[time-nuts] 5071A Cs oven

John Miles jmiles at pop.net
Thu Oct 4 22:05:54 EDT 2012

I bought a couple of 5071As on eBay recently, on the basis that the various
status messages shown in the auctions didn't look like tube failures.  They
both reported Cs oven voltages at 0.0. 

The first unit turned out to have a severe intermittent noise problem,
originating somewhere other than the tube or OCXO, that kept the unit from
being able to pass its own power-up self test most of the time.  This one
had a 10890A standard-performance tube.

The other 5071A was in much better physical shape.  It had a 10891A
high-perf tube that looked practically brand new, with almost no dust on the
HV leads.  The power-up self test also failed on this one, but for a
different reason: the tube had no beam current at all.  

I was able to get the second 5071A working perfectly with the standard
10890A tube from the other one.  At that point, I compared the resistance
and capacitance-to-ground readings between the two tubes' connectors to try
to figure out why the 10891A wasn't working.   The cause of the failure
wasn't immediately clear.  The resistance readings were comparable on the
oven heater, C field coil, and thermistor, nothing was shorted, and the
vacuum was good.  It passed all of the beam-tube tests in the 5071A

The only odd thing I noticed about the bad tube, besides the lack of beam
current from its output dynode, was the fact that its output was rather
microphonic, in a triboelectric sense.  Tapping on the bad tube's housing
with a screwdriver yields a nice ringdown voltage even with no power applied
to the tube (see http://www.ke5fx.com/cs_ring.png ).  I could even see
something resembling a speech waveform if I yelled at the tube (and believe
me, I tried that.)  With the good tube, there was no trace whatsoever of
this microphonic behavior.

So, this one has me stumped.  It's really frustrating because I've got to
assume that I've got a high-perf 5071A tube in near-mint condition that
would be a superb performer if it weren't for an apparently-simple internal
defect.   Perhaps it was dropped, breaking loose something in the EM
assembly that is now rubbing against a dissimilar material?  I'm going to
save this tube in case I ever have a vacuum rig of some kind that would
allow laparoscopic surgery to be performed on it.  Or perhaps I'll have a
chance to X-ray it at some point.

-- john, KE5FX
Miles Design LLC

> Cs depletion in modern tubes is a real failure mode!
> However some competing modes are:
> Ion pump failure (whiskers or high impedance shorts)
> High Cs backround level (Gettering saturation)
> Oven or ionizer filament failure (More common on older HP units with the
> AC excitation, (think of those neat light bulbs with the magnet inside
> where the filament jumps all over the place, less drastic mechanical
> flexing caused the filaments to eventually fail due to fatigue.)
> Slow leaks ruining the vacuum
> Running the Cs with the ovens off as described WILL delay some of these
> failures, (it's recommended that the ion pump remain energized however)
> A 5071A tube could be installed into a 5061A (with proper adaptors) but
> why?
> Later HP 5061A and 5061B "mini" tubes with the same diameter as the 5071A
> tubes can be installed into the 5071A with some caveats and proper
> adaptors.
> Corby

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