Further Adventures with the EMC 213

[Winky Dink]

In our last episode I fried a 1B3GT with excessive filament voltage. After that I spent some time learning about vacuum tubes and tube testers. Then I ordered some NOS tubes, including a 1B3GT, from Antique Electronic Supply. When I tested the newly arrived 1B3GT it failed the "short" test. Since I had an older 1B3GT which tested OK, I assumed the newly arrived tube was bad and emailed AES right away. AES replaced the bad tube immediately without question, but the new one also tested bad. This time, before blaming AES, I took a closer look at all the 1B3's I had, and I dissected the oldest one, an RCA tube from the 1950's.

For this tube, the EMC checks for shorts between pin 2 and each of the other pins, so a good tube shows a short on pin 7--i.e., through the filament. So how could the newer tubes show shorts on pins 3 and 8? A closer look at the GE and Sylvania tubes...

...shows internal connections for pins 3, 5, and 8. So I guess my manual was never updated for these modifications.

If anyone has read this far, can you explain to me why the 117L7 (below) shows no short between pins 2 and 7--either with the tube tester or a multimeter? I have an RCA and a GE, and they both show good emissions but no continuity across the filament.



Thanks much,
Winky.

[miniman82]

My tester says to ignore certain short conditions, especially on HV rectifiers. Are you sure yours doesn't have similar verbiage?

[Winky Dink]

Only instructions for 1B3 are, "Glows at 2, 7. Press to 'S'. OK over 18." General instructions are, "The N-S slide switch is normally kept in the 'N' position unless the tube chart indicates otherwise." The tube chart is a 1977 reprint. Maybe it was never updated for this tube because most users would recognize and understand the difference in the newer tubes. Thanks.
-Winky

[Reece]

The 117L7-GT heater is rated at 117 volts at 0.09 amps, which means that in operation the heater resistance is about 1300 ohms. It will be less when cold. As such, it won't be enough to show as a short on your meter, but should show on a resistance scale.

[Winky Dink]

Thanks, Reece. Indeed, the cold heater shows 196 ohms.
- Winky

[N2IXK]

The unused pins on vacuum tubes can be designated as either IC (internal connection) or NC (No Connection).

NC means exactly that--nothing inside the tube is connected to this pin. The circuit designer is usually free to use an NC socket terminal as a tie point for component support.

A pin marked IC or "Do Not Connect" is usually connected to "something" internally (typically used for physical support of the internal elements in some way). Exactly what (if anything) is connected to these pins can change with tubes of different manufacturers (who may use different structural arrangements for the same tube type) or even with different revisions of the same tube design from a single manufacturer.

In the specific case of HV rectifier tubes, the tube manuals will tell you which pins are "usable for wiring at or near filament voltage". These pins were occasionally used for a series filament resistor when a single turn on the flyback didn't produce enough voltage, but a second turn produced too much. A current limiting resistor in series with the CRT anode lead was another possible use for these pins.