명관 "45"는 어떻게 탄생했는가? – UV-201A에서 시작된 파워 3극관의 진화, How the Legendary Type 45 Came to Be – The Evolution of Power Triodes from the UV-201A
"Audion의 등장에 대해서는 앞서 알아 보았는데요 애석하게 대중화되지는 않았습니다. 그러다 1920년대에 GE와 Westinghouse 의 기술로 대중에게 처음 선보인 역사적인 오디오 증폭용 3극 진공관 UV-201이 등장합니다."
1906년 Audion이 세상에 처음 등장했을 때만 해도 사람들은 이 작은 유리관이 훗날 오디오의 역사를 바꿀 것이라고는 상상하지 못했을 것입니다.
앞선 글 「드 포레스트와 삼극관(Audion)의 탄생 」에서 살펴보았듯 Audion은 역사적으로 매우 중요한 발명이었지만, 일반 대중에게 널리 보급되지는 못했습니다.
진공관이 비로소 사람들의 생활 속으로 들어오기 시작한 것은 1920년대였습니다. General Electric(GE)과 Westinghouse의 기술을 바탕으로 제작된 UV-201이 등장하면서 라디오 시대가 본격적으로 열리기 시작했습니다. 그리고 2년 뒤에는 소비전력을 크게 줄인 UV-201A가 출시되면서 수많은 라디오 애호가들의 사랑을 받게 됩니다.
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| UV201 |
UV-201
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| UV-201내부 구조 |
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| UV-201 히터 가열된 모습 |
UV-201A
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| UV201A |
주요 특성은 다음과 같습니다.
- 최대 플레이트 전압 : 135V
- 플레이트 전류 : 약 2.5~3mA
- 증폭률(μ) : 약 8
출력은 약 0.1W 정도였습니다. 지금 기준으로는 매우 작은 출력이지만 당시에는 충분했습니다.왜냐하면 아직 종이 콘(cone) 스피커의 시대가 아니었기 때문입니다.
사람들은 대부분 헤드폰이나 혼(Horn) 스피커를 통해 라디오를 들었습니다. 거실을 가득 채우는 풍부한 저음은 아직 오지 않은 시대였습니다. 작은 출력이라도 음성을 전달하는 데는 충분했고, 이것이 당시 UV-201A가 표준 진공관으로 자리 잡을 수 있었던 이유였습니다.
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| Early 1920s radio and horn speaker |
UX-112A
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| UX112A |
UX-112A의 사진은 The UX112A 의 링크를 보면 내부 구조까지도 짐작할 수 있습니다.
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| UX-112A 내부 모습 |
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| UX-201A 와 UX-112A 히터 비교 |
숫자보다 중요한 것은 진화의 방향입니다.
표가 조금 복잡해 보일 수도 있습니다.
그러나 엔지니어의 입장에서 보면 핵심은 단 네 가지입니다.
- 증폭률(μ)은 약 8로 거의 그대로 유지되었습니다.
- 플레이트 전류는 약 2.5mA에서 7~8mA로 크게 증가했습니다.
- 내부저항(rp)은 약 10kΩ에서 5kΩ 정도로 낮아졌습니다.
- 출력은 약 0.1W에서 0.25W로 향상되었습니다.
즉, RCA는 더 많이 증폭하는 진공관을 만든 것이 아니라 더 큰 전류를 흘리고 더 큰 스피커를 울릴 수 있는 진공관을 만들기 시작한 것입니다.
UX-171A
1927년 무렵 RCA는 한 단계 더 나아간 UX-171 계열을 발표했습니다.
곧이어 등장한 UX-171A는 UX-112A와 동일한 산화물 도포 Multi-fold Hairpin Filament를 사용하면서도, 전극계(Electrode System)를 새롭게 최적화하여 훨씬 강력한 출력관으로 발전했습니다. 겉모습은 크게 달라 보이지 않습니다. 하지만 내부에서는 중요한 변화가 일어나고 있었습니다. 플레이트는 더욱 커졌고, 그리드 구조도 개선되어 더 많은 전류를 안정적으로 흘릴 수 있게 되었습니다.
출력 역시 약 0.1W에서 약 0.7W까지 증가하였습니다.
흥미로운 점은 필라멘트가 아니라 전극계가 진화의 중심이었다는 사실입니다.
RCA는 새로운 필라멘트를 다시 개발한 것이 아니라, 이미 완성된 필라멘트 기술 위에 전극 구조를 계속 발전시켜 더욱 강력한 출력관을 만들어 갔던 것입니다.
그리고 마침내 45 등장.
사실 UX-171A에 이르러 RCA는 출력 삼극관의 기본 기술을 거의 완성하게 됩니다.
우리가 지금도 명관으로 기억하는 45(Type 45)는 바로 이 UX-171A의 다음 세대에 등장합니다.
하지만 저는 45를 하나의 '기적 같은 발명'이라고 생각하지 않습니다.
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| Type 45 |
오히려 UV-201A에서 시작된 작은 변화들이 UX-112A를 거치고, 다시 UX-171A에서 하나씩 쌓여 마침내 완성된 자연스러운 진화의 결과라고 생각합니다. 돌이켜 보면 그 흐름은 매우 일관됩니다. 처음에는 더 적은 전력으로 더 많은 전자를 방출하려 했고, 다음에는 더 많은 전류를 흘리려 했으며, 마지막에는 더 큰 스피커를 울릴 수 있는 출력관을 만들고자 했습니다.
45는 바로 그 긴 여정의 첫 번째 완성형이었습니다.
다음 글에서는 RCA가 어떻게 이러한 기술들을 하나로 완성하여, 오늘날까지도 가장 아름다운 직열 삼극관 가운데 하나로 평가받는 45를 탄생시켰는지 함께 살펴보겠습니다. 그리고 그 이야기가 어떻게 2A3의 탄생으로 이어지는지도 함께 따라가 보겠습니다.
Part 1. The Birth of Power Triodes – From UV-201 to UX-112A
When the Audion first appeared in 1906, few people could have imagined that this small glass tube would eventually change the history of audio.
As discussed in my previous article, "The Birth of the Audion (Triode)," Lee de Forest's invention marked the beginning of electronic amplification. However, the Audion itself never became a household device.
Vacuum tubes truly entered everyday life during the 1920s, when General Electric (GE) and Westinghouse introduced the UV-201, the first triode widely adopted for radio receivers. A few years later, the improved UV-201A dramatically reduced power consumption and quickly became the standard tube for battery-powered radios.
The UV-201
At first glance, the UV-201 appears remarkably simple.
Its plate is relatively small, suggesting that the internal structure was equally uncomplicated. Contemporary photographs confirm this impression: a straight tungsten filament stretched through the center, surrounded by a loosely wound grid, all enclosed by a modest plate structure.
Compared with later power triodes, it was an extremely simple design.
The UV-201A retained essentially the same internal construction. The most noticeable difference was its improved filament and the barium getter coating inside the glass envelope, which often obscures the internal structure.
Typical operating characteristics were:
- Maximum plate voltage: 135 V
- Plate current: approximately 2.5–3 mA
- Amplification factor (μ): about 8
Its output power was only about 0.1 watt.
Today this seems insignificant, but in the early 1920s it was entirely sufficient.
Why?
Because loudspeakers had not yet become common.
Most listeners used headphones or horn speakers. Large cone loudspeakers capable of filling an entire room with music were still years away. At that time, the UV-201A was designed primarily as a voltage amplifier or detector tube rather than a true power output tube.
The Arrival of the UX-112A
Technology, however, never stands still.
As radio broadcasting became increasingly popular, people wanted more than just intelligible speech through headphones. They wanted music that could fill an entire room.
RCA responded by introducing the UX-112 in late 1925, followed by the improved UX-112A in 1927.
The most significant innovation was its oxide-coated filament.
Instead of relying solely on pure tungsten, the filament was coated with oxides such as barium and strontium, allowing abundant electron emission at much lower temperatures.
This was a remarkable breakthrough.
Battery-powered radios could operate longer while maintaining nearly the same amplification performance.
The warm, gentle glow that we associate with classic directly heated triodes today is also a result of this oxide-coated filament technology.
The filament itself also evolved physically.
Instead of a simple straight wire, the UX-112A employed a multi-fold hairpin filament, in which the filament was folded several times inside the tube.
This was far more than a cosmetic change.
By increasing the effective filament length, engineers created a more efficient electron source without significantly increasing power consumption.
The engineering philosophy had clearly begun to change.
Rather than merely amplifying signals, RCA engineers were beginning to design tubes capable of delivering greater power.
Part 2. The Evolution Toward True Power Triodes – UX-171A
The engineering direction became even clearer with the arrival of the UX-171 series.
Around 1927, RCA introduced the UX-171, followed shortly by its improved version, the UX-171A.
At first glance, the UX-171A looks remarkably similar to the UX-112A. Both tubes employ an oxide-coated multi-fold hairpin filament, and externally there appears to be little difference between them.
But internally, an important transformation had taken place.
Interestingly, the filament itself was no longer the focus of development.
Instead, RCA concentrated on improving the electrode system.
The plate became larger to dissipate more heat, while the grid structure was redesigned to accommodate higher current. As a result, the tube could deliver significantly more power without fundamentally changing its filament technology.
Although the amplification factor (μ) remained almost unchanged, the tube could drive much more current.
Output power increased from approximately 0.1 watt in the UX-112A to about 0.7 watt in the UX-171A.
This seemingly modest change represented an important shift in engineering philosophy.
RCA was no longer trying to build tubes with higher voltage gain.
Instead, they were building tubes capable of delivering greater power to loudspeakers.
Looking back today, this may have been one of the most significant turning points in the evolution of audio triodes.
Rather than repeatedly reinventing the filament, engineers continued to refine an already successful oxide-coated filament while directing their efforts toward optimizing the electrode system.
This approach would become the foundation for every great directly heated power triode that followed.
From an engineering perspective, the evolution from the UV-201A to the UX-112A and then to the UX-171A reveals a remarkably consistent trend.
- The amplification factor remained nearly constant.
- Plate current steadily increased.
- Internal plate resistance decreased.
- Output power continued to grow.
In other words, engineers were gradually shifting their priorities.
The goal was no longer simply to amplify weak electrical signals.
The new challenge was to move enough electrical energy to drive larger loudspeakers and reproduce music with greater realism.
Seen this way, the UX-171A was far more than just another receiving tube.
It represented the point at which the essential engineering concepts behind the modern power triode had largely come together.
The next step would not require a completely new idea.
It would require refining and integrating the technologies that had already been proven successful.
And that next step would lead to one of the most admired directly heated triodes ever created.
Part 3. The Final Step Before the Type 45
By the time the UX-171A appeared, RCA had already established most of the fundamental technologies required for a high-performance power triode.
The legendary Type 45, which would soon follow, did not emerge from nowhere.
Its essential engineering foundation had already been laid.
As we have seen, the UX-171A did not introduce an entirely new filament design. Instead, it built upon the already successful oxide-coated multi-fold hairpin filament, while engineers focused on refining the electrode system to handle greater current and deliver higher output power.
So what was the next logical step?
Surprisingly, the answer was quite simple.
If an oxide-coated filament had already proven successful, it could be refined even further.
If the electrode system could handle more current, it could also be enlarged and optimized.
Combine these two improvements, and a new class of directly heated power triode naturally becomes possible.
That tube was the Type 45.
Today, many audio enthusiasts regard the Type 45 as one of the finest directly heated triodes ever produced.
Personally, however, I do not see it as a sudden stroke of engineering genius.
Rather, I see it as the natural outcome of more than a decade of steady evolution.
The story began with the modest UV-201A.
It continued through the UX-112A, where filament technology took a major leap forward.
It matured with the UX-171A, where the electrode system evolved to deliver significantly greater output.
Seen from this perspective, the Type 45 was not an isolated invention.
It was the moment when years of accumulated engineering experience finally came together.
Looking back, the direction of that evolution was remarkably consistent.
First, engineers sought ways to emit more electrons while consuming less power.
Next, they focused on increasing plate current.
Finally, they designed tubes capable of driving increasingly larger loudspeakers.
The Type 45 became the first truly complete expression of that engineering philosophy.
Yet the story does not end there.
The Type 45 itself would become the foundation for another legendary directly heated triode—the 2A3.
In the next article, we will explore how RCA transformed the technologies perfected in the Type 45 into the 2A3, a tube that many enthusiasts still consider one of the greatest audio triodes ever created.
The journey from the UV-201A to the Type 45 was not a series of isolated inventions.
It was a continuous engineering evolution.
And the journey toward the 2A3 had only just begun.
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