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Old 06-23-2015, 05:46 PM
N2IXK's Avatar
N2IXK N2IXK is offline
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Join Date: Apr 2011
Location: Sittin' on the "Group W" bench...
Posts: 799
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, 109 views)
File Type: jpg Base end.jpg (56.8 KB, 78 views)
File Type: jpg Internals.jpg (45.5 KB, 63 views)
File Type: jpg Label.jpg (51.7 KB, 64 views)
File Type: jpg Anode Water Ports.jpg (39.3 KB, 50 views)
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