Mullard CV4004 Matched Pair

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Mullard CV4004 / 12AX7 NOS NIB Matched Pair

Some tubes are bought because they work. Others are bought because they are the one people have been chasing for decades.

The Mullard CV4004 is one of those tubes.

Also known as the M8137, the CV4004 is the ruggedized British military version of the 12AX7 / ECC83. It was built for serious use, with a stronger construction than a standard consumer-grade preamp tube. Original Mullard CV4004 tubes are especially known for their distinctive box plate construction, which makes them easy to spot and highly desirable among collectors and tube users.

This listing is for a matched pair, making it a great choice for hi-fi preamps, phono stages, stereo gear, and guitar amps where balanced performance matters. The CV4004 has the same basic tube family as a 12AX7 / ECC83, but with a more premium, rugged build.

In audio gear, Mullard preamp tubes are loved for their rich midrange, smooth top end, and natural warmth. The CV4004 version gives you that classic Mullard character with a more controlled, stable presentation. It is a great option when you want vintage tone without a noisy or overly loose tube.

Best For

  • Hi-fi preamps
  • Phono stages
  • Vintage stereo gear
  • Guitar amp preamp positions
  • Phase inverter use
  • Replacing 12AX7 / ECC83 / CV4004 / M8137 tube types

Each pair is tested and matched for proper operation, balance, and life.

A rare matched pair of Mullard CV4004 box plate tubes — the rugged British 12AX7 made for serious vintage audio gear.

Need help deciding what tubes are best for you?

Use our plate cheat sheet and vacuum tube buyers guide to determine which tubes are best for you. Likewise, you are welcome to reach out to us directly at Support@FuzzAudio.com or by using our chat located on our shop page if you have further questions.

NOS vs. ANOS vs. Test NOS

These designations are strictly used to describe the cosmetic condition of our tubes. All tubes we sell test identically, come with 100% life and are guaranteed to work in your device!

If you are picky about owning an original box or wanting a specific logo please reach out to us before purchasing.

  1. NOS (New Old Stock) Tubes- are 100% brand new vintage tubes in mint cosmetic condition with show zero signs of use. You should expect a legible logo and clean straight pins. An original box is not guaranteed. 
  2. ANOS Tubes- otherwise known as “almost new old stock” tubes are basically new, but may show slight signs of use in cosmetic condition, such as a missing logo or slightly bent pins. An original box is possible but unlikely. 
  3. Test NOS Tubes- are used but still operating to specification with 100% life. These tubes are equal in function to NOS valves, yet they cost considerably less because they have cosmetic blemishes. If you plan to disappear your tubes into the back of an amplifier or do not care what the tubes look like, then this is cost effective choice. Blemishes and cosmetic imperfections do not affect tone…

Logos: Vintage tubes have a wide variety of logos printed on them. It is not uncommon to see a Mullard made tube with an RCA or GE logo, or a GE made tube with a Mullard logo. Your tube may come with a logo such are Heath Kit, Baldwin, Fisher, Hammond, Magnavox, Fender, Gibson, Silvertone, etc. but the true manufacturer of that tube will be correct to your order. If you are looking for a specific logo please reach out before purchase.

Testing Standards

Testing Standards

At Fuzz Audio:

  • We match all tubes for free!
  • All vintage tubes are guaranteed for one year.
  • All tubes are tested on a calibrated Hickok 539C, Amplitrex, Maxi-Matcher, or other industry standard tube analyzers.
  • Preamp tubes are matched by gain and mutual conductance (Gm)
  • Power tubes matched by mA and Gm
  • All tubes tested for microphonics and are guaranteed to work in your application.
  • If you require SUPER LOW noise tubes, please reach out. We will most likely meet your requirements for free.
  • Do not hesitate to reach out if you have any questions or concerns, we are here for you.

We guarantee NOS test scores, clean pins, tight matches, and ultimately that they work in your application or YOUR MONEY BACK!

Tubes in photos represent the tube you will receive. Getter construction or logo may differ. Logo may be partially missing or cleaned off. You may or may not receive a box. Tube type, manufacture, and NOS test quality will NOT change. If logos and original boxes are a deal breaker- please reach out before purchase we are always happy to accommodate.

Shipping details

1. General
Subject to stock availability. We try to maintain accurate stock counts on our
website but from time-to-time there may be a stock discrepancy and we will not
be able to fulfill all your items at time of purchase. In this instance, we will
fulfill the available products to you, and contact you about whether you would
prefer to await restocking of the backordered item or if you would prefer for us
to process a refund.

2. Shipping Costs
Shipping costs are calculated during checkout based on weight, dimensions and
destination of the items in the order. Payment for shipping will be collected
with the purchase.
This price will be the final price for shipping cost to the customer.

3. Returns
3.1 Return Due To Change Of Mind
Fuzz Audio will happily accept returns due to change of mind as long as a
request to return is received by us within 30 days of receipt of item and are
returned to us in original packaging, unused and in resellable condition.
Return shipping will be paid at the customers expense and will be required to
arrange their own shipping.
Once returns are received and accepted, refunds will be processed to store
credit for a future purchase. We will notify you once this has been completed
through email.
(Fuzz Audio) will refund the value of the goods returned but will NOT refund the
value of any shipping paid.

3.2 Warranty Returns
Fuzz Audio will happily honor any valid warranty claims, provided a claim is
submitted within 90 days of receipt of items.
Customers will be required to pre-pay the return shipping, however we will
reimburse you upon successful warranty claim.
Upon return receipt of items for warranty claim, you can expect Fuzz Audio to
process your warranty claim within 7 days.
Once warranty claim is confirmed, you will receive the choice of:
(a) refund to your payment method
(b) a refund in store credit
(c) a replacement item sent to you (if stock is available)

4. Delivery Terms
4.1 Transit Time Domestically
In general, domestic shipments are in transit for 2 - 10 days. We ship packages Tuesday-Thursday so plan accordingly. 

4.2 Transit time Internationally
Generally, orders shipped internationally are in transit for 4 - 22 days. This
varies greatly depending on the courier you have selected. We are able to offer
a more specific estimate when you are choosing your courier at checkout.