Mislabeled Tube Misadventure
 

This is an RCA tube with no discernible identification, which I had labeled “35Z5” at some time in the past. After unsuccessfully trying to use it in an AA5 I noticed that it had only 5 pins--a good indication that it is not a 35Z5GT which has 6 pins.

https://lh3.googleusercontent.com/-l...%252520100.JPG

For high-resolution image go here and click on photo:
https://goo.gl/photos/fDdyFv4PuBPvP1dp8


So, just out of curiosity, what is it? It tested good as a 35Z5 rectifier on transconductance and emissions testers. Pins 2 and 7 are the heater, 31 ohms. Pin 3 taps the heater at 7 ohms/24ohms. Any guesses as to what it might be? And does anyone remember WHY I labeled it “35Z5?”

Apparently the next tube in the series, a 50A5, was injured. As far as I can tell, the 50A5 heater got 120 volts instead of the specified 85 volts. Now it tests as zero transconductance and zero emissions. The heater of course doesn’t light up, but curiously it still shows continuity (31 ohms) across the heater, just as it did before this misadventure.

The newer issue 35Z5's only had 6 pins, that's all the tube needs to operate.
Pin 5 is the plate, pin 8 is the cathode. 2&7, the heater with the pilot tap at pin 3, as you stated.
I looked at the tube manual and the only other tube with the same pin-out is the 45Z5, higher voltage, but the same heater current, 150ma.
Evidently, the heater is damaged, as it draws more than 150ma and wont operate in series with the rest of the tubes. I don't think, the problem will show on a tube checker, as the 35 volt setting is a tap on the power transformer. :scratch2:

I must have cleaned off legible markings when I inscribed "35Z5" on the base. The 50A5 is, (of course) the only tube I don't have a spare for, but fortunately is cheap and available. I may dissect the tube to see if the the filament defect is visible.
Thanks, that's very helpful.

I would try a thorough cleaning of the 50A5 pins.... 31 ohms sounds about right, perhaps your sharp ohm meter probes are making good contact, and the sockets are not? Pin corrosion on loktals is pretty common problem.

jr

Common problem indeed. I manually cleaned and used CRC cleaner on each pin and the sockets for every tube. But I haven't addressed the tube tester sockets, and I'll try that before destroying the tube.

Often I will first use a small scalpel blade (#11) to manually scrape the corrosion of of the pins then apply a little D-5 with a q- tip.

jr

More of the 50A Saga
 

I cleaned the pins and sockets for all the tubes before installing them, but I repeated the cleaning of the questionable 50A5 and cleaned the socket in the tube tester--600 grit sand paper and CRC Electronic Cleaner with a pipe cleaner for the tube sockets. Same result; the heater doesn’t heat even though the meter shows 30 ohms across the filament.

I took the tube apart. This what this beam pentode looks like inside.

https://lh3.googleusercontent.com/-o...%252520103.jpg

I got down to the filament enveloped by the cathode, and it still showed 50 ohms across the filament. I couldn’t extract the filament because…

https://lh3.googleusercontent.com/-W...%252520106.jpg

I don’t know what the white substance is, but it apparently coats the filament to insulate it from the cathode and the cathode is covered to insulate it from the screens. When I touched the white powdery stuff with an instrument, it released an invisible gas. Really! I could see nothing in the air, but it was extremely noxious--caused instant cough if I inhaled at all in the vicinity. I sealed it in a plastic bag, opened the windows and let the room ventilate for two hours before I reentered. I looked at a 50B5, and it has the same white coating on the filament. I waited 24 hours to be sure I was still alive before I posted this.

Two different materials with two different purposes... the white coating on the heater is alumina, applied to insulate the heater from the cathode... the white material on the cathode is barium carbonate which is applied to emit electrons. I don't recall any particular noxious fumes from either substance, when taking apart cathode assemblies.

jr

note: during processing on the pumping station, co and co2 are given off and mostly barium oxide is left on the cathode nickel surface.

Thanks. I think I dispersed some finely powdered alumina, not even visible, which was choking and stayed suspended for a minute or two. I couldn't resist taking the cathode and filament apart. I put it in muriatic acid for a minute to get rid of the barium oxide, then pried apart the cathode to remove the alumina-coated filament. For the record, it was a 36-inch staggered-apex folded filament.

A pdf about the heater coating process linked here:

PDF]The Material and Shapes of Vacuum Tube Heaters - tubebooks.org
www.tubebooks.org/file_downloads/Heater.pdf

jr

Indeed. That's where I got the information I posted.