12CU6는 2A3의 격세유전(隔世遺傳)일까? - 대를 건너 다시 나타난 2A3의 유전자를 추적하다 - Is the 12CU6 an Atavistic Descendant of the 2A3?
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부모에게서는 보이지 않던 조상의 형질이 손자 세대에서 다시 나타나는 현상입니다. 즉, 아버지와 나는 닮지 않았는데 손자가 증조할아버지를 그대로 빼닮는 경우입니다.
저는 오랫동안 진공관의 데이터시트와 특성곡선을 살펴보면서 문득 이런 생각을 하게 되었습니다.
"진공관에도 격세유전이 존재할 수 있지 않을까?"
물론 이것은 진공관의 공식 역사에 기록된 사실은 아닙니다.
그러나 수많은 데이터시트와 특성곡선을 하나씩 비교하다 보면, 오래전에 사라진 진공관의 형질이 전혀 다른 시대의 진공관에서 다시 나타난 것처럼 보이는 순간들을 만나게 됩니다.
그 대표적인 예가 바로 "12CU6" 입니다.
처음에는 큰 의미를 두지 않은 진공관이었습니다.
제가 처음 12CU6를 알게 되었을 때만 해도 그렇게 기대를 하지 않았습니다.
TV용으로 개발된 평범한 빔 출력관.
오디오 애호가들에게는 거의 알려지지 않은 이름.
그것이 제가 이 진공관을 이해하는 정도였습니다.
저는 오래전부터 잘 알려지지 않은 진공관들의 데이터시트를 모으고 서로 비교하는 것을 좋아했습니다.
그러던 어느 날 12CU6의 데이터시트를 다시 들여다보다가 이상한 느낌을 받았습니다.
"어디서 많이 본 특성인데..."
특성 값들도 익숙했습니다.
IV 특성곡선의 모양도 익숙했습니다.
그래서 다른 진공관들의 데이터시트를 하나씩 꺼내 비교하기 시작했습니다.
가장 먼저 떠오른 이름은 6AV5였습니다.
비교를 계속할수록 가장 많이 닮은 진공관은 6AV5였습니다.
아래 6AV5와 6CU6의 데이트시트를 카피해서 비교를 했습니다. 항목들의 의미가 무엇인지 알려고 애쓰실 필요는 없습니다. 수치들만 비교해도 알 수 있습니다.
※ 6CU6는 히터전압이 다른 12CU6와 동등관입니다.
IV 특성곡선도 놀라울 만큼 닮아 있었습니다. 아니, 그대로 붙혀넣기를 한것같습니다.
Top Cap이라고 진공관 윗쪽에 플레이트 단자가 뿔처럼 나와 있는 것인데 이것 말고는 생김새가 거의 비슷합니다. 인터넷을 찾아보아도 해외의 여러 진공관 애호가들이 두 진공관의 내부 구조는 거의 같다고 이야기하고 있었습니다.
저는 아직 제조사의 공식 문서를 확인한 것은 아닙니다. 그러나 데이터시트만 놓고 보더라도 두 진공관이 거의 같은 진공관이라는 인상을 받기에는 충분했습니다.
그런데 저는 이전부터 이 한 가지를 알고 있었습니다.
6AV5는 제게 낯선 진공관이 아니었습니다.
오래전부터 해외 자작 오디오 자료에서는
"6AV5를 3극 연결하면 2A3 또는 6B4G와 매우 비슷한 특성을 가진다."
라는 이야기가 자주 등장했습니다.
예를 들면 아래 사이트의 글입니다
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| http://tubelab.com/articles/tube-testing/6av5-sweep-tube/ Tube Testing — 6AV5 Sweep Tube |
이미 많은 엔지니어와 자작인들이 경험적으로 알려 준 사실이었습니다.
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| 2A3 vs 6AV5 |
6AV5 와 12CU6(6CU6)가 거의 같은 특성을 보이므로
"12CU6의 3극연결 특성도 당연히 2A3와 닮을 것"이라고 말할 수 있습니다.
그런데 더 큰 의문이 생겼습니다.
여기서부터는 저의 의문이 시작되었습니다.
왜 TV용 빔 출력관을 3극 연결하면 갑자기 2A3와 비슷한 특성이 나타나는 것일까요?
정말 우연일까요?
아니면 우리가 아직 설명하지 못한 설계의 계보가 숨어 있는 것일까요?
저는 후자일 가능성이 있다고 생각하기 시작했습니다.
저는 엔지니어이기 때문입니다.
저는 진공관을 역사학자의 시선보다 엔지니어의 시선으로 바라보는 편입니다.
엔지니어는 아무것도 없는 곳에서 새로운 설계를 시작하지 않습니다.
기존의 성공한 설계를 바탕으로 조금씩 변형하고 개선합니다.
성공도 기억하지만 실패는 더욱 가슴 아프기 때문에 오래 기억합니다.
그리고 언젠가는 과거의 아이디어가 전혀 다른 제품 속에서 다시 살아나는 경우도 많습니다.
그래서 저는 자연스럽게 이런 생각을 하게 되었습니다.
"RCA의 엔지니어들도 그랬지 않았을까?"
물론 이것은 공식 문헌에 기록된 사실이 아닙니다.
그러나 공학적으로는 충분히 가능한 이야기라고 생각합니다.
퍼즐은 나중에 맞춰졌습니다.
흥미롭게도 25L6에 대한 관심과 검토는 그 이후에 시작되었습니다.
처음에는 오래된 빔 출력관 정도로만 생각했습니다.
그런데 25L6의 6.3V 히터 버전인 6W6의 3극 연결 특성을 조사하면서 다시 한번 놀라게 되었습니다.
이번에도 2A3의 특성이였습니다.
이것은 앞의 글(6L6과 함께 만들어진 25L6, 과연 2A3의 후손일까?)에서 이야기를 드린바가 있습니다.
그 순간 머릿속에 흩어져 있던 조각들이 하나로 연결되기 시작했습니다.
- 6AV5는 3극 연결하면 2A3를 닮는다.
- 12CU6는 6AV5와 매우 유사하다.
- 6W6도 3극 연결하면 2A3를 닮는다.
- 6W6는 25L6의 6.3V 버전이다.
처음에는 서로 아무 관계없는 사실처럼 보였습니다.
그러나 하나씩 이어 보니 마치 퍼즐이 맞춰지듯 하나의 흐름이 보이기 시작했습니다.
이것은 역사가 아니라 하나의 추론입니다.
저는 새로운 진공관의 역사를 쓰려는 것이 아닙니다. 또한 이것이 이미 증명된 사실이라고 주장하려는 것도 아닙니다. 다만 엔지니어의 입장에서 보면 하나의 가능성이 보입니다.
1930년대 RCA는 2A3를 비롯한 우수한 3극관들을 개발했습니다.
이후 빔 출력관 시대가 열리면서 6L6라는 새로운 진공관이 등장했습니다. 저는 이 과정이 과거와 완전히 단절된 새로운 출발이었다고는 생각하지 않습니다. 오히려 당시까지 축적된 3극관 설계 경험이 새로운 빔 출력관 속으로 자연스럽게 이어졌을 가능성이 있다고 생각합니다.그리고 그 과정에서 25L6와 같은 저전압 대전류 출력관도 함께 태어났을지 모릅니다.
만약 그렇다면
25L6 → 6W6 → 6AV5 → 12CU6
로 이어지는 계보 속에서 2A3의 설계 철학이 살아남았다는 가설도 전혀 불가능한 이야기는 아닙니다.
그리고 흥미롭게도 이들 진공관를 3극 연결하는 순간, 마치 오래전 조상의 모습이 다시 나타나듯 2A3를 떠올리게 하는 특성이 드러납니다.
이러한 저의 가설을 아래의 그림과 같이 표현해 보았습니다.
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| J-Album Hypothesis of Atavism in Vacuum Tube Design 제이앨범 진공관 격세유전 가설 |
그래서 저는 이 현상을 '진공관의 격세유전'이라고 부르고 싶습니다.
마지막으로, 이 글을 쓰며 제가 품게 된 생각 하나를 말씀드리고 싶습니다.
Is the 12CU6 an Atavistic Descendant of the 2A3?
Tracing the Reappearance of the 2A3's Design Heritage Across Generations
Part 1 – An Unexpected Discovery
In biology, there is a fascinating phenomenon known as atavism—the reappearance of ancestral traits after skipping one or more generations.
A grandson may closely resemble his great-grandfather even though neither he nor his father does.
For many years, while studying vacuum tube datasheets and plate characteristic curves, I found myself wondering:
Could vacuum tubes also exhibit a kind of atavism?
Of course, this idea does not appear anywhere in the official history of vacuum tube development.
Yet, after comparing countless datasheets and characteristic curves, I occasionally encountered tubes whose electrical behavior seemed to revive characteristics that had disappeared decades earlier. It was almost as if an old design had quietly resurfaced in a completely different generation of tubes.
One tube, in particular, caught my attention.
The 12CU6.
An Ordinary TV Tube… Or So I Thought
At first, the 12CU6 seemed completely unremarkable.
When I first came across it, I had no reason to believe it was anything special.
It was simply a beam power tube designed for television service—a tube almost unknown among audio enthusiasts and rarely mentioned in discussions of high-quality audio amplifiers.
That was all I thought it was.
For many years I have enjoyed collecting datasheets of little-known vacuum tubes and comparing their electrical characteristics. Sometimes similarities emerge that are easy to overlook when looking at a single tube in isolation.
One day, I opened the 12CU6 datasheet again.
Something felt strangely familiar.
"I've seen these curves before..."
The operating specifications looked familiar.
The electrical ratings looked familiar.
Even the shape of the plate characteristic curves seemed oddly familiar.
So I began pulling out other datasheets and comparing them one by one.
The first tube that came to mind was the 6AV5.
The more I compared them, the stronger the resemblance became.
Eventually I reached a simple conclusion:
The 12CU6 and the 6AV5 appear to be virtually the same tube electrically.
The heater voltage differs—the 6CU6 being the 6.3-volt equivalent of the 12CU6—but beyond that, nearly every specification is remarkably close.
Their maximum ratings, operating conditions, transconductance, plate dissipation, and characteristic curves all correspond so closely that they almost seem to describe the same device.
Even without looking at every specification in detail, the overall similarity is difficult to ignore.
The plate characteristic curves reinforce the same impression.
Rather than merely resembling one another, they appear almost identical—as though one graph had simply been copied and relabeled.
Physically, the two tubes differ in one obvious respect.
The 6CU6 (and therefore the 12CU6) uses a top-cap plate connection, while the 6AV5 does not.
Apart from that feature, however, they are strikingly similar in appearance.
Many experienced tube enthusiasts on international forums have also suggested that the two tubes share essentially the same internal structure.
I have not yet found official manufacturer documentation confirming this relationship, so I cannot present it as an established historical fact.
Nevertheless, judging from the datasheets alone, it is difficult to avoid the impression that these tubes are extraordinarily close relatives.
Then I Remembered the 6AV5
The 6AV5 was not unfamiliar to me.
For years I had come across discussions among experienced DIY audio builders describing an interesting property of this tube.
When connected as a triode, the 6AV5 reportedly behaves very much like a 2A3 or a 6B4G.
This observation has appeared repeatedly in articles and DIY communities over the years.
One well-known example is George Anderson's article at TubeLab, where he explored the audio performance of the 6AV5 as a triode-connected output tube.
The conclusion was remarkably consistent with what many builders had already experienced:
A triode-connected 6AV5 exhibits electrical characteristics surprisingly similar to those of the classic 2A3.
If that is true—and if the 12CU6 is indeed electrically almost identical to the 6AV5—then another possibility naturally follows.
A triode-connected 12CU6 should also display characteristics very close to those of the 2A3.
At this point, however, a much more interesting question emerged.
Not about the 12CU6 itself.
But about why this happens.
The Question That Changed Everything
Why should a television beam power tube suddenly reveal the characteristics of a classic directly heated triode simply by connecting it as a triode?
Was it merely coincidence?
Or was there an engineering lineage that had never been fully documented?
That question became the starting point of my investigation.
As an engineer, I tend to look at vacuum tubes somewhat differently from historians.
History records what was documented.
Engineering often leaves traces that were never written down.
Engineers rarely begin with a completely blank sheet of paper.
Instead, they build upon successful designs, refining them step by step.
Past successes become foundations.
Even failed experiments are rarely forgotten.
Sometimes an old idea quietly survives and unexpectedly reappears years later in a completely different product.
Naturally, I began wondering:
Could RCA's engineers have worked the same way?
I cannot prove that they did.
No document has yet been found stating that this was their design philosophy.
But from an engineering perspective, it seems entirely plausible.
And that simple question eventually led me toward another forgotten tube—
the 25L6.
To be continued in Part 2 – Following the Engineering Trail
Part 2 – Following the Engineering Trail
The pieces of this puzzle did not come together all at once.
Interestingly, my interest in the 25L6 began only after I had already been studying the 12CU6 and the 6AV5.
At first, I regarded the 25L6 as simply another early beam power tube.
It certainly wasn't a tube that attracted much attention from audio enthusiasts.
Compared with famous tubes such as the 6L6, 6V6, or KT88, the 25L6 seemed almost forgotten.
But while researching its 6.3-volt counterpart—the 6W6—I encountered something unexpected once again.
When connected as a triode, the 6W6 also exhibits electrical characteristics remarkably similar to those of the 2A3.
I discussed this observation in my previous article:
"Was the 25L6, Developed Alongside the 6L6, a Descendant of the 2A3?"
At that moment, several seemingly unrelated observations suddenly began connecting.
- A triode-connected 6AV5 resembles a 2A3.
- The 12CU6 is electrically almost identical to the 6AV5.
- A triode-connected 6W6 also resembles the 2A3.
- The 6W6 is simply the 6.3-volt version of the 25L6.
Individually, none of these observations proves anything.
Together, however, they begin to suggest a pattern.
Looking at the Problem as an Engineer
Perhaps this way of thinking comes naturally because I spent my career as an engineer.
Engineers rarely invent completely new technologies from scratch.
Instead, they improve existing ideas.
They refine previous designs.
They adapt familiar concepts to new applications.
Most importantly, they remember.
Successful designs become foundations for future work.
Even unsuccessful experiments are seldom forgotten.
Years later, ideas that once seemed impractical may suddenly become valuable when technology or requirements change.
This is true in software.
It is true in electronics.
And I believe it was probably true inside RCA's engineering laboratories as well.
That naturally led me to ask a simple question.
Could the engineers who designed RCA's beam power tubes have carried forward some of the electrical ideas that had already proven successful in earlier power triodes such as the 2A3?
No surviving document answers this question.
Perhaps none ever will.
But engineering often leaves clues beyond written records.
Sometimes those clues survive in the tubes themselves.
A Possible Evolution
During the early 1930s, RCA developed some of the most successful directly heated power triodes ever produced.
Among them, the 2A3 became one of the defining audio output tubes of its era.
Only a few years later, beam power tubes appeared.
The 6L6, introduced in 1936, represented a major technological step forward.
Higher efficiency.
Higher output power.
Lower distortion.
From a historical point of view, it marked the beginning of a new generation.
But engineering progress is rarely a complete break with the past.
New designs almost always inherit ideas from earlier ones.
For that reason, I find it difficult to believe that the arrival of the beam power tube era meant abandoning everything engineers had learned while developing the 2A3 and other triodes.
Quite the opposite.
It seems far more likely that those years of accumulated experience became the foundation upon which the new beam tube family was built.
If that is true, another possibility naturally follows.
During the development of the 6L6, RCA engineers may also have explored lower-voltage, higher-current beam output tubes.
One of those designs eventually became the 25L6.
This is, of course, speculation.
No historical document currently confirms such a development path.
But from an engineering perspective, it is certainly a plausible scenario.
The Missing Link
If the 25L6 inherited part of the electrical behavior of earlier power triodes...
and if the 6W6 is simply the 6.3-volt version of the 25L6...
and if the 6AV5 later evolved from the same general family of beam output tubes...
then perhaps the resemblance between a triode-connected 6AV5 and the 2A3 is not a coincidence after all.
Perhaps what we are seeing is the survival of an older design philosophy.
Not the tubes themselves.
But the engineering DNA behind them.
This is where my hypothesis begins.
Not with history.
Not with documentation.
But with electrical behavior.
The tubes themselves seem to preserve traces of decisions made decades earlier.
Those traces become visible only when the beam power tube is returned to triode operation.
At that moment, something unexpectedly familiar begins to appear.
Almost as if an ancestral trait had quietly survived through generations of design evolution.
That idea led me to connect the tubes in the following conceptual lineage:
2A3 → 25L6 → 6W6 → 6AV5 → 12CU6
I do not claim that this lineage has been historically proven.
Rather, I present it as an engineering hypothesis—a possible explanation for a series of electrical similarities that are otherwise difficult to dismiss as mere coincidence.
And that brings us to the central idea of this article.
Could what we are observing be a kind of engineering atavism?
To be continued in Part 3 – The J-Album Hypothesis
Part 3 – The J-Album Hypothesis
At this point, I would like to make one thing perfectly clear.
I am not attempting to rewrite the history of vacuum tubes.
Nor am I claiming that the ideas presented here have already been proven by historical documents.
Rather, I am proposing an engineering hypothesis—one that emerges from comparing electrical characteristics, datasheets, and decades of accumulated design evolution.
History is written from surviving documents.
Engineering, however, often leaves evidence in different ways.
Sometimes the most revealing clues are found not in patents or company archives, but in the electrical behavior of the devices themselves.
When enough of those clues begin pointing in the same direction, it becomes reasonable to ask whether they are telling part of a story that was never formally recorded.
That is exactly how this hypothesis was born.
An Engineering Hypothesis
My hypothesis can be summarized as follows.
During the early years of power tube development, RCA accumulated considerable experience designing directly heated triodes such as the 2A3.
When the beam power tube era began with the introduction of the 6L6, those engineering principles were probably not discarded overnight.
Instead, they may have continued to influence new designs in ways that are no longer documented.
One possible result of that process was the 25L6, a lower-voltage, higher-current beam power tube introduced alongside the 6L6 family.
The 25L6 later appeared as the 6W6, differing primarily in heater voltage.
As television technology advanced, new horizontal sweep tubes such as the 6AV5 were developed.
The 12CU6 followed with electrical characteristics remarkably similar to those of the 6AV5.
Viewed independently, these tubes belong to different generations and different applications.
Yet something fascinating happens when several of them are connected as triodes.
Electrical characteristics unexpectedly emerge that strongly resemble those of the much older 2A3.
That observation led me to imagine the following conceptual lineage:
2A3 → 25L6 → 6W6 → 6AV5 → 12CU6
I present this not as established history, but as a possible engineering narrative.
Perhaps what has survived across these generations is not a particular tube, but a successful design philosophy.
In other words, the design DNA of the 2A3 may have continued evolving within later beam power tubes, only revealing itself again when those tubes are operated as triodes.
That is why I have chosen to describe this phenomenon as "Vacuum Tube Atavism."
Why This Matters
Some readers may ask an obvious question.
Even if this hypothesis is true, why does it matter?
To me, the answer is simple.
It changes the way we look at vacuum tubes.
For many years, audio enthusiasts have tended to divide tubes into categories.
Legendary tubes.
Rare tubes.
Expensive tubes.
Ordinary tubes.
But perhaps that way of thinking overlooks something more important.
A tube is not defined by its reputation alone.
It is defined by its electrical behavior.
Some little-known television tube may preserve the essence of a famous audio triode.
Another inexpensive beam tube may contain the same engineering ideas that once made the 2A3 legendary.
If so, then perhaps there are no "good tubes" and "bad tubes."
There are simply different expressions of the same engineering heritage.
That belief has guided much of my work at J-Album.
Rather than pursuing famous tubes simply because they are famous, I prefer to search for forgotten tubes whose potential has never been fully appreciated.
Sometimes history leaves remarkable treasures hidden in unexpected places.
Filling the Empty Spaces
Finally, I would like to leave you with one personal thought.
In digital audio, a DAC (Digital-to-Analog Converter) reconstructs a continuous musical waveform from a series of discrete digital samples.
The original waveform is never fully stored.
Instead, mathematics fills the spaces between the samples.
In a sense, I feel that my own research follows a similar path.
Datasheets, patents, engineering notes, and characteristic curves never tell the entire story.
Between those surviving pieces of evidence lie gaps—years of undocumented experimentation, abandoned prototypes, forgotten conversations, and engineering decisions that were never written down.
I have no desire to fill those gaps with fantasy.
Instead, I try to connect them using the traces that still remain.
Electrical characteristics.
Design similarities.
Historical timing.
Engineering logic.
Perhaps these fragments will never prove exactly what happened.
But sometimes they allow us to see possibilities that deserve further exploration.
That is the spirit in which this article was written.
It is not the final answer.
It is not an attempt to redefine history.
It is simply another step in the ongoing exploration taking place at J-Album.
Perhaps future documents will strengthen this hypothesis.
Perhaps they will challenge it.
Either outcome would be welcome.
Because the purpose of engineering is not merely to confirm what we already know—
it is to keep asking questions that are worth investigating.
J-Album
Engineering the forgotten stories behind vacuum tubes.
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