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 must be 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 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 a 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 Orban processor. The Orban 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 Optimod 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 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 was 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 an audio success, but a money success as the radio station ratings climbed up as listenership increased. Higher ratings mean more ad revenue for the station. There is no doubt that adding the use of vacuum tubes had improved the sound such that more people listened longer.
The design goal was to enhance the sound and not destroy dynamics. A compressor that would maintain an average level, but still allow dynamics through. The processor had a variable compression ratio, the ratio becomes tighter as the input level increases past the compression threshold. This allows soft passages to retain high dynamics while imposing more control on loud passages. There were two points of gain control. The first is at the input using an LDR (light dependent resistor) optical isolator for slow AGC connected as a variable shunt to ground, audio did not actually pass through the LDR. The second point of gain control was a 12AX7 triode in the audio path between the input and output stages for fast AGC. The LDR slow gain control at the input keeps an average level plus helps prevent the triode gain controlled stage from being over-driven. The result is a fast AGC riding on a slow AGC. The slow AGC maintains a moderate fast AGC action.