VACUUM TUBE AMPLIFIER BASICS BOOK NOTES

6V6SE Monoblock parts list
2016 Edition - page 190
Revised April 2017 Edition - page 184
The 450 ohm 5 watt resistor listed as metal film can be a wirewound type.
The 470K 1/2 watt 5% resistor should be quantity 2.

Total Amplifier Voltage Gain
April 2017 Revision - page 116
October 2018 PDF Revision - page 117
Total amplifier voltage gain should read "is voltage gain of pre-amplifier stage times voltage gain of the driver. (18.7 X 10.44 = 195)

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. 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.

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