[Amps] Horizontal or verticle?

[Ian White, G3SEK]

Richard wrote:
>There is a bit of debate between Polish and Ukrainian amp builders.
>
>The some members of the Polish school argue, based on some American 
>literature, that a valve (ceramic) should be based vertically, while 
>some well known Ukrainian builders, have there GU74b's amps horizontal.
>
>Now some Poles maintian horizontal is bad as, as for lack of a better 
>word, "pollutants" which occasionally fall from the anode land on th 
>grid, rahter than at the base of the valve as would be the case if the 
>valve was verticle, and this eventually causes a short in the valve.
>
>My feeling is that rather the cooling problem is the issue here, but I 
>wonder s the valves were designed with the though in mind of working in 
>any positon.
>

Which "American literature", one wonders?

A quick flip through the Eimac data for ceramic tubes (and also some 
Siemens data) reveals "Operating position: Any"... which is kind of 
helpful for use in aircraft.

The Russian data for the GU74b includes specific acceleration limits, 
but doesn't specifically mention operating positions. However, when the 
same tube becomes the 4CX800A, the Svetlana data sheet says "Any".

It's obviously true that any loose bits are more likely to lodge in the 
grid structure of a horizontal tube... but VHFers have been using 
ceramic tubes in horizontal cavities for many years, with no notable 
increase in failure rate. If there is a difference, it's not a big one.


Regarding Hal's awful experience with the runaway GU78, that's some 
heavy tube. If you plan to ship the amplifier with such a tube in its 
socket, it really does make sense to follow the military example and 
include some kind of mechanical hold-down or clamp.

With lighter tubes, there is also a possibility that the grid contact 
rings are trying to force the tube back out of its socket, and over many 
repeated heating/cooling cycles they can sometimes work their way out.

A simple and reliable hold-down is a teflon post screwed to the top 
cover of the amp in the centre of the vent holes, just long enough to 
touch the cap of the tube.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek