6DN7 GE Date codes

[ALL212]

I'm going to make some statements as if fact but please, argue with me as this is all stuff just gleaned from the web.

In general:
Older tubes have square getters.
Older tubes have black plates.
Any 6DN7 with acid etched dotted code on them were made by GE regardless of any other markings.

I've got 9 (one blew up) 6dn7 tubes with dotted code on them.  Some labeled GE and some labeled RCA.

As reference for the GE date codes, both as labeled on the base (if there) and the dotted codes:
http://www.antiqueradios.com/forums/viewtopic.php?f=19&t=230932

and as a reference to the coding on the RCA tube bases (table on page 18).  The few tubes I have marked RCA seemed to follow that table when compared to the acid etched dots.
http://pax-comm.com/rcadates.pdf

In short - the acid etched codes are the date of manufacture.  The label on the base was for warranty purposes.  I still haven't been able to completely figure out the acid etched dots on some of my tubes.  Some make perfect sense in that what the acid etching has is always the same year or earlier than what the label on the base has.  I have one tube with no markings on the base at all.

Tubes dated 1958, 1960 and 1962 are black/grey plates with square getters.
Tubes dated 1964, 1965 are black/grey plates with round getters.
Tubes dated 1966 and after are grey/grey plates with round getters.

Is the smaller of the plate construction the cathode? And the other, more complicated structure, the anode?

Patterns on the mica vary also but not between tubes close in years.
Cathode are all grey.  The oldest tube I have has holes in the anode structure.

It's interested that these changes were made.  I'm wondering if anyone has reference as to why?  Most folks I know prefer older tubes - black plates with square getters but I'm not sure if that's just a status or nostalgic preference?  As we all know production lines get simpler and material typically cheaper (whether for good or bad is not part of the reason - cost and profit are) so it might follow that the older tubes are better.  But does that give them a "sound" advantage?

I also have a pair of Sylvania coin based tubes.  These have 1/2 mica's that don't go all the way around the tube.  A square cathode and a very flat and simple anode.  Both plate structures are black.  I'd say these are the newest of the 6dn7's I have but I can find no date code info on them. (NR AAA and DR AAC)

Sound - I'm not sure my old ears can put one above the other...but I'll put the oldest ones in every time anyway!   

[mcandmar]

Great info, i have a dozen or so GE and RCA tubes i'll dig out and sort through to see if they match.  Cant say i ever noticed any real differences between RCA and GE tubes, and the numerous other rebrands of them.  The only tubes i found different are Sylvania and Tung-Sols, the Sylvanias having very different internal structure.

The 6DN7's are dissimilar triodes, as in there is two separate tubes inside with different specs.  The smaller plate of the two is the 1st section, larger plate is the 2nd section. Think of them as a driver and power tube inside the same glass.  As you have found the Sylvanias are very distinct in the 1st section is a thin tall plate, and the 2nd section is a large flat plate with the grid turned 90 degees, and the top/bottom mica disks are cut away.  Interestingly i have seen hybrids with the Sylvania 1st section, and regular GE/RCA plates for the 2nd sections.

You can also find GE/RCA tubes rebranded as Sylvanias which i find curious, and the Sylvania tubes can be found rebranded as International Servicemaster, Fivre, Dumont, and Mullard. Coin base were the last of the 6DN7's, and can be either Sylvanias or GE/RCA's, black plates, grey plates, or a 50/50mix.   Unfortunately the date codes for Sylvanias seem to be a secret lost in time so i've never been able to pin those down to specific years.

We should add a few pics to this thread too, i had started to document the variations in my thread here http://bottlehead.com/smf/index.php?topic=5316.0

[Paul Birkeland]

Is the smaller of the plate construction the cathode? And the other, more complicated structure, the anode?

There are two very visible plates inside the tube, they are the large pieces of metal that are basically all you see when you look into the tube.  The smaller one is the plate of the triode that is the first amplification stage in the amplifier, and the beefier plate belongs to the second stage.  The cathodes are inside the plates, and not all that visible on the 6DN7's I've looked at.

Cathode are all grey.  The oldest tube I have has holes in the anode structure.

Often times the most visible element through those holes will be the rods that hold up the grid wire. 

It's interested that these changes were made.  I'm wondering if anyone has reference as to why?  [...] As we all know production lines get simpler and material typically cheaper (whether for good or bad is not part of the reason - cost and profit are) so it might follow that the older tubes are better.

Manufacturing technology also gets better.  Tighter vacuums, tighter tolerances, and advanced metallurgy can lead to both performance increases and cost decreases.   

[mcandmar]

Just had a route through my GE/RCA tubes and i couldn't make any sense out of the dot codes, they all gave me odd numbers so i must be reading those wrong.  Going by the letter codes all my 50/50 gray and black plate tubes are 62-64 vintage with D getters, everything else up to 1968 are grey plate with round getters which follows your findings.  The 1968 vintage tubes also have additional holes in the plate structure, other wise they are the same internally.

Attached are two real oddball GE tubes i picked up recently, they are re branded GE tubes but it was the holes in the plates that caught my interest.  With 6SN7's hole plates are generally early 1950's vintage but i have yet to verify the dates of these.  Going by the above with 50/50 black/grey plates and D getters they have to be pre 1964.

[ALL212]

Let's see how these pictures come in.  It's a bit tricky trying to get no flare on the glass.  The black base is nothing more than a roll of electrical tape.  This tube dates to 1965 based on the RCA code.  The dotted code is confusing as it has two dots at the bottom of the year section.

1st stage are grey plates, 2nd stage black and a round getter.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi905.photobucket.com%2Falbums%2Fac253%2Fall212212%2F6dn7%2520tubes%2FDSC_2647_zps16ff7e33.jpg&hash=3643df2e0cff3dfeed01d37282332bfcb5c5cdc9)

Here's the one I have with holes in the 2nd stage.  I think around 1958.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi905.photobucket.com%2Falbums%2Fac253%2Fall212212%2F6dn7%2520tubes%2FDSC_2661_zps2b70e9fd.jpg&hash=50a57941fba7e5c3c8fa06d71f12b079fd1734a1)

Grey on Grey - very sloppy dots on this tube.  Series heater - you can just see the little wire above the mica.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi905.photobucket.com%2Falbums%2Fac253%2Fall212212%2F6dn7%2520tubes%2FDSC_2665_zpsd3b2fbaa.jpg&hash=21a6f9c028b3e3a6fe008e51a998e58a64899702)

Maybe a true RCA?  Both sections are black with a larger round getter.  The primary is just a black tube - no wings.  The black coating is not very even.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi905.photobucket.com%2Falbums%2Fac253%2Fall212212%2F6dn7%2520tubes%2FDSC_2667_zps84aa00da.jpg&hash=649bc596cf8d0d286fc5b72c26c446e88348e319)