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Old 18th October 2010, 10:31 AM   #1
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Default Philbrick GAP/R K2-W question

Hi, I got a Philbrick GAP/R K2-W opamp module with the original 12AX7A tubes still in place. I have read quite a bit about the module and now I'm curious how rare this thing is. You almost never see it for sale anywhere, and if, then it's without the tubes. So how rare is it, also concerning the tubes (12AX7A ribbed plates)?
I am not planning to sell the module as it's kind of awesome to have one of the first commercial opamps, plus I'll probably give the tubes a listen once my tube equipment is running.
Any input other than The Philbrick Archive?
Also, notice how the spell "computor", like they're a bunch of haxxorz
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Old 18th October 2010, 04:04 PM   #2
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Not too rare in the homeland of Philbrick-Nexus, I have at least 5 of these of various models. Common hamfest fodder - I generally pass. They are full of carbon comp resistors so I assume now they would at the least be noisy and probably not work too well. They would seem useful for VCVS (Sallen-Key) X-O filters amongst other things. The need for a +/-300V regulated bipolar supply is a possible downside.. Unfortunately cracking them open for repairs/upgrades seems impossible. A bit of history.. I have one sitting on my desk at home with Tungsol black plate 5751s in it. (I had one with GAP/R 12AX7As in it, but have no idea what happened to the tubes - it is rare to see one with tubes still in it..)

Bob Pease wrote a great article on these devices and the power supply used to power them. It may already be on the Philbrick archive site, but if not look for it.
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Old 5th September 2011, 02:22 PM   #3
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Any idea what brand they may be?

The plates measure 17mm in height, have three ribs and tiny slots at top and bottom (for grid aligning maybe?). The bottom mica is single layer, top mica has an additional (clear mica) support between the two systems. D-shaped 45 horseshoe (top) getter, bottom of one tube has letter "M" embossed, other tube "R". No seams on top. The back of the tube reads: 12AX7A Made in U.S.A. 3226030-1.

I really like their sound, so I'd naturally like to know what brand they are. I'm guessing CBS, but I'm entirely not sure.


P.S.: kevinkr, thanks for the PM concerning the phono cartrige
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Old 5th September 2011, 02:55 PM   #4
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Any idea what brand they may be?
Taking a guess by looks I would say RCA.
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Old 5th September 2011, 02:59 PM   #5
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With such a tall plate and circular micas? Hm...but they do sound similar to pair of RCA 12AX7A from '64 that I have...
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Old 5th September 2011, 03:46 PM   #6
kevinkr is offline kevinkr  United States
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<snip>

P.S.: kevinkr, thanks for the PM concerning the phono cartrige
You are more than welcome..
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Old 5th September 2011, 03:57 PM   #7
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There were a lot of tube plants here in Massachusetts, and I am suspecting Philbrick-Nexus probably did not go far from home to source them.

Local tube manufacturers included Raytheon, Hytron, and others - it is possible one of these manufacturers made these tubes. I believe Sylvania may have also manufactured tubes in Salem Ma, but not sure about this..

Varian made more specialized tubes like magnetrons, and probably never made garden variety types like the 12AX7A.
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Old 5th September 2011, 05:13 PM   #8
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After a massive amount of googling I think I can say that they are Tung Sol USA. Look at the 7025 over at https://www.tubeworld.com/12ax7.htm#ecc83tung, it looks identical to the ones I have. Also the typeset and the format of the date code are the same as with the tubes in question.

Here is a recent ebay.au auction: Tung Sol long grey plate 12AX7A 12AX7 ECC83 matched pr | eBay offering two tubes, sold to someone in HK. From the description: "You are looking at two lovely long grey plate Tung Sol 12AX7A/ECC83's with square getters. The printing and logos are very good and they are branded for GAP/R Computor. They are identical Tung manufacture tubes and are coded 3226038-1and 3226030-1."

Mine also have the 3226030-1 date code, and according to Commonly Seen EIA Codes "322" stands for Tung Sol, "6030" stands for 1960, 30th week, and "-1" is the is the shift code (1 would probably be the day shift).

So there we have it, a 1960 Tung Sol 12AX7A, currently my favourite sounding tube.
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Old 5th September 2011, 06:14 PM   #9
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I believe you are right, and a great score in their own right.
Probably made in Newark, NJ..
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