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Got a batch of 50's-60's 12ax7's

skhan007

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Joined
Feb 8, 2007
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1,670
I just got these and can't wait to try them in both my 1987x Marshall plexi reissue and my GDS 18 watt 1974x clone. I was wondering if any of you have any opinions on the following 12ax7's and recommendations (phase inverter position, first gain stage, tone stack, etc.).

L to R:

1. Black plate RCA dates to Sept. 1957
2. Grey plate RCA 1960 43rd week (Oct.)
3. Long plate Bogen by Mullard dates B9A, Blackburn Jan. 1959
4. Telefunken made in Germany, unknown date. On the top it has a "913" stamp
5. Zenith made in Holland (Amperex, I think), unknown date. Has "6 21" stamp
DSC_0003-12.jpg
 

mikev

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Joined
Oct 19, 2007
Messages
1,124
I just got these and can't wait to try them in both my 1987x Marshall plexi reissue and my GDS 18 watt 1974x clone. I was wondering if any of you have any opinions on the following 12ax7's and recommendations (phase inverter position, first gain stage, tone stack, etc.).

L to R:

1. Black plate RCA dates to Sept. 1957
2. Grey plate RCA 1960 43rd week (Oct.)

I have a grey plate like that in V2 of my Deluxe Reverb. Sounds AWESOME. And a black plate like that in V4.
And another black plate like that in V1 of my Blues Junior.
My friend and I A/B'd our Juniors in his studio today and the difference was shocking.
His sounded thin and weak with all russian tubes. The only difference was the black plate in mine.
 

Jeff West

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Jul 29, 2002
Messages
877
Hi Skhan007- All those are nice types, IMO, including in Marshalls! The Mullard f92 tends to be one of the best sounding Mullard longplates in guitar amps, I think. The Zenith looks to be a Philips Dutch Heerlen "Amperex" all right, Zenith branded quite a few of those late '50s thru '60s including '50s longplates. The "6 21" indicates 21st week of '66 but that was put on by Zenith before they shipped it, the Heerlen "delta" codes from (earlier) manufacture should be on there too, something along the lines of "I65 45L3" or thereabouts, maybe, where the "4" is the delta.

If the Telefunken has the date codes intact (they rub off), we could interpret, either a pair of small letters or a long sequence of digits in white. The "913" was for warranty purposes only. I'm thinking that's a rib plate, right?

I'm curious about the exact date code on the RCA if you don't mind posting it, is it "60-43"? or two letters? 1960 is early for short grey plate RCA, that may be one of the first ones! Incidentally, as you may know, RCA had a big plant in Indianapolis that manufactured lots of octals, rectifiers and some miniature receiving tubes. Judging from the octagon, not that particular specimen, however.

Enjoy-

Jeff W.
 

skhan007

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Joined
Feb 8, 2007
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1,670
Hi Jeff,

Thanks for this great information! Much appreciated. The Zenith Holland tube has no other markings on it besides the Zenith marketing "replace only with a genuine..." script. I do have a '66 Amperex Bugle boy with all the correct Heerlen delta coding, so I'm familiar with that nomenclature, but this tube has nothing beyond the yellow writing you see in the pic above.

With respect to the Telefunken, on the opposite side of the logo, there's a smudge that's unreadable. The "913" is on the top and opposite that, you can see three symbol/digits with the last one being a "6"- the other two are practically gone. It is a ribbed plate!

As for the RCA's, the black plate has the KO, which I'm told is Sept. 57. The grey plate clearly says Made in USA with 60-43 underneath that. This makes it an early one? I love learning about this sort of thing. Makes me feel like an archeologist uncovering some ancient clues!

As far as a tone update: I put a 1958 Mullard long plate (that was microphonic in my 18 watt combo) in V1 of my Marshall 50 watt plexi along with the Telefunken in V2 and Zenith Holland in V3 (those two pictured above). I'm not sure which is responsible, V2 or V3, but all of a sudden, my tonal range came alive in my plexi. I'm able to dial the bass up to Noon to 1 O'Clock and no flubbiness. Fat thick bass and the mids are much richer sounding. High end hasn't changed.

I put the 1957 black plate RCA into V1 of my 18 watt and the Jan. 1959 longplate Mullard into V3 (V2 is my phase inverter in this amp and my '66 Amperex bugle boy remains as does my bugle boy tube rectifier). Oh, the tone of the long plate Mullard!!! Fantastic!! My 18 watter rules!!

I cannot say enough about how old tubes have made my two amps come to life!!
 

Jeff West

Member
Joined
Jul 29, 2002
Messages
877
Thanks for info, skhan007. I'm glad the tubes are sounding good to you.

Yeah, late '60 is about the very earliest you'll see on those short plate RCAs, I think. The longer grey plate RCA 12AX7s and 7025s which immediately preceeded sometimes have date codes into 1961, and also the (different construction) long blackplate 7025s (which are exceptionally cool sounding in some amps, BTW). RCA printed these codes on in the warehouse when they determined where the tubes were going, not in the factory right when they were made (unlike the Philips etch codes).

If you're really into the archeology, there are more details in there too. For example, you're right that "KO" on the BP indicates 9/57, and the two letter date codes at that time indicate that it was an RCA branded piece for retail by a tube vendor. The fact that the shortplate has "60-43" instead of "LZ" for "10/60" indicates it was destined for OEM use instead and the dash between the digits specifies aspects of the warranty!

Also, as I mentioned the factory, during the time frame of those you have there there were four RCA factory locations producing receiving tubes, and they coded hidden info about this (and other things) onto the tube, often into the octagon. Many (but not all) of the classic RCA 12AX7s and 7025s of the '50s and into the '60s were made at the Cincinnati plant, in fact I think I can see the "tell" for that on your blackplate, it's a little extra "tab" or defined thickening on the etched line of the octagon on the lower right corner along the bottom. If that "tab" was etched on the upper left corner at the top, it would indicate Indy. I don't think I see it on that one, but if there were an etched "dot" floating above the octagon and over the tab then that would further specify Cincinnati night shift production, instead of daytime!

On the '60s Heerlens like Amperex (or certain Zenith!) the codes can get very very faint, and even rub off entirely. The type/batch codes (like "I65") always seem to go first. If you look in very bright light and with magnifier, etc., sometimes the delta can be discovered, but admittedly not always even on unquestionably bona fide. Also, they may be hiding under the later printing. To be thorough, better look underneath between the pins too. Over hundreds of mid '60s Heerlens I've seen exactly one from 1966, definitely real, that had "I65" and delta code ("46J4") underneath in black ink, and no etch codes, even though Heerlen had officially stopped putting the codes down there over 10 years before!

The way to tell that Tele is ribbed plate even though you can't see the plates is because there are no side holes of any type visible, unlike those various smoothplate versions from Berlin.

I really like Blackburn (and Heerlen) longplates selectively in some Marshalls too, and examples from both of those factories made in their final year (1959) seem often to sound especially good (to me)- just like Les Pauls, right? !

There are always more details, especially regarding Mullards, but it's getting late . . . and you already know you like the tone!

Play hard-

Jeff
 
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