How important are date codes when matching tubes in a preamp?

[Shug451]

So a local seller has a pair of 12xa7 RCA black plates that he says test 9/9 on his Orange VT1000 valve tester. He says the tubes are identical except one has a date code from 1953 and the other from 1958. I'd like to use these in the phono stage of my Dynaco PAS preamp and/or my Project Tube Box DS. How important is matching the date codes if the tubes both test the same value? Thanks for any and all advice.

 

[zackthedog]

If they're the same plate structure/design I really don't think it matters.

 

[Schmidlapper]

The tubes don't know what is stamped on them, date doesn't mean anything. Matching is about the electrical characteristics of each tube. How they are being tested for matching also makes a lot of difference. Some testing gives representative numbers, some actual values, some test tubes barely loaded, and other test fully loaded.

 

[Schmidlapper]

Straw man argument, different date code of same make and type is OP's question, not two different brands of tubes. Matching date codes are a nice bonus, but I wouldn’t suggest the OP should pass on these solely because of it.

 

[GordonW]

If the manufacturer is the same, the construction is the same, and the tubes test matched- then for all effective purposes, they are matched, regardless of date.

I've deliberately grouped tubes from a single manufacturer, but of different manufacture date, in order to match the operational parameters. It's a better solution, than measurably mismatched tubes that happen to share the same date code.

Regards,
Gordon.

 

[GordonW]

There is a fundamental difference, in comparing Russian tubes (which were known for inconsistency, to at least some extent), to tubes of identical construction, from a big manufacturer back in the 1950s or 1960s.

Back then- there is very little evidence to show that companies like RCA significantly changed anything about their tube materials or construction, for the same tube designation, in ways that weren't obvious from a glance (i.e, black plate vs. gray plate, round plate vs. square plate, etc).

It was, in fact, in their best interest, to be consistent in these, due to contracts they had with manufacturers, the US government, and other clients.

This is why I don't buy the argument, that differences in a few years of date codes, will matter whatsoever in an otherwise identical US made tube from the 1950s. It also just doesn't jibe with the experience that I've had, working with these older name-brand tubes, either.

Regards,
Gordon.

 

[GordonW]

Unless there's something that's changing in the materials, or the tolerances are badly drifting- there's likely to be more unit-to-unit differences from tubes made in the same year, than between a "bogey" tube (the average of all the tubes made) between 1953 and 1958, by RCA. In other words- unit-to-unit variations, within a single batch, will likely swamp batch-to-batch differences- both in measurements, and in sound.

I haven't measured, or heard, that sort of differences, in otherwise identical tubes from that era, with that little of a variation in age.

So, from that- in my own experience- matching the tubes INDIVIDUALLY is FAR more important than just relying on them being the same age.

 

[peterh]

So a local seller has a pair of 12xa7 RCA black plates that he says test 9/9 on his Orange VT1000 valve tester. He says the tubes are identical except one has a date code from 1953 and the other from 1958. I'd like to use these in the phono stage of my Dynaco PAS preamp and/or my Project Tube Box DS. How important are matching the date codes if the tubes both test the same value? Thanks for any and all advice.

Mfg date are not at all important

 

[Schmidlapper]

There's no straw man. The point in my response to the OPs question is better drawn into view with the example of the different brand of tube [of the same type] in each channel. The principle is consistent. In other words, a given tube type among different brands may have obvious visual differences. But, tubes of the same brand and type that were made at different times may yet have differences in their construction, even if not visible. If the measurements - the matching - the numbers on the Maximatcher or Amplitrex - are the same between two different brands of the same type of tube, and given that a certain tube type has the same specifications across different manufacturer's datasheets, then using different brands of the same tube type shouldn't make any more difference than using tubes of the same brand but from different production runs.

I've also read comments from a respected tube seller who makes a related point in countering claims that the current crop of Russian tubes are really all the same, as evidenced by what you can see, with only the labeling being different. He challenged this on the basis that we cannot see far enough into the tube to know what all differences there might be inside.

I have used matched pairs of tubes of the same type and style, made by the same manufacturer, but labeled as a different brand name, with no audible distinction between the channels whatsoever. This would have been a comparable example for arguments sake I suppose, but bringing in the different manufacturers of the same tube, is definitely a straw man tactic to bolster the validity of your position with another more plausible example.

It is funny how qualitative differences always seem to be linked to whatever nuance has caught the eye at the moment.

 

[gadget73]

claims that the current crop of Russian tubes are really all the same, as evidenced by what you can see, with only the labeling being different.

only direct counter I can offer to this is my EH KT88 do not sound the same as Gold Lion KT88 despite looking nearly identical and coming from the same factory. I always assumed it was a case of same thing, different box but my ears disagree at least in this specific case. I borrowed the set of GL KT88 specifically to test that theory.

 

[Schmidlapper]

Inserting a valid but different example to bolster that your premise is valid. Example: "Example: Chinese 12AX7 in the left channel and a Mullard 12AX7 in the right channel. Would you want to do this?"
These are two different manufactures, Mullard by New Sensor, Russia, and random Chinese tube. This is not rebranding of a same manufacturers tubes, and why I found it as hyperbole.

 

[Schmidlapper]

Exactly my point.

 

[maxhifi]

I wouldn't fuss over that difference, provided the tubes look identical and have identical internal structure. It is getting hard to find old stock from the 50s at all at this point in time.

I would draw the line at something like matching a 1950s tube with a 1970s tube though.

 

[GordonW]

I wouldn't fuss over that difference, provided the tubes look identical and have identical internal structure. It is getting hard to find old stock from the 50s at all at this point in time.

I would draw the line at something like matching a 1950s tube with a 1970s tube though.

Yes, there is a practical limit to how much difference there can be in date of manufacture, before it becomes significant, of course. Especially in the 1970s and beyond- there was more "just using what they had", to fill the few contracts that were still there- all sorts of substitutions could, and would, be made, as long as it "met spec" for the contract.

That said- I've had coin-base RCA 6NS7GTBs from the early 1970s, that measured (and sounded) exactly like tall-base RCA 6SN7GTBs from the late 50s. Of course, they had the same plate structure (long ribbed plates) in both cases...

Regards,
Gordon.

 

[Dswankey]

So a local seller has a pair of 12xa7 RCA black plates... He says the tubes are identical except one has a date code from 1953 and the other from 1958.... How important are matching the date codes if the tubes both test the same value?

My guess would be that between 1953 & 1958 major tube manufactures weren't replacing and or upgrading their tooling machines. One of the things I think is fair to say is that one of the allures of owning vintage tube gear is that the stuff was from an era were stuff was built to last and not obsolesce in a couple of years such as today's mass market/big box equipment.

So I would think that would also apply to the machinery they were using to manufacture tubes.

I've pulled original tubes from 50's/60's gear and the tubes date codes were not matched.

Here's a pair of Webcor Mullards I pulled from the same machine note the difference in codes.

They're both B (Blackburn) I believe the the 0 represents 1960 and the E would be May and the F would be June

I doubt when Webcor contracted with Mullard they stipulated that all the tubes had to be from the exact same date run IE matching date codes.

So my answer would be not important

 

[JohnT123]

only direct counter I can offer to this is my EH KT88 do not sound the same as Gold Lion KT88 despite looking nearly identical and coming from the same factory. I always assumed it was a case of same thing, different box but my ears disagree at least in this specific case. I borrowed the set of GL KT88 specifically to test that theory.

I think many of the differences are things we can't see. Grid materials and alignment, cathode coatings, plate material, etc. At least this is how I explain the difference.

 

[maxhifi]

Yes, there is a practical limit to how much difference there can be in date of manufacture, before it becomes significant, of course. Especially in the 1970s and beyond- there was more "just using what they had", to fill the few contracts that were still there- all sorts of substitutions could, and would, be made, as long as it "met spec" for the contract.

That said- I've had coin-base RCA 6NS7GTBs from the early 1970s, that measured (and sounded) exactly like tall-base RCA 6SN7GTBs from the late 50s. Of course, they had the same plate structure (long ribbed plates) in both cases...

Regards,
Gordon.

Yep, I have 1970s RCA 12AX7s labeled made in Japan, which would never have happened in the 1950s.

Isn't it wild how expensive the 6SN7 has become?

 

[GordonW]

My 1975 Lafayette catalog has 3 full pages of tubes available.

Yes, but by that time- a lot of those would be "surplus stock" and the like. IOW- left-overs that hadn't sold. Production of new tubes was definitely winding down, at that point, and by 1980, there were just a fraction of the previous tube types in production, at all...

Regards,
Gordon.

 

[GordonW]

Oh, and a note to the OP- the RCA black-plate 12AX7s- I have had some issues with RCA 12AX7 tubes- even older ones- being slightly too noisy (hiss/hum) for a Dyna phono stage. The later 12AX7A was specified out with a heater that would reduce hum- but the earlier 12AX7 (no "A") did not necessarily have that same degree of low noise as the 12AX7A. The 12AX7A was better than the 12AX7- but even so, it could still have a slight amount of audible hum in a Dyna.

That's ostensibly why they also later made the 7025- it was a "select" version, with even lower noise. Those usually are quiet enough.

Regards,
Gordon.

 

[GordonW]

They'd still have been "first run" tubes in 1979- just not guaranteed from the same era, or of exactly the same internal construction anymore. Lots of consolidation of warehouse stock, from wherever it was, to a few remaining warehouses.

I've had that happen with Realistic Lifetime tubes- open four boxes of the same tube type, and get four different construction tubes. All of them were perfectly good- just not a match to each other, either in construction or in measured parameters.

However, that would NOT have been the situation in the 1950s and 1960s. If you got four of the same type tubes from just about any major manufacturer- you'd have likely gotten the same construction, and decent matching of performance parameters, pretty much guaranteed- even if they were a few years apart in manufacture date.

Regards,
Gordon.