A Simple Project

 

I bought a pre-amp kit from Amazon. It is a pre-amp valve tube buffer and it means that it is a go between audio source (e.g. CD Player, iPhone) and the power amplifier.

The kit is assembled most of the parts and I connected the unit with plugging in to the blue and white wires. The connection diagram cam with the shipment. The connection part is not difficult if you are comfortable to do wiring. Here were what I did to the wiring worked.

The first step is to connect the transformer to the 110V household current. Two red wires go to one side and two black wires go to other side of the Household connector. The connector will then plug into the 110V electrical outlet. In order to protect people from electrical shock, I used a heating-shrink tubing to protect the exposed areas of the wiring.  Then, I turned into the Valve Tube voltage side. I connect the two blue wires to the 170V connector and two white wires to 6.3 V connectors of the assembled unit.

There two tubes came with the order. However, they looked like very fragile form the outset. Also, the tubes get very hot when I turn the unit on for a while. It looks like the tube won’t last long. I am planning to replace the tubes with other higher end type.

 

I tested the new pre-amp with iPhone and my BOSE headphone to see whether the sound quality is good. I am not a tube aficionado, I can only say that after using it for a while to let the tubes break in, and – er – switching back and forth between the new pre-amp and a solid state pre-amp – I find that I prefer the sound with the tube buffer engaged. So, to my ears, it is well worth the price.

 

The next step is to put the transformer into a housing. I brought a wooden box from Dollarama. The transformer fitted nicely into the box. With the holes setting out in place and aligning the household power cable and voltage cables, the transformer is totally enclosed by the box.

I’m happy with it and I am looking for a tube power amplifier to work with it.

As I don’t have a power amplifier to demonstrate the pre-amp performance with speakers. Here I use a YouTube video to demonstration its performance in speaker.

 

 

Power Amplifier

Valve Tube Amplifiers are good for their unique characteristics of damping factors, zero DC offset and safety to speakers.

Damping Factors

Damping factors are the measures for the effectiveness of an amplifier to control the loudspeaker and prevent the ringing of the loudspeaker. Tube power amplifiers can handle well with high output source impedances, they also have lower damping factors. The way of tube amplifier to delivery those performance because they use lower or no negative feedback and separate output transformers. Solid state amplifiers choose to use near-zero output resource impedances for damping factors and a standard tube amplifier is designed with an output impedance of about an Ohm. This subtle difference changes the frequency and transient response of the audio system when the loudspeakers are connected. The overall impact is to lower the low-frequency damping, which can produce a better bass blooming effect. Audience will find it preferable.

Zero DC Offset

DC  offset does replacement of the woofer cone and leads to the addition of speaker distortion. It is a result of unbalanced loads or problem of the audio amplifiers. Solid state amplifiers always have some amount of residual direct-current (DC) even they use the big electrolytic output coupling capacitors. The speaker will pick up the a slight trace of DC components among the amplified audio signals. A valve tube amplifiers are decoupled the output by transformers , it means there is no DC component passing to the speaker.

Safer for Speakers

If a solid state amplifier get a broken transistor which is short-circuited in the output, the speaker will be blown away. However, if a tube amplifier has a output short-circuit problem, the speakers are isolated from the valve amplifier by the transformer.

 

Tube Maintenance

Tube is the core of an amplifier. In order to maintain a proper performance of the amplifier, the first most important thing is to maximum the tube life expectancy. The average service life is about 10,000 hours for a conventional amplifier tube.  The 10,000 hours of expected tube life is an estimate, however, most of the tubes can serve that long. If the amplifier is used by 300 hours of service every year, the tube will last 33 and 1/3 years. If we include 50% discount factor, the tube will survive for about 15 years. Some tube may fail earlier than this estimate, however, most of the tubes are indeed last that long. Therefore, 15 years is a good estimate for the tube life expectancy.

It seems that tubes are far more reliable in fact than people think of them. A lot of people think tubes are fragile, therefore, they practice premature tube replacement as a precautionary measure to maintain the amplifier performance. However, the tube should be replaced when it is ailing or failing.

The most compelling and immediate criterion for tube replacement is any corruption of the tube’s vacuum simply because the consequences can be dire. This corruption can be slow and insidious as is the case with thermally induced hairline cracks or as abrupt as a burst or broken glass envelope. Both cases are extremely rare and usually occur in tubes that have some previous damage or are subjected to extreme mechanical duress, for example, when your well-meaning buddy drops your amp. You can determine if the vacuum is intact by examining the getter – that’s the metallic-, chrome- or silver-colored layer at the top of the tube – to check if it’s milky or discolored. If you spot discolorations or see that the entire top of the tube is stained with a milky coating, than air has entered the tube. It is defective and must be replaced.

 

New Old Stock (NOS) Tube Selection

 

 

If you interested in buying valve tubes replacement an amplifier, you should consider the New Old Stock (NOS). New Old Stock (NOS) refers the unit as “new on the self”. These are brand new tubes which are no longer produced by manufacturers and they may be 60 more years old. Here are famous manufactures.

• German : Telefunken
• Dutch : Amperex, Philips
• British : Mullard, Brimar, Mazda
• American : RCA,,GE,Sylvania, Ken-Rad, Dumont, Tung-Sol

However, not all the advertised NOS are really not used before. Some of them are obviously used. There are several ways to tell a new tube form a used tube.

Burn mark
The visible things to examine are burn marks (burn marks are the spot appeared on the metal part of the tube in dark colour). Also look at the base of the tube, the base will be darkened when it received the high temperature from the heating elements.

Scratches
We should also look for any scratches or marks on the plastic base of the tube or vertical scratches on the very bottom of the base. Those are the signs for used tube.

Getter
A good and hard vacuum inside a tube make a tube working reliability. In order to maintain the vacuum level right inside a tube, there is a “getter” designed to remove any residue gas. The getter is the metal in a small cup or holder built in the centre of the tube. The metal is used to react and absorb with oxygen. The metal is ignited as the final step in the production. The metal reacts with Oxygen and produces a getter flash inside the tube envelop. This is the silvery patch seen inside a glass tube. If the tube leaks, the silvery flash will turn white.

I hope those small tips can help people to find the true NOS.

Does Tube Matter

When transistor became available to the public market cheaply and plentifully, the audio amplifier market changed dramatically in 1970s. Tube amplifiers were considered as unwanted because tubes were old and hot and easy to break. Transistors are considered belonging to the future and fashionable. The similar situation just like today, who want an old dump phone versus a new smartphone.

However, the tide changes in the recent years, valve tube amplifier never die completely. A lot of music listening audiences still favour the valve tube amplifiers. The reason is simple, valve tube amplifiers sound better. Even solid state amplifiers claim that they sound like valve amplifier; they produce sound sweet almost like valve amplifiers.

The fact is that valve amplifier reduce the harsh sound in most hi-fi systems without losing the treble and musical details. It happens seriously in CD player. Solid state amplifiers tend to over-emphasise the sibilance of female singers. Tube amplifier allows the “silence” happens between the notes. Solid state amplifiers have a lot of “crossover” distortion at low levels, The crossover distortion remove the ambience, reverberations and echoes of a reproduction.

The better reproduction of music by valve tube amplifiers allow them to be relevant all the time in the Hi-Fi world.

 

 

Famous VTA Design

I would like to talk about a couple of famous vacuum tube amplifier brands in the field.

Leak is an old brand name for VTA. It started its business in 1934 and creased the production in the late 1970.  LEAK TL/12 Point One Amplifier was their signature product. The amplifier produced 15 watts output with only 0.1 % of distortion. It employed push-pull KT66 vacuum tubes over three stages of amplification.  The amplifier delivered high-performance at reduced cost. The later version of the VTA used a newer more efficient power valves and a new circuit design of “ultra-linear” in order to produce a higher power output by driving the tube capability s to its high end of characteristics. A large amount of  TL/12 to professional users and radio stations. Audio enthusiasts are still treasured the TL/12 in the Hi-fi market.

Wireless World published the design of “William Amplifier” in an article of the title “Design for High-quality Amplifier”. The design highlighted the way to reproduce high quality of radio.  After the publication, many Williamson amplifiers were made. The circuit design was simple and used negative feedback for the first of that kind.  The design also applied thought to use high performance transformer. It was a commercial success.

Mullard is a valve manufacturing company. It published  a famous book “Mullard Circuits for Audio Amplifiers”  to promote their tubes EZ80, EF86 and EL84 in 1959.  The most famous amplifier circuit Mullard 5-10. The new push-pull configuration and the good Partridge output transformer produced the great sound reproduction.
Dynaco was found in 1955 to produce hi-fi audio system. They were popular in the 1960s and 1970s for its highly affordable and excellent quality of audio reproduction.  They were also a major supplier of kit amplifier as well and produced about 300,000 units.

McIntosh Laboratory is US handcrafted high-end audio equipment founded in 1949. Their product signature is their black glass front panels. McIntosh was a supplier for the famous Woodstock Music Festival in 1969.  Their signature products are :

• Early 1980s: MC 2500 Power Amplifier, 500 WPC Power Guard and similar chassis to MC 2300
• Around 1990: MC 2600 Power Amplifier, 600 WPC Power Guard final version on MC 2300 Chassis
• Around 1990: MC 7300, 300 WPC and much smaller than MC 2300

 

 

 

 

Pitfalls of Vacuum Tube Amplifiers

A lot of music listening audience love valve tube amplifier, however, there is a couple of pitfalls for using valve tube amplifiers. Vacuum tubes are using an very old technology. The technology has some inherited issues. They are cost, quality, and burn out issue.

Vacuum tubes are relatively expensive than the solid state components. A good tube cost a about $20 to $100. You have to replace them from time to time. Some really high end amplifiers have a lot tubes as the driver, therefore, the cost of maintenance is high.

All the tubes are handmade; therefore, they are not created equal. The end result is that vacuum tubes are behave slightly different from each other. Most of the difference is not noticeable by regular audience, however, audiophile can sense the difference. Furthermore, for the low end tube, they produce a muddy sound and affect the overall amplifier performance.

Valve tube has a limited life time. The smaller tubes last about five to eight years according to the usage pattern. The large tubes will last about one to three years. The hard you drive the amplifier, the shorted the tube life will be. As the tube life depends on a lot of the external factors, one cannot predict the how long the tubes will last.

 

Compare with Solid State Amplifiers

The major performance difference between valve tube amplifiers and solid state amplifiers are signal distortion, clipping effect, and negative feedback

Distortion is the change of signal via the amplifier from its original form. Distortion is normally the undesired product during signal amplifier.  Valve tube amplifiers produce more distortion than the solid state amplifiers in term of second order, which is labelled as harmonic distortion. The distortion of that kind is welcome by the music listening audience as the even-order harmonic distortion is so pleasant to listeners. The harmonic distortion not only attracts people, the distortion increases according to the loudness of the volume. This type of distortion behaviour is called progressive distortion. It is similar to a situation that a performer strikes a percussion instrument harder, the instrument generates more even harmonic. When the notes reduce, the harmonic components fade away. Valve tube amplifier mimic the same behaviour.

Clipping is a form of signal waveform distortion when an amplifier is overdriven and tries to deliver a volume beyond its maximum capability. If the amplifier is driven into clipping, it may cause it to deliver an output beyond its rating. The signal is cut off or “Clipping” beyond the limit. Tube amplifiers overload linearity. It means that amplifiers will distort more but there is no set limited level and there is no sudden cut off of the signal or Clipping.

However, a solid state amplifier does not smooth out the signal during the overcapacity. The signal will be distorted to produce a sharp edge and it looks like the signal is clipped .The sharp edges of solid state amplifier’s signals at clipping produce an unbearable levels of very high order ultrasonic harmonics. The harmonic generates the strange sound on top of the music.

Negative Feedback is a process to use a portion of output as an input from a system in order to reduce the fluctuations in the output. As Tube amplifiers produce the output linearly proportion to the input. Therefore, tube amplifiers doesn’t use negative feedback as often as solid state amplifiers. Tube amplifiers without negative feedback tend to produce a rougher sound with more harmonic.

 

 

Tube History

 

Tube goes through three periods of development.

• Initial stage 1904 – 1847
• Advanced Stage – 1947 – 1970s
• Declining Stage 1970s – Present

Initial Stage 1904 – 1947

John Ambrose Fleming made the first tube called Diode which has two electrodes governing the float of electricity in one direction. The diode looks a light bulb with a lighting fragment and an extra electrode together.

In 1906, a third electrode was added for electronic amplifying the signal.

The ­use of feedback loop in the amplification circuit enables the amplifier operating at AB Class which improves the efficiency of the amplifiers.

 

Advanced Stage – 1947 – 1970s

Declining Stage 1970 –Present

After the introduction and expansion of solid state device, tube usage was deeply reduced.  However, tube is still popular in the area of guitar amplification and HIFI home stereos.