Steve Thompson wrote:
>
> Mike wrote:
>
>>Hello,
>>
>>Is it okay to use the GS-35B in an upside down, vertical
>>position?
>
>
> Yes - assuming it's not full of loose stuff.
>
That's OK - the GS35 has an internal sieve:
http://www.pa3csg.hoeplakee.nl/tubes/tubes.html
Most transmitting tubes have the heater/cathode in the middle and the
anode around the outside. The GS35, GS31 and GI7 are not like that: they
are UHF-style planar triodes with a flat disk-shaped oxide cathode, a
domed mesh grid like a small tea-strainer, and a slightly dished copper
anode. Electrons are emitted across the whole circular area of the
cathode, come up through the grid and hit the anode. This construction
gives a very short transit time which is important at UHF.
It also allows the tube to be operated in any orientation and even under
quite high G forces in airborne equipment. (How they ever lift that lump
of copper off the runway is a totally different question.)
If you're looking at the photographs, note the reinforcing bars to
prevent the grid from flexing as it gets hot and expands. Also note the
extremely effective grounding of the grid, which makes for high
stability - no long, inductive grid leads here!
You can also see several arc marks on the grid and anode, which PA3CSG
says are due to that particular tube having been operated at 4kV+. Some
GS35s will operate fine at higher voltages, but this particular one
didn't like it and arced repeatedly. In more highly magnified versions
of those pictures, it is possible to confirm that each arc mark on the
grid is matched by an arc on the anode. The biggest of these arcs have
gone through the grid and burned the cathode.
Steve was making a serious point: if there is any loose cathode material
sitting on the grid, it can emit electrons if the grid becomes hot.
However, a good tube should have no problems with being operated
upside-down. The main reason why they don't normally design an amplifier
that way is not the tube, but the problem of blowing hot air out into
the small gap between the base of the cabinet and the desk.
73 from
Ian GM3SEK
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