© Ernest Ruiz. 2001

This article is the result of all the messages I have received in my email box. In these messages, audio lovers from all over the world ask me questions about tube powered amplification. In most cases, the questions talk about the writer's desire of buying a tube amplifier and ask things like which amplifier to buy and how to do it. Also some questions are more technical, but in almost all the messages there's disorientation.

Due to this I have decided to write this article where I will give some advice about what to do when usign and buying tube amplifiers.

I will not talk about technical questions because there's a lot of information about that in the internet and this isn't the main goal of this article. In fact, I will understand that you know some technical aspects of tube amplification. But if you don't or if you want more information about these themes I recommend that you download a book in the Sonic Frontiers web site. These book explains easily all the tecnicalities of tubes and tube amplification. It is called "A taste of tubes" and it will be very useful even before reading this text. With it you will enjoy and also learn many new things

As you can see the title of this article is " I want a tube amplifier!! ". Many of you have written emails to me showing your interest in this kind of amplifiers. You want to buy one of them, but you don't know how to do it, what to look at, etc. Some of you, like these amplifiers because of their great looking, some for their sound. Many others talk about them because you have seen them in hi-fi magazines or at a hi-fi shop in your city. All of you have a special interest on these amplifiers.

The problem begins when you decide to buy the amplifier. All of you ask the magical question, Should I buy a tube amplifier? Mi answer is, Why not?

First, it must be cristal clear: a tube amplifier is as good as any other kind of amplifier. Of course, they have different characteristics that those of solid state amplifiers, but without any doubt, they are excellent amplifers from the audiophile point of view (some say that even better).

The main difference between solid state amplifiers and tube amplifiers is that solid state amps use transistors and tube amps use tubes. A tube is a crystal bulb, with different sizes and characteristics, that is used to amplify an input electrical signal, for instance from a CD player or a turntable, that is sent to a loudspeaker (this is how any amplifier works. Amplifies an electrical signal). In the most simply way, a tube is like a ligth bulb with no air inside of it (vacuum) where an electrical current of electrons flows between two poles, the anode and the cathode. This kind of tubes with an anode and a cathode is called diode (for example, a rectifier tube like the GZ34, that is a double diode). If we add to this two elements a third one called grid we have a triode (there're a lot of triodes: the ECC83 is a double triode used in preamplifier circuits and the 300B is a power triode). If we have another element to the tube we have a tetrode (for example, the KT66). If we add a fifth element we have a pentode (for example the EL84). And so on. As you can imagine there're a lot of tube models and every model has a specific application. Some of then are designed to give power, some to work with a very little distorsion, etc. Every amplifier will use a tube type depending on many things: designer wishes, power needed, etc. But that's enough technicalities. As I said before you will find all the technical questions explained in the Sonic Frontiers book. So if you want to know who was Lee de Forest, read that book.

After this minimal technical introduction to tubes let's point into some aspects that will help you when buying a tube amplifier. My reference when writing these lines is my own experience, but I have had some suggestions about questions to answer in this article from Bruno Anthonioz, Ignacio Basté, Eusebio Ruiz, Manuel Córdoba and Javier Naranjo. Thanks to them.

The first thing you will see when looking at a tube amplifier is its aesthetics. I admit that if the amp looks nice I like it more but I don't consider aesthetics the key, because what it is really critical is the sound. Sometimes when choosing an amplifer it is necessary to apply the WAF (Wife Acceptance Factor). My personal WAF is extremely low (no WAF, in fact) so I don't have problems when buying. If your WAF is highb, then you will have to look carefully to the aesthetics of the amplifier. But remenber that the first thing is the sound and the aesthetics is secondary.

After this first visual contact, you can start a more careful inspection.

External look:

-Chassis: Material, thickness and finish. Chassis are usually build from chromed iron and the letters are silk-screen printed on it. Thickness of chassis must be 2 millimeters or more because the transformers are heavy and the chassis must be rigid enough to resist that weigth without problems. If it bends, it is a bad starting point. You should know that a weak chassis can produce vibrations that are transmited to the tubes. The tubes amplifies the vibrations and these can be heard on the speakers (this effect is known as microphony, because the tube acts as a microphone). The chromed must be equal in all the chassis, without differences in aspect. The silk-screen printed letters must be resistant to the touch even with your nails and must accept cleaning with a wet soft cotton cloth or a metal cleaning product. Some chassis can be made of copper. Some DIY'ers use copper due to its better characteristics. If it's made from copper verify the same things.

-Front panel: Check all the selectors and buttons. Controls installed (volume, input selector, etc) and how they work (move them to see if they work fine). What are the buttons made from (plastic, aluminium,etc). Check all inputs (line, phono, etc), headphone socket, remote sensor (if present), leds, etc. Don't forget anything.

-Back panel: Check all the RCA connectors (if they're gold plated, size, etc), speaker terminals and impedances (preferably 4, 8 and 16 ohms), power cord (is it coming from the inside of the chassis or it is a detachable cord? It is better if it is detachable because in a future you can change it). Look at other details.

-Other aspects: Is there a protection grill for the tubes and it fits ok? Is it easy to remove?. Are there side panels and what are made of? Is the botton plate removable (to have access to the circuit).

After finishing the inspection of the external look, you can verify the amplifier configuration.

-Tubes: Brand and model. The brand will give you information about quality and durability. Refuse unknown brands or tubes not checked by markers (a company that verifys the tubes made by the manufacturer and that sells them under their own brand. They guarantee the correct operation of the tubes.). Unknown brands or tubes not coming from markers have less life expectance and a low quality: Some manufacturers names: Sovtek, Svetlana, SINO, EI, Yugo, JJ, Tesla. Some markers names: Ruby Tubes, Groove Tubes, Gold Aero, Electro-Harmonix, etc.

A interesting point about tubes is NOS tubes (New Old Stock). These are tubes made in the golden years of tube industry (50s and 60s, but also before and after these years) that are still new. They aren't used in modern equipment, except high-end equipment (veeery high). This is due their incredible high price. NOS tubes are of a very good quality: Mullard, Telefunken, RCA, General Electric, Western Electric, Sylvania, etc., etc. They sound great and have a excellent life expectance. Usually, I use in my tube equipment tubes from markers (today there're marked tubes of excellent quality) and when I can afford them, NOS or similar (for example JAN tubes- Joint Army Navy, tubes made by tube manufacturer for the USA Army and Navy, under strictly specificactions. A JAN tube can be also NOS). As you can see you can forget anything you've heard about tubes not being manufactured. It is not true. In fact, tubes have been manufactured always. For example, professional musicians have always used tubes in their guitar amplifiers. What it is true it that only some models are made. But almost everything can be found in the USA market, even models discontinnued for the last 20 years. The main problem is always the price. That's the reason why it is better to buy a tube amplifiers that uses tubes still in production. Some models of tubes still in production are: EL34, EL84, KT66, 300B, ECC83, ECC82, 6L6, 5881, etc.,

The model of the tube will give you information about the amplifier's output power (in combination with circuit configuration and class of operation). All the tubes have operating specifications given by the manufacturer and these are respected by the amplifiers designers. These specifications shows the operation ratings of the tube (voltage) and the power obtained at this ratings. Almost all the tube companies use the same circuit in their amplifiers (btw, circuits originally designed in the 40, 50 and 60) so it is easy to know the power of an amplifier only looking to the tubes. For example, an EL34 with and anode tension of 800 volts will give a maximum output power of 25 watts (this is an example where a the maximum anode tension is used. It is possible to obtain this output power with less anode tension. Others aspects that have their influence in output power are the impedance between the tube plate and the output transfomer primary, class of operation, etc..). If two EL34 are used per channel (this circuit configuration is called push-pull) we have a 50 watts amplifier. In general terms a EL34 tube offer betweeen 17-18 watts and 25 watts (35 watts and 50 watts is push-pull configuration). You should know that more power means more stress for the tube and also less life expectance. So it is better to design cicuits where the tube works relaxed, not at its full output power. How can you know the operating ratings of every tube? Read about them. All the specifications of the tubes (ratings, power curves, etc) are in the technical manuals of the manufacturers. Many tube and internet lovers have scanned these specification sheets and have upload them in the internet. You have to find them. Another example. The 300B power triode gives 8 watts with an anode tension of 350 volts in single ended configuration.

-Circuit configuration: I have already talked about circuit configurations. The basic configurations are: single ended and push-pull. Let's explain easily the concepts behind the terms (if you want more technical information read "A Taste of Tubes"):

-Class of operation: Mainly class A and AB. Almost all the power triodes work in class A, a class where almost all the energy dissipates into heat and the output power is low. The musical quality of this class is very high, but they run very hot. A typical design of a class A amplifier is 300B tube based single ended amplifier. It delivers 8 watts.

The class AB is the most common class. The energy is best used, so it dissipates less heat and delivers more power.The sound is perhaps less refined (it is still very good). A typical design of a class AB amplifier is an EL34 tube based amplifier in push-pull configuration. It delievers between 35 and 50 watts.

As a synthesis to clarify all this concepts.....My advice for those interested in tube amplification for the first time is to go for push-pull amplifiers with 35 watts of power output or more. They're the most frequent tube amplifiers in the market and they give the user less headaches. After that, if you are bitten by the tube bug you can try less powered amplifiers like single enden 300B, 2A3 (5 watts) and heavier things like the 45 (1,5 watts).

-Output transfomer: To me the key in any tube amplifier, the most critical component. The sound quality of a tube amplifier depends in the the quality of the output transformer, so you have to look for quality. Try to find amplifiers with this kind of output transfomers. Some manufacturers are: Hammond, Bartolucci, Sowther, UTC, One Electron, Magnequest, Tamura, Lundahl, Electra Print, etc. Usually the tube amplifier manufacturers buy output transformers made by these industries because of their quality and some of them design their own transformers that are built under their specifications also by these industries. So try to verify the brand of the output transformers of the amplifier you want to buy. If you can't verify this point be careful, except if the brand of the amplifier is well known. Also check working condition. If the transformer is too hot when you touch it, that means that it is working in stress condition or even they it has a problem (less probable). Output transformers are designed to work with a given model of tubes and to deliver a given output power. For example, a Hammond 1650P is designed to work, among others, with EL34 and KT66 and to deliver 60 watts of output power. If you use it to deliver more power it will burn.

-Power transformer: Equally important, but perhaps less. It must work relaxed. That's means that it hasn't to be very hot when touched. If it is very hot it means stress and less durability. It is better to use over-dimensioned power transformers. They will work relaxed and will last for a long time.

After you finish the inspection of all the external aspects and the amplifier configuration, you can continue with the internal inspection, if it's possible. This is more difficult because the amplifier must be opened by the seller and that means two things. First, it is dangerous because there're lethal voltages inside the amplifier. Second, warranty could be lost. It is very useful to see what there's inside the amplifier you want to buy because it gives you an idea of what you are buying.

When opened, you must look at the following things.

-Type of circuit: Ideally without printed circuits. Circuits without printed circuit are called point-to-point circuits, because they're wired and soldered point ot point. This type or circuits allows the owner to modify the electronic parts present in the amplifier to improve its quality (very common within the tube lovers). Some amplifiers have the tubes soldered to a printed circuit (ahhhggg!!). Forget them.

-Electronic parts: Try to find quality. There aren't many parts in an amplifier, basically resistors and capacitors, but they must be of good quality. In fact less is best, because all this parts modify the sound, but you will find some of them. ;-). Be ready to find parts with names like Holco, Wima, Sprague, Hovland, Jensen, Alps, Noble, Cardas, Kimber, etc. If you see this names, things are OK.

With this we finish our general inspection and we would have a close knowledge of the amplifier we want to buy.

Now, let's choose some audio components to partner with our amplifier. There aren't problems with sources. Connect the CD or the TT to the correct input and that's all. But usually, there're some problems with speakers, mainly due to a wrong combination of speaker sensitivity and amplifier output power. A bad relation is a disaster.

Today speakers are designed to work with high output power amplifiers. They are solid state designs and usually they deliver 30 watts or more. So, speakers are designed with this concept of power in mind and that's why they have low sensitivities beginning at 86db/1w/1m and reaching in the most efficient models 91db/1w/1m. A 86dB speaker with a 80 watts amplifier is not a problem, but it is with a 20 watts amplifier. The ideal combination tries to find the balance between power and sensitivity in that way: less power, more sensitivity.

So I recommend when using tube amplifiers, speakers with high sensitivity. In fact we would start with speakers at 90-91dB. With a 20 watts amplifier it is the minimum. With less power we can have some problems. We must find high efficiency speakers with 95dB or more. We can find other concepts of speakers when looking at high efficiency speakers. They are horn speakers based on Lowther drivers or similar with 100dB or more and specially designed to work with low power amplifiers like single ended triodes. We have to know that these are special designs and that any step forward in sensitivity is also a step forward in price.

My recommendation to tube novices, with the amplifier I have described before in mind (push-pull, class AB, based on EL34 with 35-50 watts of power), is a speaker with a sensitivity of a 90-91 db/1w/1m. Don't use speakers with less sensitivity because it won't work. And of course, try as many speakers as you can until you find what you want. Sometimes there's a surprise.

To end this article I will try to answer some questions I have received since I'm in the internet. They will clarify more the tube amplifier question.

How does a tube sound?

In general terms, they have a sweet sound, soft and easy to listen. But the final answer is in the hands of the user.The sound could change easilly when changing the tubes. A Mullard EL34 sounds different to a Sovtek EL34. So as you can see it is very easy to change the sound of an amplifier to our taste. This is very difficult (almost impossible) in a solid state design, because we have to change the transistors.

The sound, has dynamics or is too soft and sweet?

It depends. Usually, the sound is changed by many things like the circuit configuration. For example, if the amplifier has tube rectification the sound is sweet. If it has solid state rectification the sound is still sweet but with more dynamics and bass control (more close to the sound of solid state amplifiers). Also the electronic parts used in the amplifier, like signal capacitors, changes the sound.

What maintenance needs a tube amplifier?

Less that you can think. Basically, cleaning it with soft cotton cloth. I have seen amplifiers with a lot of dust and working perfectly. But some maintenance is recommended. Some electronic parts like electrolityc capacitors and resistors fail with the years (many, many years). This is what happens to classic amplifiers. If you buy a classic unit check all these parts or take the amplifer to an expert technician to do this verification for you. About tubes, some people say that it is better to don't touch it with the fingers because the body grease may get into the tube through the glass. I use cotton gloves.

Can a tube amplifier work with "hard" speaker (low sensitivity)?

The question here is the relation between output power and sensitivity. A low powered amplifier will need a more sensitive speaker. If the speaker has a low sensitivity it will need a more powerful amplifier.

Can a 30 or 50 watts tube amplifier work with a speaker like the Nautilus 803 or Dynaudio 3.0 and obtain an excellent sound?

Sometimes it's useful to read the speakers specifications provided by the manufacturer to see what they recommend. Somebody ask me about combining the Nautilus 803 with a 30 watts tube amplifier. The combination was a disaster. I recommended a solid state amp with 80 watts or more and that person was satisfied. Again the relation between output power and sensitivity. About sound quality, it depends on the ears of the listener and his concept of musical reproduction.

Does it exist any limit to output power for tube amplifiers?

No. If we want more power from a tube amplifier we have to design a circuit with more power tubes and try to find an output transformer that delivers this power output. The most frequent tube amplifiers are in the 35 watts-80 watts range. As you have seen there're less powerful amplifiers and also there're more powerful amplifiers. As I have said to obtain more power, more output tubes are needed and this is expensive and I believe that it looses the basic conception af tube amplification. If you need a lot of power, try solid state amplifiers

Is it easy to find tubes?

Absolutely, without any doubt. Try to buy it directly to manufacturer, not in audio shops. It's cheaper. Also you can try to find it in the USA and the UK markets.

How long time last a tube?

It depends on the operating rating of the tube in the given amplifier. If the tubes works relaxed it will last for a long time. For example, an EL34 may last for 5000 hours.

How long warming time need a tube to get its maximun capacity?

About 30 minutes. During this time the sound improves subtlely.

What is your opinion about hybrid amplifiers or the use of a preamplifier-amplifier combination with one of this components being a tube unit and the other a transistor unit?

If the sound is good to the user, there isn't any problem in combining tube and solid state components. Usually it is done to soften the sound of transistors.

The sound from a tube amplfier kit, could be the same in quality terms that the one from an amplifier made by a big amplifier manufacturer?

Yes, if the DIYer knows what he's doing. The electronics parts can be found without problems. The same parts used by the big industry area available to DIYers.

It is better to buy a classic amplifier, a new model or to go for DIY?

It depends. I love classic designs like Quad, Leak, Marantz, Mcintosh, but I respect new models. If there's any problem it is easy to repair a new model than a classic. Sometimes we have problems trying to find old electronic parts. About DIY, as I said before a DIY amplifier can be as good as any other amplifier. At the highest level. ACME.

Are tube amplifiers expensive?

This is a myth. We can find a wide range of prices. You can find a good tube amplifier for more or less 1200 euro. But, there're also amplifiers that can cause serious damages to your bank account.

After you have selected your amplifier, you have to listen to it. I can help in that question. Maybe there will be surprises, perhaps because a very well build amplifier will sound bad. Or not? And perhaps those efficient speakers will not sound. But you know, this is hi-fi and things get clear only after listening. And you decide if you like a brand or a given model or not. Or if you prefer solid state. You will suffer or enjoy your selection. I don't want this responsibility. ;-)