Vacuum tube heater

Ron H

Joined Apr 14, 2005
7,063
It's been a LONG time since I've fooled around with 12AX7 tubes, but I remember the heaters going up inside the cathode in a rather rectangular loop. You could see the middle of the loop of each of the two heaters glowing red-orange near the top of the tube.

I busted one apart some 40 years ago to have a look at the "guts".
OK, that's what I couldn't envision. If that's how they are made, then Cdrive wins.:(;)
 

Adjuster

Joined Dec 26, 2010
2,148
Vacuum Tubes are definitely fragile as compared to solid state, which makes the 'Proximity Fuse', .... (one of the United States most closely guarded secrets during WW2), seem like a <snip> near an impossible undertaking. For certain, they're not so fragile that one has to worry about breaking the vac seal or connection from the base pins merely by the pressure of test probes, as stated elsewhere in this thread.... Unless you're King Kong! :D
I'm not quite King Kong, but would admit to not being the most dexterous of individuals. Since the pins on a B9A based tube like 12AX7 are thin and quite closely spaced together, it is not beyond the realms of possibility to bend them when using meter probes. I have done this myself - the valve did not break at the time, but lost its vacuum when powered subsequently. Of course, it may have had some pre-existing flaw, but having had this experience I wanted to be cautious in my advice to the OP.
 

SgtWookie

Joined Jul 17, 2007
22,230
Metal pins through molded glass are still used for applications where hermetic seals are required, but placing undue stress on the pins can and will cause the hermetic seal to fail. One should be very gentle with tubes/valves when handling them to minimize the stress on the pins. If the vacuum seal is compromised, the tube will fail in short order.
 

CDRIVE

Joined Jul 1, 2008
2,219
I'm not quite King Kong, but would admit to not being the most dexterous of individuals. Since the pins on a B9A based tube like 12AX7 are thin and quite closely spaced together, it is not beyond the realms of possibility to bend them when using meter probes. I have done this myself - the valve did not break at the time, but lost its vacuum when powered subsequently. Of course, it may have had some pre-existing flaw, but having had this experience I wanted to be cautious in my advice to the OP.
It's more likely that current new stock are probably Chi-Com knock off-s and not quite up to the QC standards of.... uh... my day, or maybe glass is in short supply there?:confused::D
 

CDRIVE

Joined Jul 1, 2008
2,219
One should be very gentle with tubes/valves when handling them to minimize the stress on the pins. If the vacuum seal is compromised, the tube will fail in short order.
Back in the late 60s - early 70s I worked in a lab that produced thick & thin film hybrid integrated circuits (Log Amplifiers) used in missile guidance, Radar and ECM. We didn't use standard test probes, as most are familiar with. Instead the company purchased very high quality (Starrett) machinist dividers and gave them to our machine shop for modification, where they would then insulate the two legs and attach test leads to them. Great for very close and delicate work. I'll have to duplicate it one of these days. They never accidentally slipped of a connection and shorted an adjacent pin, as they had extremely sharp tips.
 

marshallf3

Joined Jul 26, 2010
2,358
We called them 'Pencil Tubes', as they were only about 3/8" in dia by 1" tall (ARC-34's) went into F4 era air craft and plate voltage was supplied by Dynamotors!

Signed.. Older Than Dirt ;)
Remember them well, quite a few were produced and many were simply embedded into modules so they could easily be changed out. You could have an entire IF stage in a module.

Now, onto the RCA Nuvistors .....
 

CDRIVE

Joined Jul 1, 2008
2,219
Now, onto the RCA Nuvistors .....
While researching the RCA Tube Manual for the 12AX7 filament issue, I discovered my old layout drawing for the 15 meter Nuvistor preamp, folded between the pages for the 6CW4. This is a bottom view of the aluminum, home brewed, chassis. In those days nearly everything was point to point wiring. PCBs were just becoming the order of the day. Because of this, layout was critical, especially at higher frequencies. The oval like shapes are the Nuvistor sockets and the rectangular objects next to the BNC IN/Out jacks are the bottom of 50 ohm IN/OUT resonant cans.

Back when I tested this amp for gain and band pass, I also measured the Input and Output impedance using a Gozinta; sometimes pronounced "Gazinta", depending on local dialect. :D

Hey, what's the status of this topic? Is it working yet?
 

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