Pre Tube Reviews: featuring CV3986/6021

I have been very fortunate to be a beta tester, for a company some of you may be familiar with, this company is Tubemonger. I know a beta tester sounds weird for tubes, but Tubemonger has done something quite impressive. They have taken a tube that most of you are probably not familiar with and made into a very impressive “plug and play” 6922/ECC88 etc. replacement option. That tube is the CV3986/6021 sub-miniature flying leads tube.

This tube has some impressive “numbers” shown on the data sheets.
This tube is rated to 60,000 ft without pressurization. Has a high “G load” tolerances to an amazing 450 max G Shock rating/impact acceleration load.
These tubes are Very Tough! These tubes because of their construction are also highly resistant to microphonics.

Here is some info from a data sheet:

Also here is a link to further information from another data sheet:

Also some pictures of the CV3986/6021 compared(for size differences) to one of my Amperex Holland long plate foil D getter 7316 / 12au7 1958 (Beckman label)

This is my current favorite tube and using my more versatile Mjolnir 2 from Schiit Audio, for tube rolling. The “Mini-Mullard’s” (for my gear and my ears) beat the likes of all of my other tubes consisting of varying types of including 12AU7, 12AT7, 12AX7, 6922, 2C51, 396A etc.

See my current list of tubes here:

Ok now onto my review below
(These are my subjective opinions based on my gear and my ears YMMV)

(Disclaimer) “being a beta test means I received these tubes for review, the owner ask me for my subjective opinions on these tubes which are mentioned below. These are my opinions and my opinions alone”.

Regarding the CV3986/6021 (Mini-Mullards)

Okay let’s talk about a phenomenal tube,(for my setup at least) that tube would be Tubemonger’s new CV3986/6021 sub-mini tube, or as a friend of mine called it, the “Mini-Mullard” As we dive into details, let’s start with.

Detail: this tube is a detail monster, it dives into your recordings and digs out detail, flat out amazing detail retrieval!

Soundstage: this tube has a soundstage that is hard to find in a tube, what I mean by that, is the soundstage hits all three axis, it has width, depth and height. Most tubes will give you width but not touch on the others some tubes will give you width and depth, but very few tubes will give you all three.(atleast in my experience). Because of the soundstage this tube is very “lifelike” it gives you a sense of being “there”.

Imaging: again this tubes shines, it has some of the most “lifelike” imaging I have heard. It will “show you” where the instruments are located within your soundstage. Together with the above mentioned soundstage, it delivers an amazing listening experience. It adds “life” to your music.

Treble: the “Mini-Mullard’s” have very detailed and extended treble response.
To my ears, it is different then a typical Mullard’s treble response. It reminds me of a Telefunken ECC801S treble response, though not harsh as the Telefunken, with even more detail.

Midrange: the midrange response on these tubes is wonderful, as this tube does have the Mullard’s “lush midrange”

Bass: the “Mini-Mullard’s” Bass takes on more of a flat or linear response. The bass is detailed and “clean”. For most tracks it’s just perfect(To my ears, with my setup), not muddy or bloated, just nicely detailed and when called for, impactful.

In summary: these really are phenomenal tubes, they offer great imaging and detail, a soundstage with width, depth and height, which gives them a “lifelike” sound! Capable of amazingly lifelike vocals, with their great treble and Mullard’s “lush” midrange response. More of a flat to linear bass response that has just the right amount of presentation and impact with out being
overpowering, muddy, or bloated.

Other noteworthy mentions, the treble on these tubes can take a short time for your ears/brain to adjust to the sound, but when they do, the results are amazing!

The above are my impressions using the Monolith Liquid Platinum by Alex Cavalli. Below is my impressions on my Schiit Mjolnir 2

I was able to do a proper listening session with my Schiit Mjolnir 2 and on low gain all of the above mentioned review statements, stand true for the Mjolnir 2. The Mini-Mullard is an absolutely phenomenal tube!

Now where it gets interesting, on high gain on my Mjolnir 2 the Mini-Mullard really “POP” now this is to its benefit on most songs. However it’s also to its detriment on others.

For the benefits(most songs): the bass level increases while maintaining detail and “cleanliness” of the bass. It does however start to go away from being linear. The midrange has a slight “volume” boost as everything else about the midrange remains the same and treble takes on more energy.

For the detriments(a few songs, more related to poor mastering quality): the bass can be to much and become bloated, resulting in the complete loss of linearity. Midrange will lose definition and detail. Treble becomes harsh and edgy.

The above is only on high gain, as on low gain the tube sounds wonderful and does not exhibit any of the “high gain detriments” on the same tracks.

Side note of importance: I volume match the low and high gain settings on my Mjolnir 2 with the use of a sound level meter and test tracks. In this process I volume match to the decibel. For as accurate reviews as possible.

Side note: you really have hit a “Grand Slam” with these tubes! As they continue to impress me each time I listen to them.

Thank you!


These tubes are exclusive to Tubemonger and can be found here:


Hi Ian @Torq, Andrew @andrew, and Taron @taronlissimore

Please check my above post, as I want to make sure I am following all of the rules in this forum. As I don’t want to break any of the rules. Because I really appreciate this forum and the great community it hosts.

If something needs to be corrected or changed please let me know

Thank you!


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One question I have; it looks like extracting the tube and the (integral I’m guessing) adapter looks like it could be easy to damage tube or pull apart the tube from the adapter. Or is that just a socket saver? Your image above makes it look like the tube has long flexible leads that need soldiering off, or some sort of permanent attachment to that base, like the iFi GE JAN 5670W.

I had the same question. I would only extract that tube from an amp, by holding the attached socket saver. So, if using this with an amp that has recessed sockets, it might be a good idea to use an additional socket saver. I really want to try this tube, but have some reservations about using socket savers, and generally don’t use them at all. I don’t like to add anything to the chain, unless it is a necessity, but might have to break my own rule for this tube.

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Yes, Schiit’s top plate tube holes are really tight. If the socket saver it is mounted on had some handle extentions to stick up above the top plate a bit that would work. Or if I could find a tool that was thin enough to get around or under the socket saver maybe. If I read the poop sheet at Tubemongercorrectly the mini-tube is potted, as in permanently embedded in that socket saver. But trying to pull it up by the tube could break something lose inside the socket saver.

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Hmmm, it’s not like using 6021 tube in audio gear is anything new. Even many years ago, I remember Audio Aero gear sounding rather pleasant using 6021 subminiature tubes.

I suppose it’s a great business idea to stick an adapter and charge $120 EACH(!)

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Correct the tube is permanently mounted in the adapter filled with silicone.

For my Liquid Platinum and Mjolnir 2 I use Tubemonger’s socket savers and only remove the 6021 by its base. As the socket savers add the height to allow me to do this, since I tube roll, I have had the socket savers in my amps from the beginning.

I can say that for me personally, I would not remove the 6021 by the exposed tube portion only as I would only remove by the base. Though I can say that the tube is attached to the base very well.

I’ll get clarification on what Tubemonger recommends and post an update.


I’ve been using socket savers under all my Schiit tubed gear from the get go.

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@FLTWS also @Wes_S I heard back from Tubemonger and this is what he mentioned:

“Base is always a better idea but it is OK to remove by holding the glass portion also. Bond between tube and base is indestructible.”

“You have to be gentle though since you are handling glass.”


That is great news! Thanks for confirming that. I will be picking up a set, for sure.

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Ordered. I’ve got a lot of other equipment irons in the fire presently so it could be several weeks before I get to seriously audition.


Look forward to your thoughts on them!