"Antonio Vernucci" wrote in message
...
Having got an old Zenith battery-receiver with no tubes, a friend of mine
bought
two new tube sets including the expensive and hard-to-find 1L6 converter.
The
receiver works, but is fairly weak.

While checking the receiver, I determined that the 1L6 filament voltage is
nearly twice than it should be (about 2.6V against 1.4V, very dangerous!),
whilst the filaments of all other tubes are at nominal (1.4V). This is
odd, as
tubes are all in series. Trying the spare 1L6 gave the same result.

I thoroughly rechecked the circuit, finding nothing abnormal. I then
removed the
1L6 and replaced it with a resistor having the same value of a hot 1L6
filament
(27 ohm). Surprisingly, the voltage across the resistor was now correct
(1.4V).

So I built a little test jig, connecting the 1L6 directly to a 1.4V source
and
measuring its filament current. Instead of reading the 50mA, the current
only
was about 25mA, just half of what it should be. The spare 1L6 gave exactly
the
same result. I also checked my test jig with a different tube (1U4),
getting the
normal 50mA value. So I was sure that the 1L6s have a problem.

Being aware that there are two different series of battery tubes, i.e. the
50mA
series and the 25 mA series, I checked my tube catalog to determine
whether a
25mA tube exists that could be pin-to-pin equivalent to the 1L6. I found
it: the
1AB6.

Having read on the internet that selling fake rare tubes is getting a
common
practice, I would tend to conclude that the dealer sold my friend 1AB6s
remarked
as 1L6s (they are branded "DEC 1L6" in white).

I wonder whether any of you had the same experience or heard something
similar.

By the way I tried to cure the problem by putting a 56-ohm resistor in
parallel
to the (fake) 1L6 filament. Voltage then dropped down to the normal value,
but
the tube quits oscillating. As the 25 mA series tubes have a lower plate
current
that the 50mA tubes, my conclusion would be that the circuit parameters
are
unsuitable for a 25 mA-series tube. Nevertheless when the filament is
largely
overheated the 25 ma-series tube anyway succeeds to oscillate.

73

Tony I0JX

Tony,
Could I suggest you post this experience on rec.antiques.radio+phono ?
It could be an important heads-up for the many who own/repair/restore
Zenith TO models, among others.
Cheers,
Nelson