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.