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Buying Hamfest Tubes



All,

Of all the Tesla coil parts that can be bought at a hamfest, tube purchasing
is probably the most challenging.  Here are some checks that can be done to
prevent the loss of your hard earned cash:   

1.  Visual inspection.  Check for a darkened  glass envelope which means the
tube had a lot of use and may be near the end of its life and may have low
emission resulting in weak TC sparks.  The darkening often occurs in an
uneven, and spotty or striped fashion.  Hairline fractures in the glass may
occur at the pins or terminals, this will let air into the tube and ruin it.
 Discoloration of the plate due to overheating; this may or may not be a
problem.  Try to look down between the plate and grids to see if any of the
grid wires are burned through or bowed inward or outward, such a tube may
short out during use, if it's not already shorted.  Sometimes a tube may have
been dropped, causing the plate to bend out of alignment, or may have even
torn loose the thin metal strap connections within the tube.  Check for
concentricity of plate and grids.   Next look for any loose metal or glass
debris within the tube  which is usually a sign that the tube is worthless.
 Many times sellers take years to sell their tubes, or store them for years
and they get dropped over and over again, it doesn't take too much shock to
break a filament.  You can often see pieces of the filament lying inside the
glass envelope.   Certain tubes such as the 3-500Z, and the 4-250A have a
smooth metal reflector beneath the grid-plate structure, this reflector will
become darkened and "burned" in tubes that have seen many hours of use.  This
"burning" occurs near the center of the reflector where it's sometimes hard
to see.  Also observe the silver getter mark, if the tube has one.  If it's
milky colored, the tube probably has air in it and is no good.

2.  Electrical tests:  Check the filament for continuity using an ohmmeter,
however some tubes use parallel filaments, they'll check good even if half
the filament is burned out--the 833A is of this type.  Maybe a good low-ohms
meter could detect the difference but I haven't tried this.  Check between
all the other elements; plate, control grid, screen grid, suppressor grid,
all should be open, or the tube is shorted.  Some tubes normally have more
than one pin connected to the same element.  Use a tube manual to determine
what to expect for a particular tube.

3.  Even after all this checking, the tube could be gassy (gassy tubes will
show a bright blue glow inside during use, and will draw tremendous current.
 A mild blue glow may just be phosphoresence of the glass envelope and is
normal for some tubes) or have low emission,etc.  So if you pay more than a
few dollars, ask the seller for his phone number so you can contact him in
case the tube is a dud!

Happy Tube Hunting,

John Freau