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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 quatity 2.

Amplifying With Vacuum Tubes

Circuit Design Guide for the Novice

Amplifying With Vacuum Tubes is a book written at a level for hobbyist with a different approach. Instead of using schematic circuit diagrams pictorial circuit illustrations are used. A custom vacuum tube experimental breadboard correlates working circuits to design calculations. All calculations can be solved using a standard 12 digit calculator, examples are provided. Several factors of circuit design including bias requirements, voltage gain requirements and power supply requirements are covered. Basic electronic knowledge of voltage, current and ohms law related to vacuum tube circuit design is included where appropriate. A table of component values for the popular 12AX7 in various operating parameters simplifies pre-amplifier stage design.

The 70+ pages of circuit design contain enough information to design high quality vacuum tube amplifier circuits. Using sound pressure levels to determine amplifier power is included. Sound pressure levels (SPL) help calculate how much amplifier power is required to produce a desired loudness.

The last section of the book follows the design of a twelve watt per channel vacuum tube stereo amplifier. Controls on the amplifier include two inputs each with their own volume control, channel balance and bass and treble tone controls.

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.

Single Ended Class A Amplifiers
Low Power Audio Systems

There is merit for the use of single ended Class A power amplifiers. Although a properly biased push-pull Class AB amplifier will not have crossover distortion it is possible for bias to drift or output tubes to change characteristics with age causing some crossover distortion. Single ended Class A amplifiers will never have crossover distortion even with bias drift or aging tubes. Distortion content differs between single ended Class A and push-pull. Push-pull outputs cancel even order harmonics reducing or eliminating the second harmonic, but pass odd harmonics. Single ended Class A outputs pass both even and odd order harmonics. The main drawback is a limited practical power output of up to about twelve watts for single ended Class A output.

Amplifier wattage is not the main factor determining loudness, it's the sound pressure level (SPL) produced by the speaker. An example of sound pressure is seen in the movie Back to the Future. Using a high power amplifier and massive speaker Marty McFly strums a guitar and blows himself across the room. Of course in the real world such a thing is not possible. However, in the confined space of a home the combination of high efficiency speakers and low power amplifiers can produce fairly high sound pressure levels.

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 ten feet from the speakers should be sufficiently loud.

Sensitivity of speakers is usually specified as so many dB @ 1W/1m (1 watt at 1 meter from speaker). For example, 90dB @ 1W/1m. Higher sensitivity speakers are more efficient and require less amplifier watts for a specific sound pressure level. For example, a pair of BIC Formula FH-65B speakers with a sensitivity of 96dB @ 1W/1m requires less amplifier power than the BIC Venturi DV62si speakers with a sensitivity of 90dB @ 1W/1m.

At crownaudio.com you can use the 'Amplifier Power Required' calculator to determine amplifier power. Listener distance from speakers and speaker sensitivity are the prime factors. Enter the listener distance from source in meters. If 10 feet, then enter 3 meters. Enter the desired level at listener distance, 80 (dB). Enter sensitivity of the speakers. For example, 90 (90dB @ 1W/1m). Amplifier headroom at 3dB to accommodate audio peaks. Entering those parameters you should find that it only requires two watts of amplifier power. If the speaker has a sensitivity of 93dB, then only one watt is required.

While searching for speakers you may find variations of stated sensitivity. For instance Klipsch states sensitivity as so many dB @ 2.83V/1m. With an impedance of eight ohms 2.83V across eight ohms equals one watt.

SPL COMPARISON

dB — SOURCE

40 — Quiet library

50 — Average Home

60 — Speech @ 3 feet

70 — Vacuum Cleaner @ 3 feet

80 — Side of Busy Street

90 — Diesel Truck @ 30 feet

100 — Jack Hammer @ 3 feet

110 — Chainsaw @ 3 feet

120 — Pneumatic Drill @ operator

130 — Artillery @ 100 feet

SPL levels stated are based on average published values.

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