[Milsurplus] Bringing up old klystrons

[J. Forster]

Let me have another go at this.

First, the power supplies do have to work properly and the voltages have
to be in spec. Let's consider that a given.

Now, onto the tube itself.

This is an extrapolation of what I've personally seen with receiving
tubes. I've no reason to believe the physics is any different with
klystrons.

First, there is no reason to believe either the cathode or any other
elements degrade over time in a cold, unpowered tube. The only thing AFAIK
that deteriorates is the tube vacuum. If the vacuum degrades a bit, the
tube goes gassy; if a lot the tube is scrap. So, let's talk about slightly
gassy tubes.

In receiving tubes, gas degrades performance, so a "getter" is built into
the tube, like Barium or Calcium...  reactive elements. However, these
reactive elements only interact very slowly, if at all, with neutral gas
molecules, like N2.

In order to increase the vacuum cleanup, the neutral gas molecules have to
be ionized. Just powering the filament will not do it. In my experience,
there has to be an accelerating potential in the tube, so that electrons
cooked off the cathode are accelerated to the point that, when an electron
collides with a gas molecule, the colision is ionizating. This means a B+
of roughly 200 VDC.

I learnt this because applying filament alone to a tube for a couple of
weeks had no effect. Filament and B+ improved tubes a lot.

What I've done with gassy receiving tubes is to run them at nominal
filament with a couple of hundred volts B+, and adjusted the grid voltage
so the plate current is about 20% of the normal spec. This provides enough
electrons for the getter to work well. Higher plate current drives too
many positive ions to the cathode, poisoning it.

I see no reason a Klystron should be any different, so I'd suggest
applying filament and about 200 VDC, current limited, to the tube.

With a klystron, i'd think it important to degas it fully before applying
a high repeller voltage to avoid damaging internal breakdowns.

FWIW,

-John

=================



> Peter:
>
> John hinted at this, but I will say it plainly:  The klystron is no
> problem, the power supply voltages are.  Make sure all of them are within
> specs (pull the critical tubes and connect temporary loads) before
> exposing the tubes to them.  They all need to be up and running, not too
> high nor at zero.
>
> 73,
> George
> W5VPQ