[time-nuts] CBT EOL

[David Welch]

Dave,
I had a similar concern about a brand new 5071a I just purchased,This unit
is dated may of 05 and was first powered up Jan 2 of this year..the ion pump
has remained constant at 6 ua but I the electron mult volt has been dropping
about 1-2 volts per day since then.it started out at 1525 and is now at 1297
and still falling all the other parameters are fine but I noticed the osc
freq control has been dropping also it is now at -6.84,started out
at -5.50..do you think these is a problem with this tube?it is a high
perf..should still be under warranty as the manual says there is a 5 year
warranty on tubes 4240 prefix..
have you ever run into this before?any suggestions..Thanks David..
----- Original Message ----- 
From: "Dave Carlson" <dgcarlson at sbcglobal.net>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Monday, February 04, 2008 9:18 AM
Subject: Re: [time-nuts] CBT EOL


> Hello,
>
> Speaking not from a physicist perspective, but from being in the business
> for over 20 years...
>
> The mechanism depends somewhat on the amount of flux that has been
traveling
> through the CBT, which determines if the cesium becomes exhausted before
the
> electron multiplier first dynoe "wears out".
>
> In general with the HP/Agilent/Symmetricom CBT there is enough getter
> material inside to trap contaminants well beyond the time that the cesium
> should run out in the oven. As long as the Ion Pump is working, internal
> contamination should be the minimal contributor to EOL.
>
> Whether the cesium runs out or the Electron Multiplier first dynode loses
> its coating is largely dependent on the amount of flux. The
High-Performance
> CBT run at a hotter oven temperature to gain better S/N and thereby
exhaust
> the available cesium at a faster rate. In these CBT the EOL mechanism is
> more likely due to that. As the cesium runs out the S/N degrades.
Increasing
> the EM voltage can help, but eventually you're amplifying more and more
> noise.
>
> In the standard-performance or long-life CBT the EOL mechanism is more
> likely due to the first dynode of the EM becoming depleted of its coating,
> than buildup of internal contaminants.
>
> Dave
>
> ----- Original Message ----- 
> From: <wa1zms at att.net>
> To: <time-nuts at febo.com>
> Sent: Monday, February 04, 2008 6:39 AM
> Subject: [time-nuts] CBT EOL
>
>
> When a CBT reaches end-of-life, is it because the source of Cs is used
> up? Or is it due to contamination of the electron multiplier?
>
> I've heard both as causes but wanted more info.
>
> I have a CBT with very low beam current and it keeps falling. I'm
wondering
> if running a higher than normal EM voltage might help keep it on
> life-support.
>
> -Brain, WA1ZMS/4
>
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