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Old 06-23-2015, 05:46 PM
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RCA 8D21 VHF TV Transmitting tube--Silver Plated Geek Porn!

Here's a few pics of the latest addition to my tube collection, and it fits wonderfully here at VK. This is an RCA 8D21, the tube that made high power VHF TV broadcasting a reality.

2 of these guys were used in the RCA TT-5 VHF transmitter, which pushed out 5kW visual, 2.5 kW aural on the VHF bands. The tube itself is a unique device, and a great example of "Industrial Art". A functional sculpture in glass, silver and copper.

Being a push-pull dual tetrode, Overall, it resembles an 832 or 829 type on steroids. Still fairly compact at ~10" high, 5" diameter. But to get that much power from the 8D21, tube element is INTERNALLY water cooled, with 14 separate water connections for the various electrodes. The underside of the metal header contains the heavy bolt on connections for the 135 amp thoriated tantalum filament, as well as the connections for the control and screen grids. Each 2 electrode has 2 water connections on it for supply and return.

Internally, you can see the cooling pipework looking like a set of custom exhaust headers, as well as the hand-tuned neutralizing stubs used to keep this thing stable. The screen grid RF bypass capacitor is built in, as well.

This tube was an eBay find, and it saw service in the St. Louis area. Tests to be under vacuum and no shorts/opens, so I may actually try to light this beast up at some point. This is the second one of these I have come across in many years of tube collecting. The first one was passed along when a group restoring a TT-5 for a museum made me a very attractive offer for it. I am glad to have found a replacement for it, as it is a real nice display piece.

The tube was pretty badly tarnished when I got it, but a quick dip in Tarn-X fixed that. The label is unfortunately faded in the middle. The other tube I had used a red foil adhesive label on it, this one is printed right on the metal surface. Tube is Serial Number T220 Not sure how that correlates to manufacture date.
Attached Images
File Type: jpg 829 and 8D21.jpg (68.5 KB, 104 views)
File Type: jpg Base end.jpg (56.8 KB, 75 views)
File Type: jpg Internals.jpg (45.5 KB, 61 views)
File Type: jpg Label.jpg (51.7 KB, 61 views)
File Type: jpg Anode Water Ports.jpg (39.3 KB, 49 views)
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Old 06-23-2015, 05:53 PM
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more pix
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File Type: jpg Serial T220.jpg (40.0 KB, 53 views)
File Type: jpg Water tubing.jpg (52.4 KB, 55 views)
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Old 08-27-2016, 09:01 PM
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Beautiful. I've been looking for one for years to display next to my 7C24's. (RCA 5762)
Regards.
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Old 09-01-2016, 09:19 PM
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I have mine displayed next to my 6448, which is another bizarre-looking RCA tube developed for high power UHF TV.
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Old 09-21-2016, 08:52 PM
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I started my TV career as an engineer with a station that used a RCA TT5 transmitter. I remember that we used to buy Kotex feminine napkins to soak up water leaks in order to stay on the air when we had tube water leaks. I was assigned to pick up cases of Kotex at a janitorial supply store, for the transmitter site.
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Old 09-21-2016, 09:50 PM
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They ARE purty things, aren't they ?!?
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Old 09-22-2016, 09:06 AM
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Yes, they are.

In the overall history of tube development, the 8D21 was a technological "dead end", though. The complex water system needed to cool multiple internal electrodes made for constant leaks (The Kotex solution mentioned above was a typical stopgap measure to prevent lost airtime), and the 8D21 was soon replaced by air-cooled types like the 6166 which needed less attention and maintenance, and made more power with simpler circuitry (single ended rather that push-pull).

I have heard that a kit was made available to convert the TT-5 for a different type tube, but haven't found any details. Anyone out there know anything about this? It would have been a MAJOR retrofit given the physical construction of the rig.

The part of the 8D21 that did live on is the focused beam electron optical design, which was scaled up, and formed the basis of RCA's line of "Super Power" tubes which included the 5831 used in the 1 Megawatt US Navy transmitter at Jim Creek, Washington, the 6949 and 7835 used in large particle accelerators, as well as the 6448 UHF TV Tetrode. These tubes used more conventional thoriated tungsten filaments, rather than the thoria coated tantalum filament that was apparently unique to the 8D21.

Last edited by N2IXK; 09-22-2016 at 02:10 PM.
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Old 09-22-2016, 11:35 AM
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Cool looking tube!
Thanks for the mention of the Jim Creek station, can't believe that as a NW radio nut I had not heard of it before.

http://www.navy-radio.com/commsta/jimcreek.htm

impressive!
jr
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