"m II" wrote in message
news:UPc9d.37987$N%.31812@edtnps84...
RHF wrote:

DG,

Here are a few Vintage Radios on eBay:

Lots of nice stuff out there. I wonder about the tube supply though..how
hard is it to get the things these days?

There's still plenty of tubes. Most radios used common tubes, which were
produced in high volume. The military has surplused out pallet loads of
mil-spec radio tubes over the last couple of decades. Also, since the tubes
were common to most of the makers, there's still bushel baskets full of good
pulled tubes nobody ever threw out. I've bought plenty of used tubes at
hamfests, and nearly all are good. I usually pay fifty cents to a buck for
used tubes I know I can use. New radio tubes usually go for two to five
bucks. Some audio output tubes and rectifiers might go for more. There's a
couple of battery set tubes which are now pretty expensive.


I haven't looked lately, so I don't know supply or pricing.

Mostly it's not a problem If the radio tube supply ever gets tight, people
will be able to modify sets to use solid state replacements or TV tubes.
Most of the TV tubes are almost give away items now. If those TV tubes
don't get thrown out, somebody might be using them in radios in a hundred
years or so.


A long time ago, someone suggested a variable power supply for the
filament supply..the idea being that you could heat up the filaments in
a slow and orderly fashion and thereby extending their life
tremendously. It seemed to make sense as most lightbulbs fail at the
moment of being turned on. There are enough similarities that a
comparison may safely be made.

Don Klipstein says a soft start device might improve bulb life by only a few
percent. I'll take his word for it:

http://members.misty.com/don/bulb1.html#mll


Anyway, the heater of a tube runs much cooler than the filament of a light
bulb and ought to last much longer. I know there's a formula for
calculating the life of a filament vs. temperature. I don't know what it
is. I think there's a fourth power relationship, however. A little
temperature reduction increases lifespan dramatically.

Tube heaters run cool enough that they don't evaporate much material during
their lifespan. I think the heater failure mode is a little different than
a light bulb. The number one cause for heater failure in the AC/DC sets
I've worked on is a heater to cathode short in another tube.

Tubes fail, but they don't usually fail by opening up the heater. Emissions
go down as the tube ages. Heater to cathode shorts are a common problem.
Gas can get into the tube Grids can get damaged by overheating. Tubes
which use soldered pins can have bad connections.

Still, radio tubes are generally reliable. I think small signal tubes have
been estimated to have an average lifespan of 50,000 hours, longer if you
can accecpt reduced performance. My Hallicrafters SX-99 (1957) still has
all it's original tubes, and they all work fine.

Frank Dresser