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CBS 12AU7?

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famousmockingbird

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Joined 2010
#1
I have a bunch of 12AU7s I was looking through to mail a couple to a fellow builder. I want to send something good but have little experience with 12AU7s, and I have no amp that uses 12AU7s to give a listen. I noticed when testing some of these that pretty much all of the CBS 12AU7s tested higher than all the other brands in regard to transconductance. The Tung-Sol, G.E., and Sylvania all test about the same, 2.5mA/V-2.8mA/V. The CBS test around 3.3mA/V. I have heard people talk about how the some of the CBS 12AU7 variants are some of the best, including the 7318, which has a slightly lower Rp and slightly higher gm. http://frank.pocnet.net/sheets/201/7/7318.pdf

Anybody have any experience with these tubes? If you had to use one in an amp which would you choose, G.E., Sylvania, Tung-Sol, or the CBS. I might just send him a couple to try but I figure I could get a little info on here.
 

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#3
kevinkr said:
I have had good results with Canadian made CBS branded 12AU7A..
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Hello Kevin, so these are Canadian made and just branded CBS? Who made tubes in Canada? Was it a G.E. plant in Canada? Sorry for all the questions.




kevinkr said:
Why not send him a couple of pairs of different tubes?
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I was giving them to him for free so I didn't want to send a bunch, but for free he could always just ship back the tubes he doesn't like in the amp. Good advice, thanks.
 
#5
kevinkr said:
I believe the were in fact made by Canadian GE, and they were actually quite good. I'm not aware of CBS being anything other than a rebrander, but they generally sourced their tubes from good U.S. and Canadian sources as far as I can tell.
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Thanks Kevin you confirmed my suspicions that they were made by G.E. The testing just kind of made me rethink who made them, they all test really strong. I will send him some of them, if you like them they must be good:up:
 
#6
G.E made tubes usually sport the tube type etched into the glass with a stencil outline. The CBS branded tubes pictured should be G.E. sourced as a result. Good tubes in general. There's also the 5814A which is a tighter spec 12AU7 and has a more sturdy structure. I use these in my 45SET amps. I've tried multiple brands including GE 5-star, RCA clear tops (getter on the side) Sylvania and likely some others (Mullard ECC82). Mostly I've stayed with the RCA clear tops, but I prefer Sylvania in many cases over RCA.

Note that several manufacturers offered a Broadcast Quality tube. GE had their 5-star line, RCA had Command Series and Sylvania had Gold Label IIRC. Some other probably existed as well, and of course the mil-spec JAN labels and commercial versions (4-digit numbers).

Regards, KM
 
#7
kmaier said:
Mostly I've stayed with the RCA clear tops, but I prefer Sylvania in many cases over RCA.
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Hello Kmaier, of the Sylvania brands I have some have green paint and some have yellow. I believe the green ones are older? They both have what looks to be the same construction, short gray plates with three ribs, and a D getter on top. I also have a couple Sylvania 6189 with 3 micas and short gray plates.
 
#8
Not sure on paint color (green vs yellow) regarding age. As Sylvania did supply other vendors with tubes, Zenith generally sourced from them and in the same vintage would always have yellow paint while the Sylvania marked one had green paint. Chances are they might have run out of paint now and then and used what was on hand.

The 6189 being a premium version of the 12AU7 would account for the additional mica (and sometimes thicker mica) and in some cases additional and/or thicker support wires.

Some tube types Sylvania excelled with and some RCA did. The same could be argued for Tung-Sol and GE, etc., then again many of the suppliers sourced from one another. I have a NOS pair of RCA 45 triodes in the original boxes with matching date codes. However, one is a true RCA made tube and the other is undoubtedly made by Sylvania.

Regards, KM
 
#11
kevinkr said:
..... I'm not aware of CBS being anything other than a rebrander, but they generally sourced their tubes from good U.S. and Canadian sources as far as I can tell.
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If I am not mistaken, CBS bought out Hytron, and was making tubes for a few years and just some types. But as you say they rebranded many other manufacturers tubes under their label to be in the market. And still later it was absorbed by GE. Cheers, Dak
 
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