing in a tube that has been off for an extended time
is almost all from the oven filaments and the ionizer filament.
The metal surfaces are virtual "sponges" and getter any gases
that are available. If you turn them on with no protective 
cycling circuit you run the risk of overloading the ion pump 
to the point where it cannot pump at all!

Don't use the 200Meg divider voltage to determine the output 
voltage of the supply. Unloaded it is a correct representation
of the output voltage. (4Vdc equals 4000V out)However any 
appreciable load causes a counter voltage that drops the 
divider voltage a lot! The divider voltage might 
then read 1 volt but the output voltage is still over 3000V. 
(it's designed that way to shut off the ovens if the divider 
voltage drops.)Use a Fluke 80K-40 divider probe or similar 
on the output as it has a very high input impedance. A recent
measurement of 4 A18 supplies shows an output voltage of
3000 V with a 50ua load.

Also if testing the supply on the bench you must ground the case
of the supply to the minus input pin. If you don't your
HV will read half of what it should be.

Ion pump supplies for vacuum maintenance like for Cesium tubes, 
Hydrogen Masers, and some high power Rf TX tubes are just for that,
"maintenance". This allows low power consumption, small footprint,
and long life.

On the EFOS Maser here the ion pump supply has a max rating
of 2ma, this for a large 20 liter pump. 

If you are running a tube at high ion pump currents you might
have 20na of beam current but check the peak to valley ratio.
Just because you can get to 20 na does not mean the tube
is performing well. Running an Allan Deviation plot is the
easiest way to know for sure.

The oven filaments are very robust, the ionizer filament due to
the required shape is not. If you are using the older A11
module then the ionizer voltage is AC and causes vibration
of the filament which can result in premature failure. Once
HP came out with the new A11 (DC on the filament) they would 
not honor a tubes warranty if you were using the old style A11.

Cheers,

Corby Dawson