Articles for Audiophiles by Steve Deckert
AUDIO PAPER #150
TUBE AMPS FOR NEWBIES by Steve Deckert April 2023
If you are new to tube amps, or own one but don't have replacement tubes on hand, this article is for you.
Tubes typically give their full level of performance for around 5000 hours. After this the performance begins to decline. If it were a light bulb, it would be like the bulb growing dimmer each day for a full year until you either noticed it's only half as bright or it burns out.
So the very first thing you need to understand is that in a tube amplifier, the tubes are NOT part of the amplifier. Just as a lightbulb is not part of the lamp. You would never purchase a lamp and then return it when the light bulb burned out, instead you would get another light bulb and install it into the lamp.
Obviously an tube amplifier will not make sound without installing the tubes, just as a lamp will not make light without installing the bulbs. Most people with lamps will have at least a few spare bulbs on hand because they don't want to be in a situation where they can't see when it's dark.
Imagine how silly it would be to wait until the bulb burned out, and call the store where you purchased the lamp and order a new bulb while you sit in the dark for 3 or 4 days waiting for it to arrive.
When a light bulb burns out, it is the light bulb that failed, not the lamp. The lamp does not burn out the bulb... and some bulbs burn out as soon as you install them and others last for years, even ones that came from the same box. Tube amps are no different. Tube amplifiers do not burn out tubes and some tubes fail right away, or within weeks, and some last for many years.
With tubes from a reputable supplier, there will be a warranty. That makes keeping everything running smoothly much easier. You simply replace a bad tube when it fails and if it is still under warranty you send the failed tube back for a replacement. This keeps you listening to music without down time.
So in summary, if you are going to own a tube amp, or already do, you need to have a spare set of tubes on hand at all times.
Because tubes that work correctly last many years under normal use and then take many more years to decline to a point of catastrophic failure, it means that a depressing number of audiophiles are listening to amplifiers using tubes that are no longer performing to spec. Symptoms of this manifest as lower power output, warmer sound, rolled off treble, loss of dynamics, loss of focus, noise, hum and even microphonics in some cases.
DIFFERENT TYPES OF TUBES
Decware tube amps can have several different types of tubes, but the most common fall into three categories: 1) Input tubes, 2) Output tubes and 3) Rectifier tubes.
1) Input tubes are the smaller 9 pin tubes that raise the signal level high enough to drive the output tubes.
2) Output tubes are the larger 8 pin tubes that drive the speakers. These are the ones that work the hardest in most cases and typically need replaced most often.
3) Rectifier tubes are similar size to output tubes, and while they only need 4 pins, they are installed into 8 pin sockets. Some rectifiers have 8 pins, some 4, some 5, some 6, some 7. Only 4 of the pins are actually wired, the extra are simply there to add stability so the tube doesn't feel loose when plugged into the socket.
On our small amplifier, the SE84 series, there is one input tube (it actually has a left and right channel inside the single tube) and there are two output tubes, one output tube for each channel. The whole show is powered by one rectifier tube.
Input and output tubes are called "audio" tubes because the music signal flows through them. The rectifier tube on the other hand is part of the power supply. Its job is to change AC into DC current that the tubes can use to make music.
Obviously changing the audio tubes from one brand to another can have an effect on how the amplifier sounds but less obvious is the fact that the rectifier tube itself can affect the sound, and in some cases even more profoundly than the audio tubes.
When you hear the term "tube rolling" it means changing from one brand to another to hear how the sound changes. Some sound warmer. Some have more resolution. Some image better. At Decware we have listened to most of the possible brands that a customer could try and have selected not the flashiest sounding tubes, but tubes with the overall best balance and longevity. They give the owner of the amplifier an excellent reference that if he or she wishes to roll in other brand tubes and try to find something that sounds better they can. If they don't want to experiment or worry about all that, they can rest easy that we have already chosen tubes we feel accurately represent the sound we're after.
This particular category of tubes needs some special discussion. When building a tube amplifier we have a choice of using rectifier tubes to create the high voltage DC we need to operate the amp, OR we can use solid state diodes to accomplish the same thing. The difference between them is how they sound and what happens if there is a high voltage spike on your line, perhaps from a nearby storm. Since the rectifier or diode is the first thing a power spike sees, that is the thing that fails. With a rectifier tube it is not such a problem because the tube itself will fail and it is located on the outside of the amp plugged into a socket making it a 10 second job to replace. If it is a diode(s) then the amplifier has to be shipped back to the manufacturer for repair because diodes are internal parts that are soldered into place and non-removable.
Sonically the two approaches sound different as well. The diodes have less voltage drop giving the amplifier a touch more high voltage which means slightly higher power. Tube rectifiers, or we could call them tube diodes, are affecting the speed and the voltage drop of the high voltage feeding your amplifier which directly affects the midrange, bass and even treble character.
For hardcore tube amp affectionados, being able to change the rectifier tubes is exactly like changing the tires on your car. Each brand and type of tire changes the ride and how the car handles.
RECTIFIER TUBE FAILURES
This is an important topic because it happens more now than it did in the 1940's and 1950's when rectifier tubes were in everything and our national security depended on their quality and reliability. Today the only demand for rectifier tubes that is large enough to warrant production is the guitar amp market. The amount of audiophile tube amps using them is not enough to support this production. There is no national security issues, laboratory calibration concerns, or color quality issues on your TV set to worry about if a rectifier tube isn't such high quality. Only the guitar player will complain, and most guitar amps hum and buzz to a point where the only thing noticed about the rectifier is that it works or it doesn't.
So current production rectifier tubes basically come from three places... China, Russia and Slovakia. The Russian ones are by far the best, but even those are no match for the US and European tubes up to the 1960's.
Many of the Chinese remakes of popular rectifier tubes as an example are actually only rated to handle 4uf capacitors in the first section of the power supply and at that value are only rated to last 500 hours. That is only 1/10th what it was before the solid state revolution took over in the late 1960's.
Today's modern amplifiers often use much higher value capacitors in the first section of the power supply because the most common and sturdiest rectifier tube called the 5U4G, is rated to handle around 47uf instead of 4uf. This means the amplifier can have much higher dynamic performance and lower noise and hum. Many Decware amplifiers have this value. But many Decware amplifiers also have low power and that is a factor that affects how much stress if any is placed on the rectifier tube.
When a rectifier tube fails it happens when you first turn on the power. There is a flash inside the tube and then a fuse blows in the amplifier. The flash is caused by the turn on surge. The size of the turn on surge is determined by the amount of power the amp draws and by the size of the capacitor in the first section of the power supply. It is that combination that dictates how likely a flash or arc is to happen when you first turn on the amp.
If a rectifier tube arcs or flashes on start up it means it has expired in the amplifier it is in. You must replace it. Otherwise you will continue to blow fuses and possibly damage the amp.
Sometimes you can take a rectifier tube that flashes in an amplifier and use it in a preamp or smaller amplifier with no additional problems because the demand on the tube is lower.
In summary, if you find yourself saying "my amp keeps blowing rectifier tubes", then please re-read this article before you continue. Rectifier tubes blow up amplifiers, amplifiers do not blow up rectifiers. The fuse is what protects the amplifier from total failure when a rectifier tube has failed or is starting to fail.
The vast majority of the time if a rectifier tube lasts for a couple weeks in your amplifier, it will likely last at least a couple years before it needs replaced.
A FEW OTHER NOTES ABOUT RECTIFIER TUBES
Rectifier tubes have two halves inside them that are used to rectify AC voltage into DC voltage. Each half is basically a tube diode or one way valve. When the rectifier is new, it is tested, each half and reads a high percentage on the tube tester. As the tube ages, this reading gets lower and lower. This means that the voltage drop across the tube will increase slowly with time and gradually drop the power of your amplifier.
If the two sections of the tube wear at a different rate, or one section fails or partially fails leaving the other section to remain functional, the amplifier will still work, but it will hum. If one half of the rectifier tube fails completely the hum will be quite loud.
BIASING TUBE AMPS
If you are new to tube amps, the circuit of the amplifier determines how much current is allowed to pass through the tube. This current = power or watts. Too much and the tube will overheat and prematurely fail.
As far as you are concerned, the only thing we need to learn about biasing is how it affects the output tubes. Input tubes and rectifier tubes are not really in this category so we will only focus on the output tubes. The output tubes remember are the ones that create the watts that power your speakers.
So there are two types of bias, actually three.
The old school way is called "cathode bias". This is a simple high wattage resistor that is connected between the tube and the circuit ground. It's value determines the current that can pass through the tube. It is bomb proof, foolproof, is not adjustable, and never fails. It never causes your tube to overheat and turn cherry red as it tries to melt your amplifier.
The next method is called "fixed bias" which means the opposite. Yes it actually means that the bias is adjustable. So you adjust the current level of the tube with a small screwdriver or a knob and a meter to set the tube to a specific value that you have to know before hand. Setting the value too high will melt the tube and probably damage the amp.
So why in Gods' name would you choose fixed bias over cathode bias? Between the two, cathode bias often sounds better and is more consistent because as the tube specs drift over time the cathode bias compensates automatically. It will keep the tube sounding it's best until way past when you should have replace the tube. Fixed bias on the other hand makes around 30% more power so that is your answer. Power specs sell amplifiers because they look more impressive on paper.
The third method is called "auto bias". This is typically mixed up with cathode bias because both are "automatic" but the difference is huge. Auto bias is a fixed bias amp with an integrated circuit or even microprocessor that measures the current draw of the tube and adjusts the bias to a preset amount. That way you never have to bias the amp.
The problem is that as the tube wears out, and the specs of the tube drift, the auto bias circuit is adjusting the bias of the tube so that you remain unaware of the condition of the tube until it actually reaches its failure point. By the time a tube reaches its failure point, say at 8000 hours, it has sounded like crap for 3000 hours compared to when it was new. Also, the complexity of circuit to handle the biasing algorithms is statistically prone to fail compared to cathode bias amplifiers.
POWER FROM TUBE AMPS vs. POWER FROM SOLID STATE AMPS
Apples and Oranges. The differences are very real. Tubes amps have more instantaneous peak power than solid state amps for a given wattage. Tube amps when they run out of power, which is called clipping, do it gracefully and gradually. We call it soft clipping. When solid state amps clip is it abrupt, aggressive and dangerous to your speakers because when it clips it puts out pure DC voltage to your speakers. Speakers are not compatible with DC voltage. A good analogy would be that tiny amounts of DC voltage on a speaker is like tiny amounts of water to toilet paper. DC voltage is what blows speakers, not watts.
Speaker cones when they move create voltage through the voice coil and magnet assembly. Just like it takes voltage from the amplifier to move the speaker cone, when the speaker cone moves it creates its own voltage that it send back to the amplifier. This creates all kinds of havoc in a solid state amplifier that drops the usable power down to a small percentage of rated power. Tube amps have what is called "output transformers" that block the voltage coming from the speaker so that it can have no effect the sound of the amplifier. This makes the usable power from most tube amps near 80 to 90 % instead of the 15 to 20% you can expect from most solid state amps.
This is why when comparing a 20 watt tube amp to a 100 watt solid state amp the tube amp can sound fuller, have more bass, more hit, more body and do it without having to be cranked up loud and at the same time be able to maintain it in a room up to a louder volume before it starts sounding bad.
HEAT FROM TUBE AMPS
Lower than you think unless you try to have high power tube amps and we don't go there because the sound quality is never as good. High power tube amps will use sometimes a dozen output tubes in parallel to get the high power and this creates the same amount of heat as a small electric heater.
The small Decware amps draw around 75 watts so you could expect the heat output to match that of a 75 watt light bulb. The larger amp we make are about double that. High power tube gear is 10 times more.
HANDLING THE TUBES
No you don't have to wear gloves to touch the tubes. That is for Halogen light bulbs, not vacuum tubes. The tubes just plug in and out with your hands. Gloves would make it dangerous as you would lose any grip.
So why do we do it?
Why do we put up with tubes that eventually all fail when we can have solid state amps that last forever? The sound. No more explanation is needed. This is assuming of course that the tube amp is designed for sound not for profit. There are many tube amps out there that I could find better sounding solid state to replace it. But the companies that design for sound quality rather than watts and profit will make amplifiers that let you hear your speakers for the first time, as well as your DAC, cartridge, cables and everything else. The resolution can be like the James Webb telescope making 95% of everything else sound inferior.
I didn't know cars could fly! Imagine if you were driving and you pressed a button somewhere and your car started to fly! You had know idea this was even possible. Same feeling when you hear a real audiophile tube amp for the first time. And of course once you can fly, walking becomes a form of suffering.
Some tube amps are like pets, with lots of output tubes, biasing that is hard to get right, and are riddled with failures or inconsistent sound. Many are lower priced. Many are not. Many are about marketing and numbers. These often drive first time buyers back to solid state who never revisit tube amps again. That is the travesty of it all, because seriously when it is done right, you can't unhear it .
When it comes to tube amp NEWBIES there probably isn't a better company than Decware to buy a tube amp from because the amps are simple to use, easy on tubes, the tubes are reasonably priced and are high quality hand selected and tested in the actual amplifier. And the amplifiers have a lifetime warranty. Decware amps are in service since 1990's and are passed on to kids and friends because they last.
Just remember when you budget for a tube amp, add in the price of a complete spare tube compliment at the time of purchase, and then plan to replace them all at least every 4 or 5 years. This is the price of admission for serious tube audiophiles. Having a complete spare set of tubes is how you tell if your tubes need replaced, even if your amp has meters because meters won't tell you how the tube is sounding. We recommend taking the tubes out of the amp every 6 months to 1 year and installing the spare tubes to see if the sound gets better. If it doesn't, put the spare tubes away and try again in another 6 months. Eventually the sound will be better indicating your original tubes are worn and should be replaced.