John T. M. Lyles wrote:
>
>That broken GS35B that Tony showed the link to, was really trashed.
>Wonder if it had a series R to the power supply?
I've handled that particular broken tube, and looked at it carefully
with a magnifying glass. The photograph was taken by PA3CSG, who once
sold a few HB GS-35B amps for 2m and 70cm. The tube was returned by a
user who had driven it to about 2.5kW output with a B+ of about 4.5kV...
and then wondered why he'd had multiple flashovers.
I don't know whether he used a series R, and there is no way of knowing
if it ever was a good tube. It certainly wasn't when it came back!
>Looks like a lot of stored energy dumped from the cathode or grid to
>anode in that tube. I thought those Russian tubes were coaxial geometry
>but they look like planar design.
>
The GI7, GS31 and GS35 are all planar. As Tony's website points out,
these tubes were made at a factory in Novosibirsk, and are clearly
original Russian designs (unlike the tubes from St Petersburg or
Ukraine, which are coaxial geometry and look awfully similar to Eimac
and other western designs).
Looking very closely at the individual burn marks, they fall into three
classes:
1. Light burn marks on the grid, with slight discoloration of the
cathode immediately underneath.
2. More severe version of #1 with dark brown or black marks on the
cathode.
3. A hole burned right through the grid mesh, and a black crater in the
cathode underneath.
My speculation is that when the arcs from B+ hit the grid, there was a
transient rise in voltage at that spot on the mesh, so the discoloration
of the cathode immediately underneath the burn mark was due to excessive
electron current density. In the really severe cases, the arc burned
away part of the grid mesh and struck clear through onto the cathode.
--
73 from Ian GM3SEK
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