Hi, I don't know what these early pre-glass vacuum tubes are called. It is an RCA 5Z4. I did not test it because I do not know if the test requirements of a pre-glass tubes are the same as the glass or metal vacuum tubes. I cannot find any info on the internet about this type of tube. It appears to be unused or NOS. Does anyone know when these were made? It is about 5 inches tall in a metal case with a lot of holes in it for air flow. Is there any value to it? Thanks for any information. Bob

RCA introduced a set of metal vacuum tubes in 1935 of which tube collectors call today the "original nine". This tube was one of the "nine" tubes introduced. Internally it has two separate long metal tubular plates with each plate evacuated and having a filament within it. The metal tube plates have a glass seal on each end with the filament protruding out. The plates are the anodes and are "hot" both physically and electrically so they were enclosed in the metal perforated envelope. It was considered a bad idea to have the electrically "hot" plates where a little piece of wire, hardware, etc. could fall in and short the plate elements. Shortly after they were introduced, RCA remanufactured the 5Z4 to have a tall non-perforated metal envelope like the envelope of a metal 6V6, 6F6, 6D5, etc.

It has the same characteristics as the later production 5Z4's and can be tested in a tube tester as a 5Z4. While they "don't grow on trees" as the saying goes, there are enough around that any serious tube collector should have been able to find one. I consider them uncommon but by no means rare.

Rereading my above post I should have said the connection wire from the filament extrudes through the glass seals. The actual lighted filament terminates inside with only the connection wire extending out. (Just trying to be technically correct.)

I might add that the British actually beat us to the metal tube with their Catkin tube. The early version of the MH-4 was a half-wave rectifier with an truly exposed anode mounted on a British style 5-pin base. The metal anode looks like an elongated space capsule and is dangerous to the touch since it is "hot" both electrically and physically. Realizing the problem associated with the exposed anode, the British put a protective perforated metal over the tube. Who knows, maybe RCA got their idea of the perforated metal envelope from the British Catkin tube.

Bob, your comment intrigued me so I looked it up.

http://www.r-type.org/exhib/aab0040.htm

Amazing someone made a tube with the plate exposed like that.
Also it appears to be a triode. I assume a power output triode?

-Steve

The MH-4 is a general purpose triode used as an amplifying triode, detector or oscillator. Here is a link to the later version that has the protective perforated metal envelope. http://www.r-type.org/exhib/abl0002.htm

Yes, and much more decorative than the version RCA came up with

-Steve

The original 5Z4 tubes with the perforated metal shells were unpopular for a couple of reasons. In humid weather there would be a tendency for them to ionize the air around them and produce corona, along with ozone. The corona was more of a nuisance as it would cause noise or static on the radio. As I recall, RCA had a replacement program for a while where anybody dissatisfied with a perforated 5Z4 could bring it back to a RCA dealer and exchange it for a new solid shell one. It's amazing as many of them survived as there seem to be. The other common problem did not turn up until more recently; some of the metal-glass seals were less than perfect and the tubes are often gassy when tested today.

Whether the tube tests good or bad, I would keep it as an oddity and not try to use it. There are plenty of newer style 5Z4s available if you need one for a radio.

IF you're interested in parting with it, I'd love to have it for my tube collection.

One other problem with these is that they are often found with broken locator pins, which were easier to break off due to leverage from the tube's height.

This was 50+ years ago and the memory isn't as sharp as it use to be. In the service I serviced the IFF/SIFF equipment for the radar. It had 3 round metal output tubes lined up in the transmitter side. that just fit in the holes. The top was the plate I think, had fins for cooling and stuck up maybe 1/2 ". It got narrower as it went down and each section touched metal sections in the socket. Like an up side down Christmas Tree. I don't remember any pins. The top exposed area was electrically hot and I mean a lot of voltage. Voltage readings had to be done on the equipment twice a day and put in the log. I used just a small pocket screw driver that had the neon bulb inside. Hold onto the pocket clip and touch each cap and it would get brighter as you moved across. I just copied the numbers that were above if brightness looked OK. Didn't need to check the other voltages.
Would this tube be classified as an air-flow tube? Ron's our tube expert and maybe he knows what I'm talking about or someone that was in the service and worked on the old GPX-8 IFF systems. Every branch of the service used this same old box.

Freeman

Freeman, would these tubes you mention look like this?

-Steve

Among tube collectors, the nickname for that 5Z4 is the "birdcage" .

I have a Stewart-Warner R-1361 which must have had a 5Z4 meltdown at some point. The rectifier tube, power transformer, and electrolytic capacitors were all replaced, and the work was done so long ago that the replacement transformer is a genuine S-W part. It was an expensive repair job for someone!

Hopefully it happened under warranty.

-Steve

The fins were a little thinner metal, but the concept is the same. A little turn I think and you could pull it right up. Any time we went on a deployment and were setup, I would always reseat them. I must have had a flaky problem sometime and reseating them cleared it. All the other tubes were held in place with the twist off tube cans, so they never moved. Google helped me find the box, but no pictures of the inside. Everything we had was classified at the time and the military is always a little slow declassifying anything.

Thanks for looking.
Freeman