Five 6AS7g type tube comparison

Discussion in 'Portable and Other Gear Measurements' started by atomicbob, Feb 11, 2025.

  1. atomicbob

    atomicbob dScope Yoda

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    Five 6AS7g type tubes were measured for comparison.
    Measurement methods and notes can be found in this post: xDuoo TA-66 technical measurements

    Index:
    Post 1: Introduction, output impedances, THD+N surfaces
    Post 2: D2 surfaces
    Post 3: D3 surfaces
    Post 4: 4+HD+N surfaces
    Post 5: frequency response, bandwidth (transient response)
    Post 6: 1 KHz 0 dBu distortion suite, 1 KHz distortion vs level
    Post 7: Residual Noise, Multitone
    Post 8: Chatham 2399 noises, measurement suite report pdf files
    Post 9: reserved for additional data and/or corrections

    Amp used for tube evaluation and measurements: xDuoo TA-66

    Output tubes evaluated:
    01 DSC_0118_small.jpg
    Left to right
    Shuguang 6n5pj
    Sovtek 6AS7g
    Chatham 2399
    Tung-Sol 5998
    GEC A1834

    Measurements for each tube will be presented in the same order as listed above. Animations will also display in the order listed above.

    Introduction

    Tube rollers have long described different auditory experiences with different tubes in their respective amplifiers. Traditional measurements have not correlated well or only hinted at audible differences. An examination with a new measurement technique, distortion surfaces, may provide greater insight.

    In this evaluation only the output tube in an single OTL amplifier will be changed.
    Driver tube will remain constant, in this case a 6n2.

    Listening was performed with a Holo Audio Cyan 2 and Spring 3 DAC, Sennheiser HD800S, HD800-JAR, HD6x0 headphones.

    Brief listening descriptions:

    Shuguang 6n5pj
    rendered music nicely, feeling well balanced, engaging
    Sovtek 6AS7g added some refinement in transient response and a little more punch in the bass.
    Chatham 2399 very similar to TS 5998 to my ears
    Tung-Sol 5998 does some additional magic that is to my liking, but I don't think it is worth today's asking prices for what is adds over the Sovtek or Shuguang tubes.
    GEC A1834 has a gorgeous midrange but gives the music a vintage presentation. Bass is slightly less punchy compared to the others and highs feel a little rolled. A great example to hear this vintage feeling is a track by the Charlie Haden Trio West called Haunted Heart. At 6:28 the song transitions from modern recording to a 1950 cut of Jo Stafford. The midrange remains wonderful but one can tell it is definitely a vintage recording. The A1834 is similar in rendering music but not so noticeable as the example recording. Ridiculous prices today and unobtainum.

    Detailed listening descriptions may be found in this thread:
    6080 comparisons
    6AS7 comparisons

    Output Impedance (nominal approximation):
    02 20250210 TA-66 tubes output impedance.png
    Output impedance is the first traditional measurement demonstrating differences existing between the tubes. Note Shuguang and Sovtek to be similar at 124 Ω. Likewise 2399 and 5998 are similar at 98 and 93 Ω respectively. Possibly one of the factors in what I find attractive in the sound of these two tubes. A1834 is in a class by itself at 116 Ω.

    Distortion Surfaces

    This new 3-D presentation of distortion measured over frequency and level contains much more information than traditional numeric specifications. Surfaces for THD+N, 2nd harmonic (D2), 3rd harmonic (D3) and 4+HD+N (residual crap) appear below. THD+N does allude to differences but is a composite of too many measurement attributes. Decomposition into the other three will provide greater insight.

    When comparing the surfaces presented pay attention to the overall shape of the surfaces. For example note all five THD+N share falling to a dip and then rising again. The overall surfaces vary in the x-axis (Dist dB) and the dip varies in the z-axis (dBu) but the overall shape is remarkably similar. Shape similarities exist in D2, D3 and 4+HD+N distortion surfaces but greater variance in shape appears. D2 and D3 are the most relevant for correlating to audible similarities and differences.

    THD+N
    03 TA-66 6n5pj THD+N surface 300R - annotated.png

    04 TA-66 Sovtek 6AS7g THD+N surface 300R - annotated.png

    05 TA-66 2399 THD+N 300R - annotated.png

    06 TA-66 5998 THD+N surface 300R - annotated.png

    07 TA-66 A1834 THD+N dist surface 300R - annotated.png

    08 TA-66 tubes THD+N surface 300R.gif
    Animation for enhanced visualization.
     
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    Last edited: Feb 14, 2025
  2. atomicbob

    atomicbob dScope Yoda

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    2nd harmonic distortion D2

    21 TA-66 6n5pj D2 surface 300R - annotated.png

    22 TA-66 Sovtek 6AS7g D2 surface 300R - annotated.png
    Observe how Shuguang and Sovtek are remarkably similar

    23 TA-66 2399 D2 surface 300R - annotated.png

    24 TA-66 5998 D2 surface 300R - annotated.png
    Observe out 2399 and 5998 are remarkably similar but distinctively different from Shuguang and Sovtek.

    25 TA-66 A1834 D2 dist surface 300R - annotated.png
    Observe how A1834 differs from the previous four.

    26 TA-66 tubes D2 surface 300R.gif
    Animation for enhanced visualization.
     
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  3. atomicbob

    atomicbob dScope Yoda

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    3rd harmonic distortion D3

    31 TA-66 6n5pj D3 surface 300R - annotated.png

    32 TA-66 Sovtek 6AS7g D3 surface 300R - annotated.png

    33 TA-66 2399 D3 surface 300R - annotated.png

    34 TA-66 5998 D3 surface 300R - annotated.png

    35 TA-66 A1834 D3 dist surface 300R - annotated.png
    Again the Shuguang and Sovtek are similar to each other while 2399 and 5998 are also similar to each other but somewhat different from the modern production tubes, though the difference is less pronounced. A1834 is more similar to the other vintage tubes than the modern production tubes yet still in a class by itself.

    36 TA-66 tubes D3 surface 300R.gif
    Animation for enhanced visualization.
     
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  4. atomicbob

    atomicbob dScope Yoda

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    4+HD+N distortion (crap factor)

    41 TA-66 6n5pj 4+HD+N surface 300R - annotated.png

    42 TA-66 Sovtek 6AS7g 4+HD+N surface 300R - annotated.png

    43 TA-66 2399 4+HD+N surface 300R - annotated.png

    44 TA-66 5998 4+HD+N surface 300R - annotated.png

    45 TA-66 A1834 4+HD+N dist surface 300R - annotated.png

    46 TA-66 tubes 4+HD+N surface 300R.gif
    Animation for enhanced visualization.


    Shuguang 6n5pj is the cleanest but they are all remarkably similar.
     
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  5. atomicbob

    atomicbob dScope Yoda

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    Frequency Response comparisons

    51 20250208 TA-66 6n5pj frequency response 5Hz - 96KHz 300R 0dBu t2.png

    52 20250208 TA-66 Sovtek 6AS7g frequency response 5Hz - 96KHz 300R 0dBu.png

    53 20250207 TA-66 2399 frequency response 5Hz - 96KHz 300R 0dBu.png

    54 20250207 TA-66 5998 frequency response 5Hz - 96KHz 300R 0dBu.png

    55 20250206 TA-66 A1834 frequency response 5Hz - 96KHz 300R 0dBu.png
    A1834 is slightly different from the rest, but if frequency response were the yardstick, nothing stands out as significantly different.

    Square wave response comparisons

    Shuguang 6n5pj
    56 20250126 SigGen TA-66 6n5pj 100Hz sqr 2Vpp 1uS div 5MHz filter 300R - BW calc.png
    BW ~= 125.8 KHz

    Sovtek 6AS7g
    57 20250208 SigGen TA-66 Sovtek 6AS7g 100Hz sqr 2Vpp 1uS div 5MHz filter 300R - BW calc.png
    BW ~= 120.0 KHz

    Chatham 2399
    58 20250207 SigGen TA-66 2399 100Hz sqr 2Vpp 1uS div 5MHz filter 300R - BW calc.png
    BW ~= 133.2 KHz

    Tung-Sol 5998
    59 20250207 SigGen TA-66 5998 100Hz sqr 2Vpp 1uS div 5MHz filter 300R - BW calc.png
    BW ~= 130.9 KHz

    GEC A1834
    60 20250206 SigGen TA-66 A1834 100Hz sqr 2Vpp 1uS div 5MHz filter 300R - BW calc.png
    BW ~= 125.0 KHz

    Chatham and Tung-Sol produce the fastest transient responses which may contribute to what I enjoy about them.
     
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  6. atomicbob

    atomicbob dScope Yoda

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    Amplifier 1 KHz Distortion vs Amplitude

    61 20250126 TA-66 6n5pj A04 Distortion 300R 0dBu.png
    6n5pj shows good matching between left and right sections
    It also has the lowest distortion numbers

    62 20250208 TA-66 Sovtek 6AS7g A04 Distortion 300R 0dBu.png
    Sovtek isn’t as well matched between sections and shows higher distortion numbers

    63 20250207 TA-66 2399 A04 Distortion 300R 0dBu.png
    2399 is better balanced between sections while having higher numbers than 6n5pj

    64 20250207 TA-66 5998 A04 Distortion 300R 0dBu.png
    5998 is not so well matched between sections. Right channel has remarkably lower numbers.

    65 20250206 TA-66 A1834 A04 Distortion 300R 0dBu.png
    A1834 isn’t particularly well matched and has higher distortion numbers.


    Amplifier 1 KHz Distortion vs Amplitude


    66 20250127 TA-66 6n5pj A00 distortion vs amplitude 300R +20 dBu start t2.png

    67 20250208 TA-66 Sovtek 6AS7g A00 distortion vs amplitude 300R +20 dBu start.png

    68 20250207 TA-66 2399 A00 distortion vs amplitude 300R +20 dBu start.png

    69 20250207 TA-66 5998 A00 distortion vs amplitude 300R +20 dBu start.png

    70 20250206 TA-66 A1834 A00 distortion vs amplitude 300R +20 dBu start.png

    As these are all the 1 KHz slice from the distortion surfaces they allude to differences but do not provide as much insight as the distortion surfaces above.
     
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  7. atomicbob

    atomicbob dScope Yoda

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    Residual Noise

    71 20250128 TA-66 6n5pj Residual Noise 300R t2.png

    72 20250208 TA-66 Sovtek 6AS7g Residual Noise 300R.png

    73 20250207 TA-66 2399 Residual Noise 300R.png

    74 20250207 TA-66 5998 Residual Noise 300R.png

    75 20250206 TA-66 A1834 Residual Noise 300R.png

    Greatest differences are AC mains at 60, 120, 180, 240 and 300 Hz


    Multitones

    76 20250126 TA-66 6n5pj Multitone 300R 0 dBu total level.png

    77 20250208 TA-66 Sovtek 6AS7g Multitone 300R 0 dBu total level.png

    78 20250207 TA-66 2399 Multitone 300R 0 dBu total level.png

    79 20250207 TA-66 5998 Multitone 300R 0 dBu total level.png

    80 20250206 TA-66 A1834 Multitone 300R 0 dBu total level.png

    6n5pj is the cleanest but otherwise these measurements are substantially similar.
     
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  8. atomicbob

    atomicbob dScope Yoda

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    Tube Noise

    Buying vintage, NOS tubes is always like playing roulette. One tube may be quiet, well matched between sections and render music representative of the particular tube family. Another may have considerable section mismatch and/or suffer from audible hum, microphonics, and various tube noises during warmup and/or post warmup.

    The Sovtek 6AS7g I acquired has a low level glassine shimmer in its left channel, near threshold of audibility. So far a Herbie’s Audio Lab Rx damper appears to have reduced the shimmer to below audible threshold.

    Chatham 2399 warmup and residual noise
    Tinks can be heard during first 10 minutes of warmup at approximately 1 minute intervals. After warmup there are residual glass like sounds in right channel, very close to audibility threshold. When no music is playing and the room is VERY QUIET I can hear them. Even the slightest room noise such as HVAC running softly masks these glass sounds.

    Attached are recordings of the tinks and glass sounds. Each one has been recorded at 94 dBSPL = -6 dBFS and then digital gain applied to create a version 30 dB louder. To hear in normal playing context, play a typical music track and adjust sound level for normal perceived loudness. Then play the 94 dBSPL recording. It may sound faint or not be audible at all in your specific environment. Play the 64 dBSPL with the same amplifier sound level and the sounds will become apparent along with AC mains hum.

    Standard measurement suites pdf files for the amps attached

    Please note GEC A1834 shows a FAIL result. Several tests results failed due to the high 60Hz and 120Hz AC mains noise being above -72 dBu or 0.025%.
     

    Attached Files:

    Last edited: Feb 11, 2025
  9. atomicbob

    atomicbob dScope Yoda

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    reserved for additional data and/or corrections
     
  10. Beefy

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    Fantastic stuff!

    The similarities and differences between the 2399 and the 5998 are interesting, because they are the exact same tube. All 5998 and 2399 were were manufactured by Tung Sol, with different labels. The highly desirable Western Electric 421A is almost universally considered to also be the same tube, but cherry picked for high Gm and matched sections.

    I've never really seen (or sought for) data on the same tube being measured for distortion across tubes and individual sections, so this is really very handy. And 'regular' tube measurements/tests likely tell us very little.
     
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  11. dasman66

    dasman66 Self proclaimed lazy ass - friend

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    I always thought that as well, but have never seen them side by side before. The photo sure makes it look like the plates on the 2399 are shorter than the 5998... but that could just be an optical illusion...
     
  12. Beefy

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    There's lots on the Bottlehead forums about this. Differences in construction seems to be present across manufacturing dates, but both tube types manufactured at the same time are almost certainly the same.
     
  13. Wilewarer

    Wilewarer Almost "Made"

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    The multitone charts are interesting to me. It's not just that the Shuguang 6n5pj is cleaner, it has a descending pattern on the grass that doesn't seem to level out within that frequency range, whereas the other ones all level off by 1k.
     
  14. atomicbob

    atomicbob dScope Yoda

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    dual triode assembly is positioned higher in the 2399 than the other tubes. Look closely at the top neck on the 2399. Plate size is the same as 5998.
     
  15. purr1n

    purr1n Desire for betterer is endless.

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    The awesome thing about the 2399 and 5998: Post-pandemic, they are unobtanium! So less to worry about!
     
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  16. joch

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    I’m thinking tubes are sometimes like expensive wines. There’s probably some bias because of a vintage or justification of the price spent on rarities, and I bet most people can’t tell the difference or would care if they didn’t know. That bloominess is a feature? Those recessed mids give out a fun v shaped sound? That warmpoo is a signature warmth? Ooh I like that, uh huh.

    Joking aside, I think you guys are right…it’s the implementation, the caps, the OPTs, etc. If the amp doesn’t sound good with so-so tubes, putting WE, Mullard, Tung-Sol etc probably won’t help that much. And let’s not look down on a Shuguang, or a Gold Lion, or a JJ. These probably pair well with the right equipment, and without the NOS nervosa.

    Anyway, pass me the Two Buck Chuck while having a nice dinner with good friends.
     
  17. Wilewarer

    Wilewarer Almost "Made"

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    Honestly, the TA-66 these tubes are being compared in probably -is- the right equipment that the Shuguang pairs well with. I can't believe someone would design an amp like that for mass production if they didn't have access to a large stock of reasonably-priced tubes that they know for sure will allow the amp to sound good.
     
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  18. atomicbob

    atomicbob dScope Yoda

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    Updated first post to include links to very detailed comparisons in 2015 thread on Bottlehead Crack
    Detailed listening descriptions may be found in this thread:
    6080 comparisons
    6AS7 comparisons
     
  19. atomicbob

    atomicbob dScope Yoda

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    Thomson-CSF 6080WA

    Thomson-CSF 6080WA (abbreviated T6080 for convenience) was measured to compare with previous 6AS7g tubes in posts above and can be seen in the picture found here

    Measurement methods and notes can be found in this post: xDuoo TA-66 technical measurements

    Amp used for tube evaluation and measurements: xDuoo TA-66
    DAC used for listening evaluation: Holo Audio Cyan 2
    Headphones: HD800S, HD800-JAR, HD800-Jmod

    Brief listening description:
    T6080 has a wonderful midrange similar to the A1834 while maintaining transient response and bass punch of the Sovtek 6AS7g yet having an airy quality like the Tung-Sol 5998. Each of those tubes performs the specific attribute better, however the T6080 is a very good balance of all attributes. This balance probably accounts for my current affinity towards the T6080 during listening sessions. As of this writing, it is still an available tube, though not at bargain prices anymore nor outrageous either, yet.

    Output Impedance (nominal approximation): 117 Ω
    Very similar to the A1834 at 116 Ω.

    Distortion Surfaces
    01 TA-66 Thomson-CSF 6080WA THD+N dist surface 300R - annotated.png

    02 TA-66 Thomson-CSF 6080WA D2 dist surface 300R - annotated.png

    03 TA-66 Thomson-CSF 6080WA D3 dist surface 300R - annotated.png

    04 TA-66 Thomson-CSF 6080WA 4+HD+N dist surface 300R - annotated.png


    Frequency Response
    05 TA-66 Thomson 6080WA frequency response 5Hz - 96KHz 300R 0dBu.png


    Bandwidth (transient response)
    06 SigGen TA-66 Thomson 6080WA 100 Hz square 2Vpp 1uS div 5MHz filter 300R - BW calc.png
    BW ~= 128 KHz


    1 KHz 0 dBu distortion suite
    07 TA-66 Thomson 6080WA A04 Distortion 300R 0dBu.png


    Residual Noise
    08 TA-66 Thomson 6080WA Residual Noise 300R.png


    Multitone
    09 TA-66 Thomson 6080WA Multitone 300R 0 dBu total level.png


    Measurement suite report pdf file attached below
     

    Attached Files:

  20. Johnny the Nose

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    I think Atomic Bob's work here is really cool. I've always wanted to see something like this with tubes. I've owned RCA 6as7g, Tung Sol 7236, Tung Sol 5998, Svetlana and some tubes just seem to work better than others - I wondered sometimes with 5998s if it was just the output-impedance-lowering-factor cleaning the sound up a bit on these OTL amps but there is just a lot going on, subtle quality differences that can ad up. I recognize that on a well-designed amp the tube shouldn't make or break it but there are obvious differences that can become strong preferences. As impractical and basically impossible as it may be - I'd love to see something like Atomic Bob's work on 6as7 equivalents with NOS 6sn7 tubes. It's such a crap shoot and hard to say if a lot of tube-specific-lore is just inherited subjective nonsense passed around to streamline sorting through the mess of it all like a bad game of telephone or sellers just hyping things up to profit on FOMO.
     
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