A Historic Turn of Events
A very interesting thing happened in the electronics industry during the middle of the 1970s. This was a time of great turmoil of which few people were aware. The mighty vacuum tube had been the mainstay of the electronics industry for over 65 years. It was the technology that had helped win World War II. It was the technology that had put men on the moon. It was the key technology which had brought ease, connivence, and comfort to millions of people. Virtually every element of industrialization and modernization had been built on the vacuum tube. It was also a doomed technology.
Back in 1947 a team of American physicists named John Bardeen, Walter Brattain, and William Shockley had developed an interesting little device called the transistor. While the mighty vacuum tube was bringing earth shaking changes to the lives of millions, it’s replacement was steadily studied and improved. By the middle of the 1950s, products utilizing the transistor began to emerge. But the technology was new, expensive, and not highly reliable. So the vacuum tube marched on as the electronics technology of choice for most consumer products. Slowly but surely, transistors improved and gained ground. By 1970 it was much cheaper, much more reliable, and making inroads in almost every area.
At this time, one of the last remaining hold outs of the vacuum tube was the television. The very high voltages and the power required for television picture tube circuits were an ideal fit for the inexpensive vacuum tubes and were difficult areas for transistors conquer. At the same time televisions had undergone a major evolution. The large oak and walnut cabinets of the 1950s had given way to lighter and easier to manufacture materials. And televisions had gotten much cheaper. As such, the number of televisions per household was increasing rapidly. Virtually every American household had at least one television and many had more. Sales were booming! So the manufactures did the only thing they could to keep up. They ordered millions of vacuum tubes from suppliers to meet the ever increasing demand.
They it happened. Almost overnight, the price of transistors plummeted and their reliability soared. New break throughs in the application of transistors were happening daily. And the burgeoning Japanese electronics industry fully embraced the new technology. In order to compete, television manufacturers were forced to abandon the tubes and embrace the new technology. The virtual death of the vacuum tube had arrived.
However, there was a catch. The industrial supply lines were full of vacuum tubes. There were warehouses filled with millions upon millions of vacuum tubes destined for televisions which were now being built with transistors. These tubes were sold for scrap. Millions were destroyed; melted down for the metals and glass of which they were made. However, millions were also bought up by entrepreneurs, romantics, and mass collectors. They have been held for years and are still stacked high in old warehouses across the US. And they are now for sale. Thanks to the internet (another gift from the transistor) these tubes are now being made available to people who can make use of them. And this is were we come back to this project.
This amplifier is based on a tube intended to drive the vertical deflection circuits in larger televisions, the 6EM7. It has two dissimilar triodes in one envelope. One is a high gain, low power unit intended as an amplifier or oscillator, and the other is a higher power unit intended to drive the magnetic deflection coils on the back of a CRT. At audio frequencies the combination is just what we need to build a nice little audio amplifier. One tube per channel. And there are many thousands of these tubes and tubes like them readily available today.
The Electrical Design
When using one of these tubes obviously there is no option to make substitutions. As such, the key to a good design is to understand the differences between the triode units and to make the most of the characteristics of each.
Section 1 of the 6EM7 is a very linear high gain triode. Section 2 is a low gain high power triode. As such, section one becomes the signal amplifier and driver, and sec 2 becomes the amplifier power stage. Below is the simple schematic for this amplifier.
Each channel only requires one tube making for a very simple design. The power supply uses a “split rail” design which provides independent filtering for the left and right channels. The helps provide good channel separation and a nice wide sound stage.
This design uses section 1 as a single ended triode voltage amplifier with a gain of about 32dBv to drive section 2 as a single ended voltage amplifier. The peak output power is about 2Watts per channel. With the volume control at max, it requires about 0.88v peak to drive the amplifier to full power. Most small devices like iPods, phones, and portable CD players will easily drive this amplifier.
What really drove me to build this amp was an idea about amplifier layout. Looking back at most of my designs, and most of those on the internet, revealed a very typical pattern. Amplifiers tended to be flat chassises with tubes and transformers above and electronics below. From a historical perspective this makes perfect sense. Metal chassis were built this way to facilitate packaging and assembly. When people started to make tube amplifiers again, they just removed the upper case and built in the same way.
But I wanted something a little different. I wanted a case design that would really showcase the tubes and hide the rest of the electronics. I also wanted a more old time look. Something that harkened back to early tube equipment of the 1920s. My primary design elements were shadow box alcoves for the tubes themselves, panel meters for monitoring operation, large old style controls, a jeweled power indicator, and a nice accent of leather in the handle to showcase portability.
I started by choosing a nice dark walnut that would reflect the early tube equipment of the 1920s. From here I decided to go in an entirely different direction; vertical. By making this decision, the amplifier footprint becomes much smaller, and the tubes can be showcased at different levels. Here is a view of the chassis dry fit prior to glueing, sitting on my work bench. This was going to be something totally different from anything which I had built before.
From here I took to fitting all the various elements together. I settled on black metal accents for all the component mounting surfaces as well as the bottom and back covers. This translated in to a fair amount of metal plate. Here are all the metal parts, cut and drilled, prior to final finishing and painting.
Here can also be seen the pieces of aluminum angle which are used to mount the wood alcoves into which the tubes are mounted and the long set which hold the power transformer suspended over the bottom access hole. Here is a picture showing how major assemblies fit in the chassis. In this picture the primary power supply filter components have also been mounted.
The bottom access plate (to the right in this photo) is necessary to wire up the bottom side of the rectifier socket and the power transformer. In order to wire the primary amplifier I removed the upper tube assembly and wired up the two amplifier stages outside of the chassis. This way all that was left was connecting a few wires to the assembly. Here is a picture of the wired 6EM7 assembly prior to installation in the main chassis.
By installing all the major wires to the assembly, it minimizes the number of connections that have to be made with the unit installed. Finally here is the inside of the chassis with everything wired up and ready to go. The inclusion of the power transformer and three filter chokes at the base of the chassis make this a very stable build that is in no way top heavy or prone to tipping.
The adjustment control on the back of the rectifier housing is actually a B+ voltage trimmer to keep the dissipation of the power stage plate circuits within acceptable limits.
I didn’t submit this amp to a lot of formal testing. This design (or variations thereof) has been built by many different people since I first developed the electrical design back in September 2011. And then revised it in December 2013. The clean output power is slightly over two watts per channel and the frequency response is consistent with the advertised performance of the output transformer (40Hz to 18kHz).
The amp sounds wonderful! There is really nothing which can compare to the sound of a single ended triode amplifier. Bass is well articulated without being boomy, mid-tones are clear and even, and the highs are crystal clear. The amp has exceptional transient response; reproducing my favorite classical recorder piece (“Frederick The Great : Sonata in B-Flat – Allegro” performed by Michala Petri) with clarity, lightness, and vibrancy.
It is a success on two different accounts. First, the electrical design and execution has resulted in a fine sounding amplifier. And second, the physical design of the amp, with it’s different approach to showcasing the tubes, has resulted in a unique and beautiful amplifier with just the early 20th century feel which I wanted.
Some Design Postscripts
There are two additional items about this amplifier which are worthy of note. The first is its thermal design. With the tubes contained in small cubby holes, there was some concern with respect to overall heating of the tubes and chassis. This is especially true with the power tubes where the quiescent power dissipation is over 32 watts. The design approach was to have the tops of the compartment ventilated with holes. The thought was that the air flow pattern would be in through the front, through the vent holes into the chassis, up through the inside of the chassis, and out the vent holes in the chassis back. This would create a convection cooling flow which would keep chassis heating down.
This approach was successful in the lower rectifier compartment where peak dissipation was less than 8 watts. In the upper compartment the design is just marginal. The heat load due to direct infrared radiation was not fully appreciated. The tubes themselves still remain well below their maximum bulb temperature. The impact is on the chassis itself. The black top plate on the power tube compartment has an equilibrium temperature of approximately 156°F (≈ 69°C) in normal operation. This is still well below my standard internal chassis derating temperature of 185°F (≈ 85°C) but it is still higher than I would like. If using this design approach again, I will endeavor to keep these temperatures somewhat lower through larger compartment size, better ventilation, and perhaps the use of a higher emissivity finish on the metal parts.
When building up this amp I decided to use a jewel power indicator with a typical 6.3v indicator bulb. However, when testing out the various design elements I decided that even the 0.5 candela type 47 bulb (6.3v @ 150mA) was far too bright. However I did find that the 0.9 candela type 44 (6.3v @ 250mA) could be dimmed across a large portion of it’s range by a simple 50Ω variable resistance. So a small 50Ω, 5W wire wound rheostat was put in line with the indicator to taylor its intensity to match the glow of the tubes. It is purely an esthetic addition to the amp but it is one of those little things that can make a piece of equipment just “look right”.