Vacuum Tube Amplifiers

"they sound good and keep you warm at night"

A Brief Bio of EJ Jurich
by
EJ Jurich

Originally From Chicago
Current Age -
Profession
Radio Broadcast Engineer, retired

Started tinkering with electronics in 1955 at the age of ten. I had a table in my room where I started experimenting with tube amplifiers. The tubes I used were 35W4 rectifier, 12AX7 and 50C5 output tubes. These were mostly tubes used in AC/DC radios of the time. Filaments were wired in series and connected directly across the 115VAC power main. The 35W4 rectified AC directly from the 115VAC power main, there was no power transformer. This would mean working with a chassis where ground circuits connected directly to the AC power main. AC plug prongs were the same size allowing a plug to be plugged into an AC outlet either way. Depending which way the amplifier was plugged in circuit grounds might actually be on the hot AC main.

Sound dangerous? Yes and I got my share of shocks. I learned really fast how to safely work with high voltage circuits. The AC/DC concept was a throwback to an earlier time when some parts of a city might have DC power. It was not a good concept for high quality audio. To prevent dead shorts across the AC main, circuit grounds to the chassis and input/output connections were through a capacitor bridged with a high resistance of around 220K ohms to limit current. This created some hellacious ground loops requiring AC plugs to be reversed. Even then ground loops would haunt.

Around twelve years old my father bought me a Space Spanner Knight Kit. He wanted me to experience a radio with a regenerative detector like he had when he was young. I built a few more Knight Kits and continued coming up with audio circuits of my own. My favoriate home built project from around 1964 was a stereo amplifier using push-pull 6CA4 output tubes. About the same time Knight Kit came out with a new soild state amplifier kit claiming great specs for the time including virtually no hum. I decided to build one of these new high-tech amplifiers. I took a trip down to Allied Radio on Western Avenue and bought one.

After a couple days of wiring the kit together, I connected the amplifier to my system and powered it up. The first thing I noticed was a significant hum when the volume was turned up. In fact, a much higher hum than my 6CA7 amplifier. The wiring was rechecked and was correct. Some of the audio wiring was unshielded so I tried to reposition wires, but made no difference. I decided the hum was below normal listening levels and let it be. Although I did wonder what Knight Kit thought virtually no hum means.

Started playing an album on the turntable and after a couple songs I felt the amplifier sounded flat, no life to the sound. I tried several albums and results were the same. Connected my 6CA7 amp back up and the sound improved 300% (well sounded a lot better). Set the high-tech amp aside and went back to my tube amp. I've been a tube person ever since. If you read the entire Allure of Vacuum Tube Amplifiers article below you will see that the sonic difference of vacuum tubes still held true in 1990.

WROK, Rockford 1969
Me in 1969
WROK, Rockford

Denver, 2009
Me in 2009
(and wife)
Last Radio Gig
Denver

In 1969 I developed an interest in radio broadcast engineering. During the summer I studied, took the FCC tests and got my FCC First Class License. I landed my first assistant engineer job at WROK in Rockford, Illinois. A few months later I heard CBS in Chicago was looking for staff engineers. I stayed with CBS for three and half years. Then, WROK was looking for a new Chief Engineer and offered me the job. I was back in Rockford for my first Chief Engineer position. Those were fun times. WROK-AM was a high rated top-40 station still spinning 45s'. Slightly more than forty years were spent working at various radio stations around the country as Chief Engineer. Some of the cities include Milwaukee, Miami, Mobile, Baltimore, Birmingham, Grand Rapids, St. Louis and Kansas City.

After I retired, I had more time to work on tube amplifier designs. While browsing around the Internet I noticed there was not much vacuum tube information available for hobbyists. Web sites and books had material, but not at a level for hobbyist. In particular, the math was far more detailed than necessary to build a working tube amplifier. I decided to take a stab at writing a book geared towards the home electronic hobbyist. I did finish one in Kindle eBook format titled Vacuum Tube Amplifier Basics. Not being an experienced writer that first book was full of grammar errors and not well written. It did, however, sell fairly well and I was inspired to fix up the book.

The Allure of Vacuum Tube Amplifiers

There are a few possible reason for the allure of vacuum tube amplifiers. Perhaps it is nostalgic, status symbol of owning a vacuum tube amplifier or possibly because of the way audio sounds amplified through vacuum tubes. If you ask audio enthusiasts who prefer vacuum tube amplifiers, more than likely they will say it's the sound. The opinion of a few audio enthusiasts may not mean much. How about the opinion of a few thousand ordinary people. Click here to read rest of the article

In 1990 an experiment was performed by this author while Chief Engineer of WWMX-FM in Baltimore, Maryland. Without the knowledge of anyone an all vacuum tube gain controlling amplifier was installed in the on-air audio chain. The following is a summary and result of the experiment.

Although the radio station audio was in stereo and had a clean sound it lacked realism and depth, something that this author remembered from mono Hi-Fi systems of the 1950s. The studios and audio chain were all analog from music source to transmitter. After considering any differences in equipment configuration, it was decided that the primary difference was the use of vacuum tubes back in the 1950s.

Using a few decades of vacuum tube experience a project was started at home to build a vacuum tube gain controlled amplifier more commonly called a compressor. Audio compression is used by most radio stations to maintain a steady average loudness. The design was all 12AX7 triodes including a gain control stage; triodes were selected because of their second and third harmonic characteristics. The reason for building a gain controlled amplifier rather than just a simple buffer amplifier was for loudness. One of the pitfalls of radio broadcasting is the fact that every station Manager and Program Director want to be the loudest station on the dial. This usually results in a lot of clipping and processing of the audio with the resulting harsh high end. Using a triode as the control stage requires controlling grid bias and varying stage gain. Using grid bias to control gain has about a 30db useful range sufficient to maintain an average level. As the triode goes farther into biased gain reduction it produces increased second and third harmonics, the second harmonic adds warmth to the audio while the third adds loudness.

The plan was to create the vacuum tube gain controlling amplifier, place it in front of the existing ORBAN™ Optimod 8100A audio processing, then reduce the processing and clipping in the existing processor. The existing processor would be limiting the audio signal only enough to prevent over-modulation without adding a harsh edge to the sound, the vacuum tube processor would then make up for the loudness.

In order to get honest listening results to see if anyone would notice any difference the new gain controlled amplifier had to be installed without anyone's knowledge. One night after midnight the all tube gain controlled amplifier was secretly installed in front of the existing Optimod 8100A processor. The Optimod was set so its internal input broadband compressor did very little processing instead letting the vacuum tube gain control do all the broadband processing. The high frequency clipping was also reduced.

The next day listening at home the sound of realism that was missing could now be heard. It was a subtle difference with the addition of second harmonic content generated as the tube processor performed gain control. Listening also revealed that although the amplifier was controlling gain, louder passages still sound louder even though the actual level is being reduced. This is attributed to the extra harmonics adding loudness.

A few days went by and then compliments started coming in on how good the radio station sounded including calls from other radio station engineers. One day a music consultant once employed by the station walked in and said he was driving through town and wondered what we were doing that sounded so unique. Opening the back of the equipment rack his mouth dropped open when he saw all the glowing vacuum tubes.

Not only was this a success, but a big success as the radio station ratings climbed up and beat out most other stations in listenership. Higher ratings mean more ad revenue for the station. There is no doubt that by just adding the use of vacuum tubes had improved the sound such that more people listened longer.

One last listening test:
Several years later I was working for a group of radio stations in St. Joseph, Missouri. While talking with the program director I explained my project with the vacuum tube amplifier mentioning I still had the amplifier. He was curious to hear what it sounded like so I installed it in the audio chain of the country FM station. A couple of days later he tells me that now he can hear instruments in the music he never noticed before. A month later, without saying anything I removed the tube amplifier. A day later he came to me and asked if I removed it saying the station no longer sounded like it did.

6SN7 Headphone Amplifier

This is an experimental headphone amplifier using one section of a 6SN7 as the output tube. Power output at 1KHZ is ½ watt sufficient to drive headphones loud and speakers in a small room to a moderate listening level. With the tone control set in middle rotation frequency response measured 40HZ to 18KHZ -3db points.

6SN7 Amplifier

Speaker Binding Post

Gold Plated
Speaker Connect Posts
4 for $11.48

free shipping

Custom Panel Engraving

Single Panels
Prototypes
Production Runs

Tube Amp Transformers

Power Transformers
Output Transformers
Custom Wound Transformers
Made In U.S.A.

Vacuum Tube Amplifier Basics
8½ X 11 Paperback
$14.25
LULU Bookstore

Principles of vacuum tube operation includes function of grids, effect of tube capacitance, tube resistance, heat dissipation and voltage gain. Projects include a 30 watt monoblock amplifier designed for tube rolling.

Vacuum Tube Amplifier Basics
PDF eBook
$7.00
Secure Payment & Download

PDF is an excellent format for eBooks. They can be opened in many tablets and on a PC or laptop using Adobe Acrobat Reader.
This is the only up to date eBook version of the print book available direct from the author.

Acrosound Amplifier Circuits

These are circuits from the 1955 Acrosound transformer catalog. The circuits are updated using current available transformers, component values and have a parts list. Building using only a circuit drawing as guide requires some amplifier building experience.

6L6 CIRCUIT

6L6 Push-Pull
Center-Tap Output Transformer
6SL7 Driver/Phase Inverter

KT66/807 ULTRA-LINEAR

Williamson Type Circuit
Pentode or Triode Output Option
6SN7 Driver/Phase Inverter

6V6 CIRCUIT

6V6 Push-Pull
Ultra-Linear or Tetrode Option
12AX7 Driver/Phase Inverter

The Rejuvenation of Vacuum Tubes
by
Lane S. Upton

This is a good reference source for those who wish to try bringing weak tubes back to life. It does require the use of a tube tester.

Rejuvenation of Vacuum Tubes PDF

Secrets of Output Transformers
by
Menno van der Veen

A lot of good information about output transformers, amplifier performance and selecting an optimal output transformer for a special application.

Secrets of Output Transformers PDF

RoHS beware the Grim Reaper
RoHS, beware the Grim Reaper of electronics

vacuum tube amplifiers
High Voltage Audio
New to electronics or experienced, when working with vacuum tube equipment be careful where you put your hands.

DISCLAIMER

ejjurich.com assumes no responsibility for damage, injury or otherwise related to any use of information on this site or given by other means. Many electronic circuits, in particular vacuum tube circuits, operate with dangerous voltage and current. Always excercise care when working with electronic circuits.

EJ Jurich has no association with sites offering free downloads of my book. A site that offers free downloads or wants you to register beware. If the download is free, then why would they require you to register? Free download or torrent sites are notorious for passing malware on to you. Be very careful.

For those wondering what happened to the resource links at the bottom of the page. Those links needed to be verified every few weeks. In an effort to reduce web site maintenance and have more time to work on circuit design the site was reduced to primarily my own material. EJ Jurich