***welcome to The Tube Fairy's Ebay store!!***

your source for vintage vacuum tubes of all sorts,

tested, screened and surveyed to exacting and extensive standards!

i'm a tube amp modding and customization specialist from Sacramento, California,

selling tubes i don't need on Ebay!

(please message me if you'd like to hire me for amp work <3)

i use my Etracer curve tracer in tandem with research and experience

to comprehensively evaluate vintage tubes for performance, reliability and lifespan!

i mostly focus on high quality audio tubes and some common radio tubes,

but i have a large supply, so feel free to message me with requests!

have fun shopping!!

***PLEASE SCROLL TO THE BOTTOM OF THE PAGE FOR DISCLAIMERS

AND FULL REFUND/RETURN POLICY***

Sylvania 6SN7GTB

Inventory # 6SN7_1

Quick Test Results

Overall categories: STRONG/high gain, LOW microphonics

Likelihood/extent of use history: not used

Triode 1 (pins 1, 2, 3): STRONG/high gain

I(p): 109%

rp: 94.6%

gm: 127.5%

U: 120.7%

microphonics: VERY LOW

corners: PASS

Triode 2 (pins 4, 5, 6): STRONG/high gain

I(p): 99%

rp: 100%

gm: 128%

U: 128.1%

microphonics: LOW

corners: PASS

Test results summary and notes: results consistent with other NOS Sylvania 6SN7's, which are typically high in gm and U even as brand new tubes. This is not any sort of an issue, and if anything, will give you a bit of extra gain and audio resolution, which translates to a bit extra volume and detail. Sylvania was all but the authority on the 6SN7 iterations and this tube is no exception - with the extra high transconductance values, it's no wonder they're the most popular vintage USA manufacturer to try for hifi!

Low microphonics make this tube suitable for any application or environment, including hifi, headphone or guitar amps, even the most demanding and vibration-prone amps such as Ampeg Reverberockets.

*note: 6SN7 datasheets commonly refer to the triode connected to pins 1, 2 and 3 as 'Section 2', which I find immensely confusing and pointless. Please note I've referred to this triode as 'Triode 1' above, which makes more sense to me but is technically in contradiction with the datasheets.

Model notes: the 6SN7GTB is an up-rated version of the 6SN7GT, with a maximum plate voltage of 450V, maximum cathode current of 20mA, maximum power dissipation for one triode of 5W, and 7.5W for both triodes together. It is suitable for even the most demanding circuits calling for a 6SN7 type, and when installed in circuits designed for the 6SN7GT, will give extremely long life and reliable performance.

Sylvania notes this tube is a medium-mu double triode suited for use as a horizontal multivibrator, phase inverter, or combined vertical oscillator and detection amplifier, also featuring a controlled warmup time for series string operation.

These days, this tube is extremely popular for hifi use due to its excellent linearity. It makes a fantastic headphone amplifier, phase inverter, or driver for a power amp. It also may be useful in a guitar amp as a phase inverter, power amp driver, reverb driver and recovery amplifier, or moderate gain cascaded amplifier for a lovely clean tone.

Variant notes:

Manufacturer: Sylvania

Brand: Sylvania

Box: original, decent condition

Bottle: tall, 43mm long

Plates: 'T', 135 degree angle, black, 2 horizontal rib, 5 holes in wings, short top and bottom cutouts on broad side and wing corner

Getter: top, type unknown due to extensive getter flash

Micas: circles, 8 prongs on top none on bottom, bottom thick, top width not visible

Base: 12mm tall black plastic

Label: octagon on top, yellow on base, both near perfect

Date: unknown, likely mid 1950's, codes EJ, BEJ

Full Etracer Testing Parameters and Results

Quick-scan results

============== Test Setup ==============

HV1: 250V, swing: 5%

HV2: Linked to HV1, 100%

NEGV: -8.00V, swing: 5.00%

=========== Measurment Data ===========

(-7.60,237,9.19,237,8.35)

(-7.60,249,10.93,250,9.97)

(-7.60,262,12.77,262,11.71)

(-8.00,237,8.15,237,7.36)

(-8.00,249,9.81,250,8.91)

(-8.00,262,11.57,262,10.58)

(-8.40,237,6.74,237,5.89)

(-8.40,250,8.28,249,7.31)

(-8.40,262,9.96,262,8.88)

========= Section 1 Parameters =========

current:9.81 mA, reference:9.00 mA, percent:109.0%

rp:7287 ohms, reference:7700 ohms, percent:94.6%

gm:3314 umhos, reference:2600 umhos, percent: 127.5%

mu:24.1 V/V, reference:20.0 V/V, percent: 120.7%

========= Section 2 Parameters =========

current:8.91 mA, reference:9.00 mA, percent:99.0%

rp:7698 ohms, reference:7700 ohms, percent:100.0%

gm:3327 umhos, reference:2600 umhos, percent: 128.0%

mu:25.6 V/V, reference:20.0 V/V, percent: 128.1%

Corners test results:

============== Test Setup ==============

Case 1:HV1:400 V, HV2:400 V, NEGV:24.00 V.

Pass criteria: HV1_I < 1.0 mA, HV2_I < 1.0 mA

Case 2:HV1:500 V, HV2:500 V, NEGV:24.00 V.

Pass criteria: HV1_I < 8.0 mA, HV2_I < 8.0 mA

============= Test results =============

Case 1: pass

Case 2: pass

Quick reference notes on policy, testing procedure, test result categories and more…

• Please reach out with any questions, issues or concerns! I’ll always go the extra mile to make things right, and I love to help and advise where I can.
• Tubes are tested on the Essues Etracer curve tracer, the most accurate modern machine for matching tubes, and one of the most accurate for measurement of a wide range of parameters over the tube's entire operating range.
• Diodes are measured for plate current with a moderate DC voltage on the plate.
• Triodes are measured for plate current (I(p)), plate resistance (rp), transconductance (gm), and amplification factor (U). Matched and/or balanced triodes measure within 5% of each other in either I(p) and gm, and/or rp and U, across their entire operating range.
• Pentodes/beam tetrodes are measured for plate current (I(p)), plate resistance (rp), transconductance (gm) and screen current (I(g2)). Matched and/or balanced pentodes/beam tetrodes measure within 5% of each other in plate current and transconductance across their entire operating range.
• Used tubes generally demonstrate high rp, along with low I(p) and gm, as well as possibly high U. These measurements are evaluated in tandem to determine extent of use and remaining lifespan.
• Tubes marked NOS generally measure within 80-120% of nominal values, which was the standard manufacturing tolerance when they were produced, and are electrically equivalent to brand new tubes. They've seen very little to no use and still have their full life ahead of them.
• Tubes marked STRONG generally measure within 120-140% of nominal values for at least one of the following: I(p), gm, or U. Other values will be within 80-120% of nominal, though rp may be as low as 70%. These are also brand new tubes that generally have extra long life and strong performance, and stand even less chance of being used than NOS tubes. Occasionally these tubes can cause issues with the most sensitive circuits, which would be noted in the Testing notes. Please reach out with details about your intended application (brand and model of equipment, and position it's to be installed in) if you are uncertain or concerned about use of a STRONG testing tube in your system.
• Tubes marked GOOD generally measure within at least one of the following brackets: 120-130% of nominal values for rp, or 70-80% for I(p) or gm. Values not in those brackets measure 80-120% of nominal. These are used tubes that still have about 50-75% of their lifespan ahead of them, usually at least several years of moderate use.
• Tubes marked LOW generally measure within at least one of the following brackets: 130-135% of nominal values for rp, or 65-70% for I(p) or gm. Values not in those brackets measure 70-130% of nominal. These are used tubes that still have about 25-50% of their lifespan ahead of them, usually at least 1-2 years of moderate use.
• Microphonics testing is conducted for some tubes, and uses a high gain center biased class A amplifier and a frequency response analyzer to place the tube in one of 7 different categories, from VERY LOW to HIGH levels of microphonics. HIGH testing tubes are discarded, but MODERATE microphonic tubes can still be suitable for non-sensitive applications, and anything from AVERAGE to VERY LOW can be relied upon for no microphonics in most applications.
• Corners testing ensures triodes perform to spec at the highest voltages of their operating range. Failure conditions may or may not have a noticeable effect on operation depending on application, and are more likely to have an appreciable effect on hifi circuits. 200-250% failures stand some small chance of overdissipating the most demanding circuits and should be evaluated for safety.
• Contact pins on all tubes are de-oxidized, cleaned and polished! (gold pins not polished, of course)
• Best efforts have been made to clean tubes, preserve their labels, and provide original boxes, but vintage tubes commonly have some dust or dirt on them, labels are commonly eroded, and boxes will commonly be substitutes or in eroded and sensitive condition. Even the best testing and performing tubes usually have some cosmetic defects. Please inspect pictures carefully.
• Refunds of any sort must be applied for within 30 days of receiving the tube. If the tube arrives broken or otherwise non-functioning, please message me and send it back, and I will be more than happy to refund the purchase cost and both shipping costs if I can confirm it is non-functioning. For complete refund and return policy covering all eventualities, please scroll to the very bottom of this page.

Why purchase your tubes from The Tube Fairy?

Integrity in pricing, testing and advertisement - many Ebay tube sellers use dishonest tactics, such as: writing incorrect model number equivalents in the title to make it seem as if the tube is higher grade, or just to catch more search results; inflating prices far beyond the market value of the tube and using misleading language in the description to manipulate the customer into believing the absurd pricing somehow equates to better performance; advertising tubes as ‘matched’ (which should mean matched electrical performance) when they are only matched for date codes (no guarantee of electrical performance) and may even be untested; advertising untested or poor testing tubes as NOS simply because they may be in the original box or in good cosmetic condition; and posting misleading testing results without references, to sell weak or dangerously over-performing tubes at NOS prices.

The Tube Fairy skips all that nonsense, giving you nothing but hard data from professional performance tests, well researched and documented information about the model and any features of the construction, manufacture or vintage, and fair pricing that is directly related to these previous factors, as related to a modern market average. The title will have only the correct model number(s) and equivalents, and the testing results and categorizations are related directly to clear information on how they relate to performance. I have a lot of respect for my buyers, and it’s my mission to make sure they get exactly what they pay for. In addition, my disclaimers on testing parameters and refund conditions are clearly stated on each listing, to ensure each purchaser is clear on the terms of sale beforehand.

Tube refurbishing - vacuum tubes produced back in their heyday are likely 50 years old at least, if not 100 years old or even more! At this age, even if they were stored well in their original boxes, tubes’ condition may have suffered from the years in a number of ways. The primary victims are the connection pins, which often have oxidized or may be covered in other substances, which can dramatically affect conductivity and quality of connection. In addition, deposits may have built up on the electrodes internally, which can potentially cause shorts or decreased emissions. The Tube Fairy addresses all of these ills by giving special treatment to the pins, de-oxidizing, cleaning and polishing them back to perfect conductivity (gold pins are not polished as this would remove the gold coating), and using the Etracer curve tracer to provide pulsed DC to different electrodes, using the highest safe voltages to help remove impurities and prevent shorts.

The most accurate professional grade testing and matching available - ‘testing’ a tube can mean many things. There are many types of equipment, from those that test all possible performance parameters, to those that more or less only determine if the thing turns on! Even most competent testers usually only evaluate a tube at a single operating point, or measure a single value, which may or may not be relevant to your equipment. In addition, many sellers list incomplete information about their testing results, usually something like “testing at 94 where 45 is the minimum value”. This says nothing about what the testing parameter is or what it relates to! It only ensures that the tube is functioning above minimum ratings in some capacity, and makes tubes that are testing far beyond their nominal values seem acceptable, which could potentially be bad news for your equipment if the parameter happens to be something like screen current on a power tube, for example! These sorts of results do nothing to evaluate the tube across the high voltage operating range that it may experience in your gear, do nothing to evaluate tube longevity, and have very limited matching accuracy.

My testing device is the Essues Etracer, an unparalleled modern curve tracer, capable of measuring all relevant performance indicators across the tube’s entire operating range, including: filament current, plate current, transconductance, amplification factor, plate resistance, and screen current. The comprehensive resulting data can be evaluated to accurately determine not only the quality of regular performance, but also how much life a tube has left, the quality of its linearity, and how it performs in the extremes of its operating range. One of the only other modern testing devices at this level would be the Amplitrex AT1000, but even that unit cannot develop full spectrum graphical analysis the way the Etracer can. This device is able to represent measured information graphically across the entire working range of the tube, and this is the most accurate possible way to ‘match’ tubes for performance, meaning that their electrical characteristics will be identical within a certain tolerance across their entire operating range. This is crucial for push-pull power amplifiers and phase inverters, as well as many other applications. A final added benefit of this system is, because it supplies pulsed voltages in fractions of a second, tubes can actually be evaluated safely up to 1.5x their operating range, doubly ensuring that it will handle high voltages well. These pulsed high voltages also help to clear impurities from different electrodes within the tube, which helps to prevent shorts and may even increase longevity. 

Friendly, honest service - I sell these wonderful little buggers because I love them, I love the world of audio and audio enthusiasts, and I dearly want to bring excellent sound and performance to your system, whether it be a hifi amp, a mic pre, an effects unit, a studio compressor, or a roaring guitar amp! Each tube is packaged lovingly with extra insulation material and a strong, fresh mailing box to ensure they get to you safely. If you're ever unsatisfied with your purchase, simply return the tube within 30 days of receipt and I'll be happy to send you a full refund or replacement if available, no muss no fuss.

One of the biggest reasons why I love tubes and tube gear is because they belong to an era of technology where full comprehension of the systems involved was not far beyond the resources and knowledge of the average consumer. Our high-tech world has brought us a lot of wonderful things, but I firmly believe it benefits everyone when the understanding of how our devices work is close at hand. I hope to do my small part in making that information, and tube technology in general, accessible to everyone, but especially low-income and underserved communities that have been priced or gatekept out of being able to take advantage of this technology. That’s why I am ALWAYS happy to disseminate information and answer questions! I am still learning, myself, but I am careful not to speak on anything I am not certain about and researched on, and I’m happy to relate my answers to whatever your level of experience and knowledge may be, so don’t be afraid to come to me as a casual consumer or beginner!

Details of testing procedures, test result analysis, and testing category standards

There are three major types of tubes I am selling and can fully test - diodes, triodes, and pentodes/beam tetrodes. Other types either will be tested as one of these types (pentagrid tubes tested as pentodes), or have a method of testing that is self-explanatory, non-numeric, and/or fairly uniform across all testers (magic eye tubes tested for strength of neon glow, and width of aperture opening at two different voltages). Some tubes contain multiple sections, which will be tested individually, and individual categories given for each section if they test differently.

Diodes have two electrodes (electrical elements), triodes have three, and pentodes/beam tetrodes have five. There is an additional element in nearly all tubes called the filament, and its purpose is simply to get hot to cause the cathode to emit electrons. In some tubes the filament and the cathode are the same element (directly heated), but usually they are separate. As the filament serves no purpose beyond this, it is not included in electrode counts and testing procedures. The only relevant testing parameter for the filament is how much current it draws, and this is a pass/fail condition. All tubes being sold will only have very minor, if any, filament current deviations from the nominal value.

Tubes that test within 20% of their nominal values for all critical measurements are considered ‘NOS’, which means New Old Stock, indicating that the tube will essentially function as if it were brand new, and is regarded and priced as such. All critical measurements must be within the appropriate tolerance range, or I will not consider them NOS. NOS testing tubes may have been very lightly used in the past, it is impossible to determine with certainty, but they can be relied upon to provide the performance and longevity of a brand new tube. Some measurements may be outside the 20% tolerance range even for brand new tubes; for example, screen current or I(g2) on beam tetrodes and pentodes is usually only required to be below 120% to meet NOS standards for audio applications, and values as low as 30% still meet NOS standards for most models.

Test categories and corresponding deviations from nominal values:

LOW - 65%-70%

GOOD - 70%-80%

NOS - 80%-120%

STRONG - 120%-140%

The main exception is rp (plate resistance), for which values 120-135% denote moderate to heavy use and place the tube in the Good or Low category.

Tubes with critical values measured at less than 65% or greater than 140% are typically not recommended for use and are not listed. Exceptions detailed below.

The overall testing category is generally denoted by the lowest or highest value outside of the NOS range. However, a small amount of flexibility (2% or less) on a single less critical value may be allowed for when placing a tube into a testing category. For example, a triode that is well within a 20% tolerance on U, rp and gm, but where plate current is at 78% of nominal, may still be categorized as NOS even though the plate current is not technically within that range. This is because U and rp are by far the most crucial values for a triode, and if those values are close to nominal, a tube with 78% or 122% of nominal plate current will still perform as new. This is not the case for tubes that show significant signs of use (high rp, low I(p), and possibly low gm and high U); these will never be marked as NOS.

Tube electrodes and basic functions:

Cathode - emits electrons when heated by the filament, to be collected by the plate, and regulated by the grids.

Plate (anode) - receives positive DC voltage to attract and collect electrons emitted by the cathode, usually high voltage.

Control Grid (grid 1, or just 'grid' in triodes) - between cathode and plate, receives various static or fluctuating voltages to control the flow of electrons through the tube. 

Screen Grid (grid 2) - between control grid and plate in pentodes/beam tetrodes. In an amplifier, it receives positive DC voltage to accelerate electrons towards the plate, collecting a small amount of them itself. Usage varies widely across applications.

Suppressor Grid (grid 3) - between Screen Grid and plate in pentodes/beam tetrodes. Usually grounded at 0V to reflect electrons that bounce off the plate, back at the plate. Sometimes connected to the cathode internally.

Parameters measured and what they indicate:

I(p) (plate current) - the amount of current flowing through the plate. This is the most essential and general sign of a tube's health and remaining longevity, and the only parameter that most basic tube testers measure. The amount generally translates to the amount of detail, power and clarity it's able to lend to audio. It is more crucial to power and driver stages than to preamp tubes, but important to every type of tube.

- Values 80-120% are normal for a brand new tube.

- Values 120-140% will give extra long life and strong performance, but may occasionally over-dissipate some more marginally designed circuits. If you're unsure if a tube with strong plate current is safe for your system, feel free to reach out with the brand and model of your equipment as well as the position the tube is to be installed in, and I'd be happy to evaluate it for safety.

- Values 65-80% indicate mild to moderate use, and a decrease in detail, power and clarity which may or may not be noticeable, depending on the application and value. For example, a power tube testing 65% will be noticeably less powerful than a brand new one, whereas a preamp tube testing at 75% may sound exactly the same as a brand new one.

rp (plate resistance) - a measurement of the resistance the plate offers to AC signals such as an audio signal. Determines the linearity of the tube and the amount of harmonic distortion it adds to the audio. Besides I(p), it is also the most reliable indicator of the amount of use the tube has seen.

- Values 80-120% are normal for a brand new tube.

- Values 120-135% generally indicate a significant amount of use, increased harmonic distortion and decreased linearity.

- Values 65-80% are somewhat rare but generally unproblematic, and may indicate along with other measurements that the tube will have an extra long life, as well as indicating excellent suitability for hifi, with improved linearity and low harmonic distortion; measurements in this range will usually still quality the tube for NOS or STRONG testing categorization.

- All these standards generally only apply to triodes, while beam tetrodes and pentodes are different: rp measurements for them are always inaccurate, and values from 50-250% may still be accepted as NOS or STRONG without any indication of use, if other values meet those criteria.

gm (transconductance) - the control that the grid voltage has over anode current. in other words, the tube's potential to translate very small and fine input signals into large, robust, detailed output signals; it can be thought of as the 'audio resolution' value. for preamp tubes, this is generally the least important measurement, but for driver and power tubes, as well as beam tetrodes and pentodes in general, this is the most crucial measurement along with I(p), especially for matching, as tubes with the same transconductance value and I(p) value at any test point can be relied upon to produce the same I(p) value at any other point across their operating range. it's also a very important characteristic for hifi, as higher transconductance usually means a lower noise floor.

- Values 80-120% are normal for a brand new tube.

- Values 120-140% are generally unproblematic and may feature improved output detail, power and clarity similar to high I(p) values, usually with a lesser chance of overdissipating marginal circuits than high I(p) values may bring, and values even up to 150% can be acceptable. They will also likely result in a lower noise floor.

- Values 65-80% indicate some degree of use similar to I(p) values, and will almost certainly indicate a higher noise floor.

U (amplification factor) - the ratio of the maximum possible voltage gain from the tube's amplification stage. In reality, most circuits will bias the tube for voltage gain of about 50-70% of this number. This value is only measured on triodes, as its measurement for beam tetrodes and pentodes is too inaccurate to be a reliable indicator, though there are occasional exceptions.

- Values 80-120% are normal for a brand new tube, and many triodes measure 90-110% even when heavily used, as U is the most consistent factor over most tubes' lifespans, and therefore is the least reliable indicator of use history.

- Values 120-140% may indicate use, but only if accompanied by high rp, low I(p) or gm, or some combination of those measurements. These high values are classified as 'high gain' and will mostly create added signal gain from those amplification stages, which is usually desirable but will have different results in different applications, as it will not only make the output signal louder, but may also drive one or more other amplification stages closer to, or further into, overdrive. In guitar amps, this will cause an increase in amount of available distortion and a sooner distortion onset at lower settings (lower headroom), which may or may not be very desirable depending on what you're looking for tonally. In hifi amps, this will increase output signal strength and will usually also decrease the noise floor which is very desirable, and though it may also decrease the headroom and lower the point at which distortion onsets, this can usually easily be compensated for completely by a small adjustment of the volume control.

- Values 65-80% are very rare, but may indicate a tube with extra long lifespan, and will increase headroom and decrease available distortion and potential for overdrive. This may be very desirable in a guitar amp depending on what you're looking for; many amps are designed with too much gain on tap and a tube like this can 'open them up', increasing their range of usable settings and making them more expressive, dynamic and less fizzy. For hifi, though, it is rare that a low U tube would be advantageous and it would likely increase the noise floor, which would be very undesirable.

I(g2) (screen current) - the amount of current flowing through the screen grids of beam tetrodes and pentodes. For audio use, very low values are acceptable, as the screen grid only serves to accelerate and direct electrons towards the plate and does not carry the audio output signal, so measurements 30-120% are acceptable as NOS as long as other measurements meet that criteria well.

- Values 120-140% generally do not change the characteristics of the audio at all besides perhaps a nominal decrease in output signal strength and headroom, but occasionally have the potential to overdissipate sensitive or marginal circuits. This is generally a non-issue in hifi amps as their components tend to be generously rated, but it may be a concern in some guitar amps, especially vintage ones, as some have no screen resistors or ones that are marginally rated on power tubes. Feel free to reach out with the brand and model of your equipment and the position the tube is to be installed in, and I'd be happy to evaluate the suitability of a tube with high screen current for that application.

Tube types and type-dependent methods and analysis:

Diodes - these are extremely simple. A positive voltage is applied to the plate, and the resulting plate current is the only relevant measurement to make.

As diodes are usually meant to have AC applied to the plate and not DC, and the Etracer provides DC, which draws much higher current, they are only tested in a fairly low voltage range, but this is sufficient to extrapolate high voltage performance. As an added measure, graphical representation of plate DC voltage vs. plate current is provided up to the current limits of the tube, and gives an even better estimation of performance at close to the tube’s maximum rated values. The graph should show a smooth curve all the way to the current maximum, with an even slope.

Triodes - these have a cathode and a plate like diodes, but with a thin wire control grid superimposed in between to regulate the current flow. Various large positive DC voltages are applied to the plate, usually up to several hundred volts, while various much smaller negative ‘bias’ voltages are applied between the control grid and cathode to reduce the current to specific amounts. Various measurements are taken:

I(p) (plate current)

Rp (plate resistance)

Gm (transconductance)

U (pronounced ‘mu’, also called amplification factor)

Triodes are matched by a combination of U, rp, and plate current.  In addition, the graphs of the plate voltage vs. plate current must show matching curves within 5% across the entire operating range. I will not call a tube matched unless all 3 of these parameters are matched within 5%, and the curves are as described. The only exception is that I will allow a match of perhaps 7% or 8% on plate current, if the U and rp are matched well under 5%, and the curves are identical in shape and difference across their entire range. 

‘Balanced’ triodes are single tubes with multiple triodes in the same glass envelope, where all the triodes are matched. The same standards apply. Multiple tubes may be balanced within themselves and also matched to each other, meaning all sections of all tubes are matched. Or they may be matched and unbalanced, meaning the different tubes’ multiple sections are unbalanced identically - section 1 of tube 1 matches section 1 of tube 2, and section 2 of tube 1 matches section 2 of tube 2, but sections 1 and 2 are not balanced. Generally, guitar amps work best with phase inverters balanced and power tubes matched, both to a tolerance of about 10%, more like 5% for bass amps. For hifi, the tolerance on matching should be as small as possible, and the needs are very circuit specific. For example, the same preamp position on two monoblocks may require a pair of tubes that are matched, but unbalanced tubes would be fine. Which parameters are matched is also important - for example, if the tubes are dual triodes, the first triode being a preamp stage and the second a driver stage, it's more important the first stage is matched for rp and U, whereas I(p) and gm are more important for the second stage. For another example, a long tailed pair phase inverter position on two monoblocks would require a pair of tubes that are both matched and balanced.

Triodes show age and use by demonstrating very high plate resistance, along with low plate current and transconductance, and often somewhat high U. Tubes that display these characteristics, even if they meet the tolerances to be rated as NOS, will be listed as ‘used’ as this is likely the case. Conversely, tubes with low plate resistance, high plate current and transconductance, with all values within the NOS or STRONG ranges, can be relied upon for extra long life.

Pentodes/beam tetrodes - these two tube types only differ in the physical construction of the third grid; their electrical function is identical. The function as an amplifier is similar to the triode, but with the addition of a second grid called the screen grid to accelerate electrons towards the plate, and a third grid called the suppressor grid to reflect electrons that bounce off the plate, back at the plate. The suppressor grid may also focus the electrons into more efficient 'beams' aimed at the plate, and these may alternatively take a different form factor than the first two fine wire grids, and be called 'beam forming plates. Various measurements are taken:

I(p) (plate current)

Rp (inaccurate for most pentodes, measurements from 50%-250% can be accepted as NOS, and lower or higher measurements than that can be accepted)

Gm (transconductance)

Screen current (importance depends greatly on application, values as low as 30% can be accepted as NOS)

Pentodes and beam tetrodes are matched for transconductance and plate current, with the same 5% tolerance and graphical matching standards as triodes applying. Use extent is determined similarly, though plate resistance is less of a reliable factor.

Microphonics testing:

This is accomplished by installing the tube in a class A amplifier circuit with the maximum possible gain and signal swing before the onset of distortion, while subjecting the tube to different methods of physical vibration, including the sound from a speaker and a shouting voice at close proximity, as well as (gentle) physical strikes to the chassy and tube itself, and then running the resulting audio output into an analyzer to evaluate microphonic frequencies and amplitude. the tube is then placed in one of 7 categories based on overall presence and intensity of microphonics:

VERY LOW: microphonics will never be encountered in even the most sensitive applications

LOW: microphonics are very unlikely to be encountered in even the most sensitive applications

LOW-AVERAGE: may exhibit nominal amounts of microphonics in the most sensitive applications which can be mitigated or eliminated

AVERAGE: may exhibit very small amounts of microphonics in the most sensitive applications which can usually be mitigated or eliminated

AVERAGE-MODERATE: may exhibit small amounts of microphonics in sensitive applications which may be able to be mitigated

MODERATE: sensitive applications should be avoided

HIGH: tube is discarded and not sold

Some application factors that affect propensity for microphonics:

- Tube base type. Most plastic octal base tubes are generally more likely to produce microphonics, especially for preamp tubes.

- Gain factor. Higher gain tubes, and amplification stages that are setup for higher gain, are more likely to produce microphonics.

- Overall amplifier type. Most guitar amplifiers are high gain and feature overdrive, so they are more likely to produce microphonics. Hifi amps usually feature more moderate gain and no overdrive with better physical isolation, so they're variably sensitive to microphonics, but less sensitive than your average guitar amp. A headphone amp would be the least sensitive example, as there's no loudspeaker to vibrate the tube.

- Local circuit type for tube position. Different positions in different amplifiers will have a higher or lower propensity for microphonics based on their circuitry and position in the block diagram. For amplification stages, the closer the stage is to the input, the more sensitive to microphonics it will be. A tube used entirely for a tremolo in a guitar amp would likely be immune to any effect from microphonics. Some circuits such as phase inverters, cathode followers and line drivers are inherently less likely to produce microphonic effects.

- Tube's proximity to, and isolation from, speaker. The further a tube is from the speaker, the less possibility for it to cause microphonic feedback. The amount and types of substances that exist physically between the tube and speaker also matter greatly - an amp on the same table as the speakers will be more likely to produce microphonics than if the speakers were on the floor, or other tables. Substances such as glass or metal will be more likely to contribute to microphonics, than would other more shock absorbing substances like rubber, felt or foam.

- O-rings and tube shields/harnesses. Many companies sell rubber O-rings to put around tubes to dampen vibrations. Some tube shields also assist towards this end by means of a spring in the top, and many other methods of bracing the tubes with harnesses exist, such as a metal ring for the top attached to two springs at the base.

An example of an extremely sensitive application would be a high gain octal base tube in the first input stage of an overall high-power, high-gain combo guitar amp with ample bass response, where the tube is physically mounted very close to the speaker. This would be about the most likely combination to be sensitive to microphonics.

Examples of the least sensitive applications would be a later amplification stage, phase inverter or cathode follower in a hifi amp where the speakers are far away from the tubes, and the amp itself is physically well isolated from the speakers by substances that can absorb vibrations like rubber feet or mats. The least sensitive possible audio circuit would be a headphone amplifier, in which only low or moderate gain is needed and there is no loudspeaker to vibrate the tube.

Corners testing:

Only offered for triodes. This test measures plate current at different bias voltages in the highest plate voltage section of the tube's operating range. Many tubes test on-spec at the standard test point, but may behave erratically at the highest voltages they see. Failure measurements are taken as percentages of the normal maximum current for those bias voltages:

100-150% failures are unlikely to be noticed or cause significant issues in any but the most demanding hifi circuits

150-200% failures may cause very minor changes in audio in demanding hifi circuits, and run a very small risk of overdissipation in demanding circuits of all kinds

200-250% failures will likely cause some minor changes in audio in all amplifiers with power supplies that push the tube's maximum ratings, and run a significant risk of overdissipation in most of those designs

250%+ failures are rejected and not sold

The first test ensures that the control grid can still cause the tube to reach cut-off at the appropriate bias voltage in the high plate voltage range. This is especially important for hifi use, as it affects amount of harmonic distortion and efficiency of class B, C and D operation modes.

The second test ensures the very highest plate voltages the tube can tolerate do not result in a drastically high current measurement, which could possibly overdissipate the tube if it is already biased close to its dissipation maximum. This is relevant to the most demanding designs of all kinds of amplifiers, and if you're concerned about applicability and safety with your amplifier, I'd be happy to advise you.

DISCLAIMERS

Test and/or performance variations

Tubes are temperamental and sensitive little things. Their measured performance values may change even up to 5% or sometimes more based simply on the temperature, amount of warm-up time, or time of day at which they are tested, as well as wall voltage. I do my very best to test all parameters thoroughly and redundantly before selling a tube, and post my testing parameters and results on each listing, as well as including a printout of the test results with the tube purchased, and/or custom testing parameters, if requested. However, the golden rule of vintage audio is: Your Mileage May Vary. I cannot fully predict or control variations in how tubes may perform in your gear, or test in any other tester, or any differences between those testing results and the ones I’ve listed. Tubes perform differently in different circuits, and there is no way for me to ensure one will perform the same in every single one. These tubes are tested as amplifiers and, for diodes, rectifiers; this will not ensure ideal performance for a tube used as an oscillator, for example. This is NOT usually an issue, however, as tube circuits will allow for a wide tolerance of performance parameters from a tube (at least 20% is common) and small fluctuations will make little to no difference. In addition, testing a tube’s full range performance as an amplifier will give the best general estimation of performance for all applications.

Microphonics

some tubes I sell are tested for microphonics, if stated in the test results, such as the 6SN7's I sell. If not stated in the test results, no microphonics testing has been conducted. Microphonics are a tube's tendency to couple physical shock and vibration into the electrical audio signal. In other words, when the tube is vibrated by the sound from the speaker, if it is badly microphonic, that vibration will add noise to the audio going through the tube and send that added noise to the speaker. At worst, this can be anything from a rattling sound, to a dull hum or buzz, to a high pitched ringing or even a feedback loop. All tubes have some level of microphonics. My testing setup is meant to replicate the most sensitive and likely circuit to produce microphonics, but every circuit is different, and there's always a possibility that your system will produce more microphonics than my testing setup, especially if the speaker is physically close to the tube or housed within the same cabinet, such as in a combo guitar amp. High gain circuits and tubes, and octal preamp tubes, and tubes installed in sensitive positions in the circuit such as the first input tube, are also especially susceptible to microphonics. If you're experiencing microphonics, try moving the trouble tube to a later stage suitable for that type, moving the speaker away from the tube or isolating the chassy the tube is installed in on rubber feet or a mat, or using rubber O rings sold by a number of tube distributors around the tube to dampen vibrations.

Cosmetic condition

Tube boxes never have their condition guaranteed. Whenever possible I will send the tubes in their original boxes, or a period and brand appropriate original box, but these have often decayed beyond the point of use, and in those cases, a brand new blank white box with the model number written on it will contain the tube. If vintage boxes are included, they may fall apart with very minor perturbation or a single opening. I am happy to meet customer requests for boxing as I am able, at no additional charge.

Tubes themselves will be cleaned to whatever extent possible, without compromising whatever labeling remains, but no guarantees are made to perfect cleanliness or condition of the labels. Often, cleaning the surface of the glass completely would remove what remains of the delicate silkscreen, and often the silkscreen may be removed even more when the tube is taken out of, or placed into, a box. Pictures taken will represent the level of cleanliness the tubes have been restored to, but any labels pictured may be slightly more eroded by the time they reach the customer. All attempts to prevent this will be made. If customers prefer I prioritize cleanliness over label preservation, or vice versa, I am happy to oblige as able. Please inspect the pictures carefully, and know that often these very old vintage electronic devices have some cosmetic defects or levels of dirt built up on them, but it is the electrical testing results that are important, and even tubes in the roughest cosmetic condition can still test and perform brand new.

Refund, return and replacement policy

There are many scenarios in which you might request a refund, return or replacement, and I aim to cover them all within these Policies and Scenarios. Please read and adhere to all Policies, find your Scenario, and follow instructions to its Resolution.

Policies

Policy #1: ANY and ALL refunds, returns and replacements must be submitted for within 30 days of you receiving the tube to be accepted. 

Policy #2: it is necessary that you message me with the reason for your return. It can be as simple as "it doesn't work" or "i don't like it". Some Scenarios may require further detail.

Policy #3: I generally accept any refund, return and replacement requests that meet Policies #1 and #2, EXCEPT if you've damaged the tube by installing it in an improper circuit. please message me before purchasing if you have any doubts about a tube's suitability for your gear.

Policy #4: exact replacements may not be available, in which case less expensive equivalents may be an option, but replacements of any sort are never guaranteed.

Scenarios

Scenario #1: the tube arrives physically damaged, like the glass broken or pins badly bent.

Resolution: please message me with pictures, and I will be more than happy to send you a full refund. I may ask you to send the tube back to me at my expense.

Scenario #2: the tube arrives with a white powdery spot on the inside.

Resolution: this indicates the vacuum seal has been compromised and the tube has been damaged during shipping, so we'd use the Resolution for Scenario #1.

Scenario #3: the tube arrives non-functioning or poorly functioning.

Resolution: please message me with the brand and model of the amplifier you've installed the tube in, the tube position you installed it in, and the issue you're experiencing, in as much detail as possible. Once I've confirmed you're using the tube in gear and a position that's safe for that model, I may still offer advice as to how to lessen or eliminate the issue, but either way I will be more than happy to issue you a full refund if you return the tube. If we can diagnose for certain that there is an issue with the tube itself, I'll also pay the return shipping.

Scenario #4: you're unsatisfied with your purchase for any other reason.

Resolution: we'd use the Resolution for Scenario #3.