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