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.

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.

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.

This particular design uses all triode amplifying stages including a triode gain control stage. The original prototype was used in radio broadcasting operating 24 hours a day seven days a week for over a year. During this period the unit never drifted and never required any service, it operated flawlessly. When the vacuum tubes were tested after this period they all tested good an indication that the tubes were operating well within safe limits.

Most of the circuits use common available parts. There is a specific type VTL5C2 LDR optical isolator that must be used. As of 6/2016 the VTL5C2 is discontinued, but available as surplus requiring a little searching to find suppliers. Two 6AL5 dual diode tubes are used in AGC DC control circuits. The 6AL5 tubes are no longer manufactured, but are easily obtained as new old stock (NOS).

The primary intent was to enhance the sound and not destroy dynamics. A goal was set to produce a compressor that would maintain an average level, but still allow dynamics without artifacts. Common artifacts with many compressors include ‘pumping’ when bass notes vary AGC gain or ‘ducking’ from sudden high input levels. The basic design concept is simple, an input stage followed by a gain controlled stage, then followed by an output stage. All the stages that audio pass through, including the gain control stage, should be triode vacuum tubes. Triodes are desired because of their predominant second harmonic distortion characteristic. Additional stages would be used to amplify the audio signal high enough to drive stages that convert the audio to DC control voltages for the AGC circuits.

This amplifier is not designed to be a brick wall processor. In fact, during listening test it seemed as if the processor was not having any effect on controlling levels until the test/operate switch was switched into test mode and the output level began to rise significantly. Also, the sound quality remained the same in operate or test mode.

The processor has 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 are two points of gain control. The first is at the input using an LDR (light driven resistor) optical isolator for slow AGC connected as a variable shunt to ground, audio does not actually pass through the LDR. The second point of gain control is the 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. Both the slow AGC and fast AGC have attack/release adjustment controls.

Several of the stages used for AGC control could have been designed using solid state components. Vacuum tube circuit characteristics are different from solid state so it was decided to make the amplifier design 100% vacuum tube.

This could be a project for the advanced builder. Having an audio generator and oscilloscope is necessary to test operation of the amplifier. Circuits and parts list are provided in the following PDF file.

Vacuum Tube Gain Controlled Amplifier PDF

Original Amplifier

This is particularly important with power output tubes. The filament in a vacuum tube gives off heat. More voltage applied to the filament runs the filament hotter. Heat from the filament increases the temperature of other elements inside the tube including the plate. By applying more than the optimal filament voltage value you are in effect increasing plate dissipation. If plate dissipation is already near maximum value, added heat from the filament could shorten tube life or cause a failure.

This heating effect was proven while running experiments. With the filament at the upper voltage limit and plate dissipation near maximum one tube failed and a second started popping. Lowering the filament voltage to its optimal value remedied the problem.

Consider building a lower power system. Components such as transformers and output tubes used in lower power amplifiers are significantly less expensive. Besides costing less, lower power transformers are physically smaller requiring less chassis space.

Amplifier power output does not affect sound. An amplifier with 10 watts of output power will sound the same as a 100 watt amplifier. The difference is loudness. Rather than using amplifier output watts as a gauge for loudness use sound pressure levels, (SPL).

Sound pressure levels above 85dB are high enough to cause some hearing loss depending on exposure time. The higher the SPL the more chance of hearing loss. At an SPL of 100dB you really should be wearing ear protection. At 120dB without ear protection the chance of hearing damage is high even for short periods of exposure. At 140dB ear damage will most certainly happen without ear protection. If you would wear ear protection out in the environment, then why would you want such high SPL in your listening room. Unless of course you are planning on wearing ear protection.

Assembling an audio system using 85dB as reference for maximum SPL would provide substantial loudness. If paired with the right speakers, 12 amplifier watts can provide an SPL of 85dB. Sensitivity of speakers is usually specified as so many dB @ 1W/1m (1 watt at 1 meter from speaker). For example, 93dB @ 1W/1m. Higher sensitivity speakers are more efficient and require less amplifier watts for a specific sound pressure level. A pair of speakers with a sensitivity of 96dB @ 1W/1m requires less amplifier power than speakers with a sensitivity of 93dB @ 1W/1m. With speakers having a sensitivity of 93dB @ 1W/1m 12 amplifier watts will provide an SPL of 85dB 20 feet from the speakers.

At crownaudio.com you can use the 'Amplifier Power Required' calculator to determine amplifier power output. Listener distance from speakers and speaker sensitivity are the prime factors. Enter the listener distance from source (speaker) in meters. If 20 feet, then enter 6.1 meters. Enter the desired SPL level in dB at listener distance, for example 85. Enter sensitivity of the speakers, for example, 93 (93dB @ 1W/1m). Amplifier headroom at 3dB to accommodate audio peaks. Entering those parameters you should find that it only requires 12 watts of amplifier power.

For music a normal SPL listening level is around 70dB, 80dB is loud. Sound pressure levels above 85dB can cause hearing loss especially with long continuous exposure. A home audio system designed to produce an SPL of 80dB to 85dB ten feet (or twenty feet in larger rooms) from the speakers should be sufficiently loud. If you pay a little extra for more efficient speakers and drive them with a lower wattage amplifier, over time you will save money. Besides the savings in amplifier costs lower power amplifiers draw less current with lower electric power costs. Vacuum tubes used in lower power amplifiers are less expensive, a savings each time you replace tubes.

More information:
Noise-Induced Hearing Loss
Hearing loss and music
Loud Noises Damage Hearing