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The New & Improved™ Tube FAQ for Newbies!

post #1 of 24
Thread Starter 
ver2 is in da house!!!


A quick introduction: Who is this guide for? (Plus a little bit of history...)

This guide was written with Average Audiophile Joe in mind. Simple(r) language was deliberately chosen in order to stay true to that aim. While some concepts explained within have been vastly simplified, I'm sure the purists out there will forgive me in view of my intended contribution towards the Greater Good.

To get the most out of this guide, it is best that you have some basic knowledge of physics and electrical concepts - basic atomic theory, electrical potential, resistance, capacitance, impedance, AC, DC, power transfer, transformers, rectification and other assorted ghouls will make their guest appearances within this guide.

If much of the previous paragraph resembled Cryptology 101, perhaps it is better that you take a gander over at Wikipedia (or a nearby library) to brush up on all the theory you slept through in high school.

The previous version of this guide not so much was a `guide' per se as it was a collection of links. While easy to set up, this left a little black spot on the author's conscience; like us, links frequently go dead suddenly - in my opinion a most unsatisfactory arrangement.

Setting aside a little time each day, I wrote this guide over 3 months. I hope you enjoy it and gain as much out of it as I did while writing it.


-= Legend =-

Chapters are marked in bold large red fonts.

Sections are marked in bold underlined red font.

Sub-sections are marked in bold and underlined font.

-= Contents =-

[click on chapter titles to go there directly]
[sections marked in red are incomplete]


1.1 What is a tube?

1.2 Tube types and general structure

1.3 A pictorial guide to tube structure

1.4 Tube naming conventions

1.5 Why tubes?


2.1 Amplifier output stage topologies: SET vs Push-Pull vs OTL

2.2 Tube biasing

2.3 Regarding tube-based rectifiers…

2.4 Tube datasheets

2.5 Tube testers

2.6 The proper care and feeding of tubes


3.1 What is 'tuberolling'?

3.2 Why do different tubes of the same type sound different?

3.3 What are NOS tubes?

3.4 USD$3000 for a single tube??! Why are some NOS tubes so expensive?!

3.5 Do NOS tubes really sound better?

3.6 Matched sections vs matched tubes

3.7 Buying and selling tubes

3.8 Relabeled tubes

3.9 Tube tweaks


post #2 of 24
Thread Starter 


1. ESSENTIAL KNOWLEDGE [aka t00bs for n00bs ]

1.1 What is a tube?

To know where we are now, we have to have a reference point - where we began. Hence this section will start out with a quick history lesson (with a quick science lesson embedded within).

The vacuum tube as we know it (short form: `tube') (also known as a 'thermionic valve' or just ‘valve’ to our European friends) descended from the first light bulbs. In those, air was evacuated from a glass cylinder tube in order to prevent the rapid oxidation (i.e. burning) of a white-hot filament via reaction with atmospheric gases. The glass was then sealed to maintain the vacuum.

Thomas Edison first noticed that as a bulb neared the end of its useful life, its glass envelope would darken with a thin layer of tungsten (you can observe the phenomenon today in some halogen bulbs). The tungsten filament was evaporating and depositing on the inside of the glass envelope. To rectify this, a positively charged `plate' (positively charged relative to the filament) was introduced within the bulb in order to `attract' the tungsten vapour over to it instead. But something unexpected happened - a current flowed across the vacuum.

In 1904, John Ambrose Fleming invented what is essentially a diode - a device that allows current to flow in only one direction. He named his invention the `electrical valve'. Two carbon electrodes were placed in a vacuum tube - one being heated till it glowed white-hot (the `filament') and the other left cold (the `plate'). When a source of AC was connected between these 2 filaments, current would only flow in one direction.

Science lesson: Electrons are negatively charged particles. They are strongly attracted by positively charged objects. All conductors have free electrons within their structures that are bound by attractive forces to adjacent (and positively charged) atoms. If enough heat energy is applied to the conductor, some electrons gain enough energy to escape these attractive forces. Thus electron emission increases extremely rapidly with a corresponding rise in surface temperature. Hence, a hot glowing carbon filament will emit a large amount of electrons relative to a cold carbon filament. Now link this with the `electrical valve' - during one half of the AC cycle, electrons escape off the hot filament, fly through the vacuum and enter the cold filament, but during the next half-cycle, nothing escapes the cold carbon filament to enter the hot filament. So there is an electron flow in a single direction, which means a current flow in a single direction.

In effect the hot filament has becomes a cathode (negative electrode) and the cold plate, an anode (positive electrode). If we keep the plate at a positive electrical potential relative to the filament, and ensure that the filament is heated sufficiently so that it emits electrons reliably, we can consistently pass a large current through a vacuum.

Lee de Forest patented the Audion in 1908. A platinum wire was placed between filament and plate, creating a triode (contrast this with the diode, which has 2 elements). By applying a weak AC voltage to this platinum wire, he realised that he could modulate the large (and previously uncontrolled) flow of electrons from filament to plate. Lee de Forest had invented an amplification device, albeit a crude one - the end signal at plate was roughly the same as that applied to the platinum wire, the main difference being that it's magnitude (i.e. amplitude) was much larger.

Lee de Forest's Audion

While comparing a modern tube to a light bulb is akin to comparing Modern Man to Homo Erectus, the Audion can be said to be, at most, the grandfather of the modern tube. Obvious physical differences apart, they are identical in both idea and implementation.

1.2 Tube types and general structure

Directly vs indirectly heated

As mentioned above, in earlier tubes the filament was also the cathode. In other words, the cathode was directly heated. Such tubes glowed white-hot at 3000K, even though a lighting effect was not really necessary to their proper function. These tubes were known as bright emitters.

The next evolutionary step was to add minute amounts of the element thorium to the tungsten filament, creating the thoriated tungsten filament. Not only was running temperature reduced significantly to -2000K, electron emission was also improved by a factor of roughly 10. These tubes are known as (drumroll!) dull emitters, and are still in use today in audio - examples include the 300B and 2A3 tubes.

But a real marked improvement came about with the invention of the oxide-coated cathode. The cathode was now a thin coating of a mixture of various metal oxides which was heated by a completely separate heater filament. The 2 were separated by a thin layer of aluminium oxide insulator - hence creating an indirectly heated cathode. Why a metal oxide cathode? Such cathodes run cooler at roughly 1100K, yet have an emission efficiency of more than 100x that of bright emitters! Most of the tubes in audio use are indirectly heated types.

Despite their vastly improved emission efficiency and cooler running temperature, indirectly heated tubes have their weaknesses - amongst other things, they are extremely fragile, and the oxide cathodes are easily poisoned. These will be covered in more detail in the section "The proper care and feeding of tubes".

The triode

Triode structure has already been covered briefly above. There are 3 main active elements in a triode - the cathode, anode and control grid.
The control grid is the direct descendent of the platinum wire in the Audion. It has been refined to what is typically a helical coil of extremely fine wire (<l0um thick; a human hair is about 100um thick) that encircles the cathode.

Most audio tubes in use today are triode types. Many have two triodes in a single glass envelopes, and are known as twin-triodes. Example of triode types commonly seen in audio include the 6SN7, 6DJ8 family and the 6080/6AS7.

The tetrode

The tetrode differs from the triode in just one way - it has an additional grid, between the anode and control grid, known as the screen grid.
The purpose of the screen grid is to accelerate even more electrons toward the anode. To this end it is kept at an even more positive potential than the anode, so that electrons that would otherwise spiral off into space after leaving the cathode would be strongly attracted towards it. The coils of screen grid wire are wound more coarsely to avoid absorbing these electrons, so the most of the electrons pass right through the screen grid and straight into the anode.
As can be imagined, tetrodes usually provide more voltage gain than triodes. Yet, they are relatively uncommon compared to triodes or pentodes. Why? Read on!

The pentode

The pentode in turn differs from the tetrode in just one way - there is yet another grid between the screen grid and the anode. This grid is known as the suppressor grid.
While tetrodes provided higher voltage gain, an unfortunate side effect of increasing electron velocity was uncovered. At higher anode voltages, an electron may be so sped up by the screen grid that when it hits the anode it dislodges two electrons. The anode has now effectively emitted one electron, which leads to a net fall in anode current. This phenomenon is known secondary emission. As can imagined, secondary emission detracts somewhat from the linearity (the ability to amplify without distortion) of the tetrode.

The pentode (invented by Philips/Mullard) solved this problem. The suppressor grid is kept at an electrical potential that is very slightly more negative than the anode. Hence electrons emitted via secondary emission are repelled by the suppressor grid, and back towards the anode. Voila! Examples of pentodes commonly seen in audio include the EL34 and EL84.

The beam tetrode

While the pentode solved the linearity problems associated with tetrodes, it was under patent to Philips/Mullard. In an effort to circumvent this, the beam tetrode was invented by MOV.

The beam tetrode works just like the tetrode, but with a few small differences. The cathode is formed in such as way as to emit electrons in parallel `beams' focused towards the anode. The control and screen grids are wound at the same pitch (coils per unit of vertical height) so as to be optically aligned. The cathode is aligned so that the electron beams pass between the grid wires (still influenced by grid potential), but never intersect with them (hence never absorbed).

This reduces grid current (current generated when an electron hits and is absorbed by a grid) by around 20%. As a result, more electrons reach the anode, contributing to higher anode current and higher overall tube efficiency. Examples of beam tetrodes commonly seen in audio include the 6L6, KT66 and the KT88.

Tube rectifiers

Rectification refers to the process of converting AC to (pulsating) DC. Tube rectifiers are simply tubes that accomplish this.

Please refer to the section `2.3 Regarding tube-based rectifiers...' for more on tube rectifiers.

Other Tubes

There are other tubes used in radio transmitting. These (typically) directly heated tubes are sometimes watercooled or may have a ceramic substrate replace the vacuum. These tubes are irrelevant to consumer audio and will not be covered within this guide.

1.3 A pictorial guide to tube structure

Please click here for this section.

1.4 Tube naming conventions

There are 2 main types of tube naming conventions – American and European. They will be covered separately (and in detail) below.


European tubes follow a letter-letter-letter-digit -digit system. E.g. ECC88, ECC83, ECL82.

Letter 1, heater type:

A – 4V
B – 180mA
C – 200mA
D – 0.5V to 1.5V
E – 6.3V
F – 13V
G – 5V
H – 150mA
K – 2V
P – 300mA

Letter 2 (and subsequent letters), valve type:

A – Small-signal diode
B – Double small-signal diode
C – Small-signal triode
D – Power triode
E – Small-signal tetrode
F – Small-signal pentode
H – Hexode or heptode
K – Heptode or octode
L – Power tetrode or pentode
M – Fluorescent indicator
N – Thyratron
Q – Nonode
X – Gas-filled rectifier
Y – Half-wave rectifier
Z – Full-wave rectifier

Digit 1 (ignore digit 2 unless specified otherwise), base type:

1 – Use 2nd digit
2 – B8B
3 - Octal
4 – B8A
5 – B9D
8 – B9A
9 – B7G


1. The ECC81 has a 6.3V heater, and is a twin small-signal triode using a B9A base.
2. The ECL82 has a 6.3V heater, and has a triode and a power tetrode/pentode within the same envelope. It has a B9A base. One would have to refer to the datasheets to verify that the ECL82 is a triode and a pentode.
3. The GZ34 has a 5V heater and is a full-wave rectifier using the octal base.

A few European companies however, ignored this naming convention completely, preferring to use their own proprietary naming systems. Ediswan/Mazda, Marconi Osram and STC are known to have different naming systems.

British tubes had their own separate naming system, distinguished by their ‘CV’ (Common Valve) prefix.


American tubes for the most part followed the Radio Electronics Television Manufacturers Association (RETMA) naming system. This system followed the digit-letter(s)-digit naming pattern.

1st digit: Approximate heater voltage (A 7 or 14 here refers to Loktal base types)

Letter(s): Arbitrarily assigned. Related to individual valve design.

2nd digit: Number of electrodes (heater included)

For octal tubes, an additional suffix system was used. These suffixes can be tacked on to the end of any American octal tube.

G: ‘Glass’ indicating likely usage of the ST14 (Shouldered Tube) glass envelope. Shoulder Tubes resemble soft drink bottles

GT: ‘Glass, Tubular’ later glass envelops were simply cylinders with a hemispherical end.


1. The 6SN7GT uses a 6.3V heater and being a double-triode, has 7 electrodes (2 triodes + heater). It has a cylindrical glass shape with a hemispherical end.
2. The 12AT7 uses a 12.6V heater and being a double-triode, has 7 electrodes (2 triodes + heater).
3. The 5AR4 uses a 5V heater and being a full-wave rectifier, has 4 electrodes (1 cathode, 1 heater, 2 anodes).

There were also military tubes, either having a ‘JAN’ (Joint-Army-Navy) prefix, following a VT-digit-digit-digit (e.g. VT-231) system or a 4 digit system (e.g. 5692, 6080). In the later 2 both cases, the digits were chosen arbitrarily, hence attempting to define any logical pattern is futile.

Western Electric followed its own numbering system - usually 3 digits followed by an alphabet. E.g. 300B, 300A, 421A, 417A.

A note on American JAN/VT designated tubes:

While these tubes were subjected to a more stringent set of quality control criteria to attain military 'approval', more often than not these tubes were completely identical to non-military tubes of the same vintage. Often both civilian and military versions were made in the same factories in the exact same production line.

A single tube may have many different designations – for examples, the 12AX7 tube is also known as the ECC83 or CV492. Please remember to contact your amp manufacturer before doing any substitutions.

1.5 Why tubes?

There are many people out there who would lead you to believe that tubes are the greatest. Then there are people who think the same of solid-state equipment. Both sides have valid points to bolster their arguments. In the author's experience, the difference between high-end solid-state and tube equipment is typically imperceptibly small.

Tubes are fragile, bulky, produce excess heat, have more noise, objectively give more measured distortion when compared to solid-state, and are more susceptible to stray capacitances. They are also subject to the vagaries of production by hand, are more expensive to make and are also dwindling in supply. Taking all these into account, why even consider tubes? In short - their superior clipping characteristics and different distortion characteristics.

Clipping occurs whenever the amplification device literally runs out of juice to amplify a given signal with an initial amplitude that is too large. This occurs often during musical instances known as transients - examples include the crack of a snare on a drum or a full-out symphonic crescendo.

Imagine a sine wave:

Now we shall amplify it till it clips. For solid-state devices in general, the output would look like the sine wave A: the top section is lopped off completely, forming a plateau that is parallel to the horizontal axis. Not only does this sound terrible, the plateau is in effect DC applied to your headphone drivers (very bad).

sine wave A

Compare this now to sine wave B: the top of the sine wave, while not like the original signal, is now a curve with a slightly different rise/fall gradient.

sine wave B

This sounds a lot less offensive and is certainly easier on your headphone drivers than the `hard' clipping offered by the above solid-state example. It is also far less audible than the audibly obvious solid-state clipping. This has led to the popular myth that a`tube watt' is twice as powerful as a 'solid-state watt'. Clipping still occurs - you're just less likely to hear it.

As mentioned earlier, tubes have more measured distortion. However this distortion is primarily 2"d order, lessening greatly as we go through the higher harmonics. Distortion characteristics for solid-state however, tend towards higher odd order harmonics (5"' 7`n etc), albeit in smaller amounts.

Scientific studies have shown that humans perceive even order distortion as being musically consonant while odd order distortion is perceived as musically dissonant. Anecdotal evidence shows that while up to -5% of 2°d order distortion is audibly tolerable, only -0.5% of 5`h order distortion is audibly tolerable.

Tubes win!

What that means in plain English is that a tiny bit of odd order distortion is going to sound a lot worse than large amounts of even order distortion to the average human ear. Compare this to the distortion characteristics of tubes and solid-state.
post #3 of 24
Thread Starter 



2.1 Amplifier output stage topologies: SET vs Push-Pull vs OTL

Often, amplifier components are arranged in certain particular patterns – these patterns are known as topologies.

This section covers the common tube amplifier topologies.


SET stands for `Single Ended Triode'. SET designs are always Class A and usually cathode biased. [Refer to section 2.2 for more information on tube biasing]

Triodes typically output low current into a wide voltage swing. This is not especially useful – speakers and headphones need a high current into a narrower voltage swing. The solution is to use an output transformer to convert the former into the latter. The secondary of the output transformer is connected to amplifier output.

Being connected directly to triode anode output, there is usually a large extraneous DC current flowing through the output transformer. Hence output transformers for SET designs are usually beefy (and extremely expensive) affairs with gapped cores that are able to handle this DC offset without saturating. One may infer (correctly) that output transformer quality is of paramount importance in SET design.

Given that the triodes are biased into Class A, there is disparity in the amount of gain between the positive and negative half-cycles. Hence most of the distortion that occurs is even order (primarily 2nd). Given that chosen triode operating parameters are usually happy compromises, SET designs in general have poor measured distortion figures.

Despite their poor linearity, SET designs are extremely popular. The likely cause of this is the relatively benign 2nd order distortion - many feel that it is that which gives SET amplifiers a ‘magical’ sound.


Push-Pull (PP) designs can be Class A, AB or B. PP topologies always involve pairs of tubes, with more pairs of tubes paralleled for higher power output if necessary.

In PP amplifiers, pairs of tubes are driven in opposite phase to each other -

In Class B amplifiers, one half of the tube pair handles the positive portion of the cycle (input signal) while the other half of the tube pair handles the negative portion of the cycle.

In Class AB amplifiers, one half of the tube pair handles the positive cycle plus a little of the negative cycle. The other tube handles the negative cycle plus a little of the positive cycle.

In Class A amplifiers, both tubes handle the entire signal cycle – each tube handles both positive and negative completely.

Each side of a tube pair is connected to opposite sides of a centre-tapped output transformer – in effect one ‘pushes’ the signal through the transformer, while the other ‘pulls’ it through the transformer. The signal is in effect ‘re-stitched’ within the output transformer.

Class B PP amplifiers are usually avoided in audio. Why? When output signal ‘re-stitching’ occurs, a particular form of distortion known as crossover distortion often occurs with Class B amplifiers. Crossover distortion sounds extremely bad and is extremely obvious.

Class B PP

Class AB PP amplifiers are the most common variation of PP design. When ‘re-stitching’ the output signal, crossover distortion is avoided due to the ability to ‘overlap’ each portion of the output signal handled by each individual tube.

Class AB PP

Class A PP amplifiers ‘re-stich’ the 2 separate but complete ‘push’ and ‘pull’ waveforms in the output transformer. As can be inferred, Class A PP designs are also the most inefficient. Many feel that they give the best sound though.

Class A PP

PP amplifiers have some advantages over SET amplifiers. As mentioned earlier, two signals are summed in the output transformer. If some care is taken to match the tubes or bias them appropriately, this means that DC offset current can be more or less cancelled out during output signal summation. The output transformer hence only has to handle signal current. We would then be able to use a smaller (and much cheaper) output transformer.

This also means that PP amplifiers can get along with cheaper power supplies – any anomalies in the signal due to the power supply (e.g. power supply hum) are also cancelled out in the output transformer.

All this means that PP amplifiers typically have better measured distortion levels than SET amplifiers.

However, PP amplifiers have more higher order odd distortion than SET amplifiers. Why? As mentioned earlier, even order distortion occurs when there is uneven gain on either the positive or negative portion of the signal. Given that one half of the tube pair amplifies an inverted signal, and assuming that both tubes in the tube pair are reasonably matched, even order distortion cancels. On the other hand, odd order distortion sums.


OTL stands for `Output Transformer-Less'. Despite a recent surge in popularity, OTL topologies have typically been regarded as ‘fringe’.

While properly designed output transformers perform superbly, OTL designs attempt to avoid the inherent non-linearity of output transformers (no matter how little) by avoiding them completely.

Tubes designed for audio use typically have high output impedances and are not suited for driving low impedance loads like headphones or speakers directly (i.e. without an output transformer). OTL designs use special tubes to get around this problem. However, another problem replaces the one just solved - these tubes (examples include the 6AS7/6080 and PL509) are typically not designed for audio, and so may have questionable linearity.

Not having output transformers, OTL designs usually have abysmal efficiency.

Hybrid amplifiers

Hybrid amplifiers attempt to combine the best of both worlds - the prized tube `sound' with the extremely low output impedance and high current capability of solid-state. Often, a tube stage is placed before a final solid-state output stage.

There are countless variations of this topology, the only thing common to them all being that they involve both tubes and solid-state. Hence, they will not be covered in this guide.

2.2 Tube biasing?

Tubes need to be biased in order to function properly as low-distortion amplification stages.

But what is tube biasing? I'll quote a commonly used analogy - imagine a car engine.

We want to tune the engine so that the revolution speed (RPMs) at idle is neither too fast nor too slow. In other words - while idling - we do not want it to be consuming more fuel than it should (overly high RPM), nor do we want it teetering on the brink of stalling (too low RPM).

Now apply the same logic, and you are "biasing" a tube.

Biasing a tube involves setting the 'default' DC voltage (i.e. when no signal voltage is applied) for the control grid. Remember that a tube is a device that is a `nominally on' device - that is, electrons will always attempt to flow from cathode to anode. The grid (and the voltage applied to it, known as the bias voltage) is the only thing standing in the way of that.

Bias your grid to too positive a voltage, and too many electrons will flow from cathode to anode (consuming more fuel that it should). Bias your grid to too negative a voltage, and too few electrons will flow from cathode to anode (teetering on the brink of stalling). [Remember - different charges attract, like charges repel - electrons are negatively charged particles]

To sum up, the amount of DC bias voltage you apply to the control grid determines several things about that tube amplification stage: power output capability, distortion figures, maximum headroom, gain, efficiency and most importantly, the class of operation of the amplification stage (A, AB, B etc).

There are 2 "normal" methods (and 1 proprietary method) of biasing - cathode biasing and fixed biasing. The proprietary method is PrimaLuna's "Adaptive AutoBias", and will be elaborated upon in a bit.

Confusingly, fixed biasing is the biasing that is actually user-adjustable! In fixed biasing, a trimming potentiometer (i.e. trimpot) is adjusted by the user to determine DC voltage to the grid. As the operating parameter of tubes change as they age, increasingly frequent adjustments will need to be made to the bias adjustment. One would also need to take into account manufacturing variances between tubes of the same make and type, and bias each one individually.

Cathode biasing involves the placement of resistors between cathode-ground and grid-ground. While not needed any user maintenance, it effectively means that there is now a big capacitor in your signal path - not necessarily the best compromise for perfect sound.

Be sure to check with your amplifier manufacturer which type of biasing your tube amplifier uses.

PrimaLuna's "Adaptive AutoBias" is a proprietary "on-the-fly" autobiasing. Essentially, it operates like fixed biasing, but with multiple sensors that monitor audio signal voltages, tube voltages, etc and feed a microchip controller this information. The microchip then figures out the "best" bias for the moment for the power demand and applies it.

While this is certainly a novel idea, and possibly one of the bigger (biggest?) leaps in tube circuit design in recent times I have not heard it personally - and thus cannot comment on the audible (as opposed to theoretical) improvements it brings.

2.3 Regarding tube-based rectifiers...

the GZ32 tube rectifier, this example made by Mullard

Tube rectifiers are inefficient, expensive, have high output resistance, have large voltage drops across them, need separate heater circuitry, and in the case of some examples of recent make - are notoriously unreliable.

Taking the above into consideration, why are tube rectifiers still used?

Tube rectifiers have one clear advantage over silicon, and that advantage is so convincing as to win many converts - rise time. Rise time refers to the time needed for output voltage from the rectifier to rise from zero to operating levels when fully loaded. For tube rectifiers this is a gentle curve that extends over 5s or so. This greatly reduces inrush current generated by various capacitors down the line and potentially saves many a regulator.

For silicon rectifiers, the voltage rise line is a hard, almost vertical line (that has led to many a blown regulator for the author of this guide).

Many die-hard proponents of tube rectification claim that tube rectifiers `switch' more cleanly than their silicon counterparts, leading to less resonance within the power supply circuit. Mr. Morgan Jones, author of "Valve Amplifiers, 3rd Edition " debunks this, stating "the author's [M. Jones] experience is that both types of rectifier produce switching spikes and it is the smoothing/snubbing arrangements that are important."

Finally, there is another undeniable advantage - albeit a small one - that must be accorded to tube rectifiers: they certainly are much prettier to look at when compared to their silicon cousins.

2.4 Tube datasheets

Tube datasheets are useful things. Tube datasheets were the technical notes released by tube manufacturers. Every tube type has its own datasheet.

Amplifier designers refer to datasheets in order to choose an appropriate tube, determine operating points, design optimizations, component selection and the like. For end-users like us, the main use of tube datasheets is in researching tube substitutions.

Below are a couple of sites that have comprehensive databases of tube datasheets. Some have pdf scans of the actual datasheets, others have compiled the relevant data into html pages, and yet others have done the dirty work for you and listed out the tube substitutions for each tube.


2.5 Tube testers

Often while buying tubes, you will often come across readings supplied by 'tube testers'. What are tube testers? In the distant past, tubes were sold 'as-is', with no returns or refunds. Tube testers were used then to prove on-the-spot tube operability.

There are 2 major types of tube testers - emission testers ('dynamic/plate/cathode conductance' testers) and mutual conductance ('GM') testers. The latter utilises a superior method of testing a tube than the former, though neither type is useful at all if the tester is not recently calibrated.

Emission testers work by simply measuring `emission', which is the ability of the cathode or filament to emit electrons. Results are typically displayed as mA (milli-amperes). However, the cathode's ability to emit electrons is at best secondary to the importance of the grid's ability to modulate this same electron flow. Sometimes electron flow may originate mostly from a single `hot spot' on the cathode/filament. Emission testers do not take this phenomenon (which to some extent negates the modulating capability of the grid) into account. In addition, emission testers usually utilise a low current when testing tubes - in audio applications, current draw is almost always higher than in an emission tester. Hence at best, emission testers are useful only as a quick way to separate dud tubes from working ones.

On the other hand, mutual conductance (or GM) testers work by analysing the tube's ability to amplify an AC signal (typically 1kHz). AC plate currents are then read off and may be averaged for some time to give a GM value. Results are typically in µmho (micro-mhos). Usually such testers incorporate scales which indicate where your tube stands in a REPLACE-FAIR-GOOD scale. Some better GM testers test give results over a range of frequencies while other even better ones utilise the rated plate voltages (as opposed to a compromise lower voltage) when obtaining results.

Excellent GM testers were made by Hickok and B&K. eBay is the usual source for tube testers.

the author's B&K 747 GM tester

Remember that the ultimate test for any tube is still the actual amplifier you're putting the tube into. Many tube testers will NOT tell you anything about microphonics, real life span or how a tube sounds.

Tube testers are useful only for testing for faulty (and hence dangerous) tubes, matching tube sections and giving rough estimates of remaining lifespan.

2.6 The proper care and feeding of tubes

• The greatest chance for tube failure is at turn-on. Minimize on/off cycles for your tubes to prolong their lives. For example, if you will be listening 3 times within 4 hours, instead of switching the amp on/off 3 times, leave it on for the entire four hours! Better still, ask your amp manufacturer whether he can implement a ‘heater standby voltage’ mode.

• Avoid storing your tubes in drawers. The brief shock whenever you close the drawer is not good for tubes at all – a grid wire may break, or a oxide cathode might crack, or a anode might fracture, or a mica plate might crack, or etc...

• Original tube boxes were rarely, if ever, made of acid-free paper. To slow their inevitable degradation, store them in an airtight box with dehumidifier sachets in a dark and cool place.

• NEVER EVER tap your tubes with anything to test for microphonics while they are running. At higher temperatures (like that found in a running tube), everything is more fragile.

• For tubes with metal bases that are rusty: light rust can be wiped off with a cloth soaked in lemon juice or vinegar before being cleaned with a dry cloth and coated with some dielectric oil. For heavy rusting, gently sand off affected areas, then follow the steps for combating light rusting

• A rattling sound from your tube base is bad? Wrong! The sound is usually due to some dislodged glue, and is usually harmless.
• For tubes with cracked bases, you may attempt to secure the base by wrapping it with some electrical tape. This is a temporary band-aid type measure at most though.

• Clean off any external deposits on the glass envelopes. When running, these deposits heat up and cool down at a different rate from the glass, promoting uneven expansion and stressing the glass at that point. Your glass envelope will crack in time if you ignore them.

• Tube markings and base markings are soluble in most solvents. Be careful around them.

• You can clean tarnished tube pins with some fine wire brushes, isopropyl alcohol and cloth. Remember to clean away all the dirt you dislodge though!

• A tube that can rotate slightly (roughly 1 mm or so) within its base is usually perfectly alright. Be careful with it though.
post #4 of 24
Thread Starter 



3.1 What is 'tuberolling'?

'Tuberolling' refers to the process of swapping a tube, listening, and then swapping again in order to determine your favourite tube.

The moniker 'tuberolling' comes from the gentle circular rocking motion that is used to properly remove tubes from sockets. Remember - this is NOT a twisting motion (like turning a dial) but a gentle side-to-side rocking motion. If you yank a tube upwards or twist it like a dial, you could possibly break the glass envelope right off the base!

3.2 Why do different tubes of the same type sound different?

There are literally an infinite amount of factors that can affect the electron flow from cathode to the anode plate, which means that there are an infinite number of factors which directly affect how a tube `sounds'.

Here are some obvious factors: grid wire material, grid wire coil pitch, grid wire gauge, distance between cathode/plate, plate material, plate design, plate coating, glass coating, wiring, heater placement, triode separation distance, distances between tube parts which affects the capacitance between them, heat dissipation rate, etc etc...

To sum up, as the different tube manufacturers each had their own way of manufacturing the same tube, it can be expected that there would be variations in how tubes of the same type sound.

3.3 What are NOS tubes?

NOS stands for New Old Stock. NOS tubes are generally defined as tubes that were made before 1970, and more importantly, never used. They're old stock, but new - hence New Old Stock.

3.4 $3000 for a single tube??! Why are some NOS tubes so expensive?

engraved base WE 300Bs (circa 1930s) - people pay BIG $$$ for these

The short answer - supply and demand.

The long answer - Some tube amps are quite old, and their owners may prefer to maintain a completely `vintage' setup. Others believe NOS tubes sound better. Others are simply sheep with too much money to throw around who jack up the prices.

The NOS tube supply by its very nature finite - and dwindling each day. Some NOS tubes are very much sought after and are hence in particularly short supply. All these factors contribute to the much higher prices of NOS tubes when compared to plain new stock tubes.

You may want to consider what types of tube an amplifier uses before purchasing it - this WILL more or less determines the scale of your future expenditure when re-tubing the amplifier.

3.5 Do NOS tubes really sound better?

NOS tubes are reputed to sound much better than new stock tubes. The author of this guide believes, to a certain extent, that this is true. There are many theories floating around as to why NOS tubes sound better. I have cherry-picked a few that I am certain have at least a grain of truth in them.

1. The 1930s-1950s were the golden era of tubes. Equipment used to manufacture tubes and more importantly, skilled people to operate them were plentiful. All of these have been lost with the advent of the transistor and automation.

2. Tube R&D has come to a virtual standstill since the 1960s.

3. When compared to transistors, tubes are relatively expensive and time-consuming to manufacture (to achieve a good vacuum for a SINGLE tube, some vacuum pumps run for as long as 3 days!). In the highly competitive niche market for new stock tubes, quality may not be the top priority anymore. One amplifier manufacturer has been quoted as having to reject 1 in 4 6AS7 tubes for bundling with their amps due to unacceptable levels of microphonics/hum/channel imbalance!

Even then, it is strongly recommended that you try at least a few tubes of recent make before putting down some serious money for NOS tubes. Typically for the price of a single NOS tube you would be able to obtain at least 3-5 new stock tubes. You never know what you might stumble across...

3.6 Matched sections vs matched tubes

While purchasing tubes, you may come across the terms ‘matched sections’ and ‘matched pairs’. The meaning and implications of these two terms are covered in detail below.

Matched sections

There can sometimes be more than one active amplification element within a single tube. For example, there are triodes and then there are twin triodes - the 6J5 being an example of the former and the 6SN7 being an example of the latter. In other words, the 6J5 is virtually electrically identical to half a 6SN7.

With multiple sections (triodes, tetrodes, pentodes, etc) within a single tube comes the problem of variation. As a result of material and construction variation, there may be slight mismatches between the two sections of a twin triode like the 6SN7. Or the two sections of a twin beam tetrode like the 6DY7. Or the two sections of a twin pentode like the 6MK8.

Using a suitably equipped GM tester, the 2 sections within a tube can be tested and have their amplification ability measured. As a perfect match between 2 sections is exceedingly rare, a 5% match is the usual ‘gold’ standard with 10% and 15% being lesser grades of matching. This is the meaning of having a tube with ‘matched sections’.

Remember – knowing that a tube has ‘matched sections’ is useless without knowing how close percentage-wise they are matched! One retailer’s ‘matched’ may be 5% while another’s may be 10%!

Matched tubes

Matched tubes take the concept of matching a step further.

With matched tubes, all the sections in all the tubes to be matched must fall within a certain percentage of each other. For example, if we want to have a matched pair of twin triodes, it would mean that we would need to match 4 sections from 2 tubes. If we want a matched quartet of twin triodes, it would mean that we have to match 8 sections from 4 tubes.

Confused? Here’s an example to help you sort things out:

• Imagine we have 3 twin-triode tubes: A, B and C.
• Tube A has GM results 98/95, tube B = 96/97 and tube C = 62/60
• ALL 3 tubes have sections matched to 5%. Hence all 3, when sold individually, can be advertised as having matched sections.
• But ONLY tubes A and B can be sold as a matched pair as all their sections match to within 5%.

As you can see, matching tubes can be a challenge - which is why vendors typically charge you a (sometimes hefty) premium for matched tubes. This premium increases sharply with the number of tubes that need to be matched. Ouch!

But wait - why do we even need matched tubes for amplifier use?

In general, only power tubes need to be matched. Rarely will you come across an amp that requires its preamp tubes be matched. Many amplifiers are designed to require power tubes that have similar cathode currents. Even tubes of the same type made at the same time will have variations in their cathode current, and this is why testing and matching is required.

While few amplifiers actually need matched tubes, it never hurts to have them. Please consult your manufacturer to determine whether your amp needs matched tubes.

3.7 Buying and selling tubes

Buying tubes

As with anything out there in the world, buying tubes involves 2 words – caveat emptor (‘buyer beware’). There are literally millions of tubes out there, and there may be hundreds of versions of a single type. Below I have attempted to summarize some things you should keep in mind while buying tubes.

Do ask for photos of the tube, paying special attention to:

1. Mica shape
2. Plate shape and color
3. Support rod pattern on mica (if applicable)
4. Glass color and proportion of color (if applicable)
5. Base material and color (if applicable)
6. Label or glass prints – size, font and color
7. Date codes (if applicable)

Do your homework. Know what the tube you want looks like. There can never be enough research with tubes. It's a cruel world out there and there are people who counterfeit tubes - however it is impossible for them to get everything identical. Once armed with prior knowledge, the battle is half won for you.

Once in a while (if you're lucky enough) you might stumble upon relabeled tubes. What are relabeled tubes? They are tubes made by a particular manufacturer but with another manufacturer’s labels printed on. Something like OEM tubes. Refer to section 3.8 Relabeled tubes for more on this.

Some more buying tips:

• If the tube does not have a top getter flash, but there are tiny getter 'splotches' on the top, this indicates that the tube is used. Some filament metal evaporates with use and usually condenses above the heater openings.

• Are black or ‘rainbow’ getter flashes bad? Most certainly not! In fact, black getter flashes are better than shiny silver getter flashes. Getter particles, being more finely dispersed, absorb light instead or reflecting it. This also means that black or ‘rainbow’ getter flashes are better at absorbing gas than shiny getter flashes. You can sometimes see a rim of ‘rainbow’ with shiny silver getters flashes.

• Even if a tube has black glass, it still is possible to observe the plate structure. Simply shine a powerful torchlight at the glass and you should be able to see right through it. This is useful if you are unsure whether 2 particular tubes with black glass are the same or if you have a couple of unlabeled black glass tubes and can’t tell them apart.

Reselling tubes

Anyone can get into the business of reselling tubes - especially NOS tubes. Here are some useful pointers on factors that increase or decrease the value of tubes.

The following INCREASE the value of a tube:

• Having closely matched sections - within 5% is usually the most desirable
• Having other tubes with similarly matched sections (to make matched pairs, trios, quads, etc)
• Being NOS
• Intact labels
• Intact bases (or having no rust in the case of metal bases)
• Having nice shiny opaque getters
• having military or original boxes - these vastly increase value. If you are taking a box from another source and packing your tube in it, it is unethical to not mention so.

The following DECREASE the value of a tube:

• Sections that differ by >15%
• Previous usage or a hazy history (ie unsure whether tube is used/new) having no boxes or white boxes
• Having no labels
• Cracked or rusted bases
• Lacking a guide pin (when applicable)
• Faint or translucent getters (indicates getter is partially used up)
• Metallic splotches above the heater openings (indicate tube is used)

3.8 Relabelled tubes

Separated at birth? The story of a Ken-Rad JAN-CKR-6SN7GT and a Tung-So16SN7GT...

Do note that both tubes have...

• Black bases
• Copper grid posts
• Bottom getters in similar amounts
• Blackened glass in similar proportions
• Glass envelopes of the same height (not one tall-bottle and one short-bottle)
• And most incriminatingly, they both share the exact SAME 'staggered' grid post & cathode heater pattern on the mica plates!

They are the same tube!

3.9 Tube tweaks

Many people believe in getting the maximum from their tubes, and why shouldn’t they? The following tweaks are of the ‘last 5%’ variety, and should best be considered after your setup is complete. All the tweaks listed below are also linked in the Tube Links chapter.

Cryo-treating tubes

Tubes are immersed in liquid nitrogen for extended periods (up to 5 days) and then allowed to warm up gently. Proponents claim that this allows the metal atoms within the tube to realign themselves and stresses within the tube to relieve, extending tube life and helping the tube sound better.

Tube Dampers: Top Hats, Hal-Os, silicon and sorbothane rings

Tube damping is a tweak that definitely works.

Microphonics (the conversion of mechanical movement to measurable electrical phenomena) is a well known problem with tubes. All tubes are microphonic – it only becomes a problem if they are audibly so.

If you can ‘hear’ your tube without any music playing, it’s microphonic. Microphonics may take the form of rippling sounds, ringing noises or hum and can be set off by something as trivial as walking across the floor. Remember – never tap your tube to test for microphonics – if it’s really microphonic, you will know without needing to tap it.

Tube dampers help rectify this by either damping the mechanical movement (vibration) of tubes or by lowering its resonant frequency by adding mass. Either way, microphonics are lessened.

A good tube damper has excellent damping ability, has its physical properties remain constant even with the application of high heat for extended periods and does not cause the tube to retain excessive heat. There are dampers that offer various combinations of the above 3 criteria at many price points.

Tube Heatsinks

Tube heatsinks claim to extend the lifespan of your tubes by helping them dissipate heat. Tube heatsinks come in 2 forms -

1. Glue-on: a thermally conductive epoxy is used to literally glue heatsinks on. Rarely used as they
reduce the resale value of tubes - like diamonds, epoxy is forever.

2. 'Jacket' types: literally a slip-on heatsink for your tube. Think prophylactics.

A good tube heatsink is always matte black in color. Black surfaces radiate heat easily. Avoid heatsinks that are silver or shiny in texture – they may actually reflect heat back at your tube, making it hotter! Always locate your amplifier in a well ventilated place – otherwise your expensive heatsinks might just end up helping your tube retain heat.
post #5 of 24
Thread Starter 

1.3 A pictorial guide to tube structure:

post #6 of 24
Thread Starter 

this post intentionally left blank...

your advertisement here! PM me for rates.
post #7 of 24
Thread Starter 

Tube Links


Online Tube Sellers

Here is a selection of some tube sellers on the net submitted by your fellow forum users. There are countless more out there, so happy googling!

http://www.valves.uk.com/ [UK site]
http://www.evatco.com.au/ [AUS site]

Tube Tweak Links

http://herbiesaudiolab.home.att.net/ [Herbie’s Hal-Os]
http://www.thetubestore.com/tuberings.html [Duende Criatura Tube Rings]
http://www.partsconnexion.com/audiog...AGES/PEARL.htm [Pearl Tube Coolers]
http://www.vintagetubeservices.com/Dampers.htm [Original Mini Tube Dampers]
http://www.jenalabs.com/ [Tube Cryo-ing service - JenaLabs]
http://tubeman.com/ [Tube Cryo-ing service - ATSI Advantage Tube Services]

A useful link for Tube Tester owners

http://bama.sbc.edu/index.htm [TONS of scanned manuals for various tube testers] (Thanks Pieman!)
post #8 of 24
Thread Starter 

Credits and Version History



Many thanks to these forum members – these few have taught me much and for that I am eternally grateful.

(arranged in alphabetical order)

Hirsch (especial thanks)
Len (especial thanks)
tom hankins

And special mention goes to:

Mr Morgan Jones for his 2 superbly written books - Valve Amplifiers and Building Valve Amplifers
& the rickmonster, rickcr42 for his encouragement and advice.

Version History

1.0 – 280505 – Original post
1.0.1 - 280505 - On Len's suggestion, some modifications made
1.0.2 - 290505 - Added a ton of information, courtesy of braillediver. Many thanks!
1.0.3 - 310505 - A correction made to the 'tube dissection' post (item #5) which solves a MAJOR mistake! Thanks mastergill.
2.0 - 030605 - FAQ undergoes MAJOR RECONSTRUCTION
2.1 - 040605 - Some large changes made to tube 'dissection'.
2.1.1 - 040605 - FAQ Addendums added
2.1.2 - 050605 - Minor formatting changes made
2.1.3 - 050605 - Chapter Tube Substitution and Specs added
2.1.4 - 130605 - Chapter 3 has been completely reshuffled and a new sub-chapter added (Matched Tubes vs Matched Sections)
2.1.5 - 180605 - Completed all of chapter 3's sub-chapters. Finally!
2.1.6 - 190605 - Started work on a new sub-chapter 4.1.

3.0.0 - 021105 - Original FAQ deleted. Ver2 is in the house!!
3.0.1 - 031105 - Lots of stuff added. Color photos too!
3.0.2 - 280307 - Small correction made to the section on tube dampers. Thanks WopOnTour!
3.0.3 - 040707 - Yes... believe it or not, updates are still being made. Small section added on PrimaLuna's Adaptive AutoBias (tm). Thanks nsundin!
3.0.4 - 150707 - Added a link to a website hosting scanned tube tester manuals to the "Tube Links" section. Thanks Pieman!

post #9 of 24
That is true! I used my Joann's coupon at Michaels the other day..... as long as they are the 40 % off a single item coupon.
post #10 of 24
A terrific resource -- many thanks for taking the time to put this together!


post #11 of 24
thanks for this thread!

Am I the only one not seeing the pictures?
post #12 of 24
Would be great if you could get the pictures up and running again.
post #13 of 24
I don't see any image too...
post #14 of 24
Thank you very much, AdHoc. Very good guide!!

P.S. I'm also can't see the picture.
post #15 of 24
Thank you. What a great FAQ. It sure made a whole lot of things clearer. Still missing some simple terminology though.
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