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How to Buy Hamfest Tubes
All,
Of all the Tesla coil parts that can be bought at a hamfest, vacuum
tube purchasing is perhaps 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