VINTAGE 1977 MULLARD 12AX7 / M8137 / CV4004 / M8137 / BOX PLATE MIL SPEC MADE IN THE UK MITCHAM FACTORY - VERY GOOD CONDITION

This vintage Mullard branded CV4004 / M8137 is a ruggedized version of the 12AX7 preamp tube. The box plates are immediately discernible from other 12AX7 variants. This makes for a more bold and upfront soundstage while retaining that famous Mullard smoothness. Made in Great Britain at the Mullard Mitcham factory for the English Military, the CV4004 / M8137 is one of the best 12AX7 variants you can use for any type of audio. Whether it's for your favorite guitar rig or HiFi system, you'll be pleased with the performance of this tube. A brilliant choice for the V1 tube slot on any amplifier for uncompromising tone.

Other screened codes on the tube read as follows 7744 KB/QDD Please see the photos.

The tube was manufactured in the 2nd week of July, 1977 at Mullard's Mitchham UK factory.

I bought the tube new as NOS production. Has less than 2 hours of use in its entire life.

TESTED VERY STRONG AND BALANCED ON MY MAXI-PREAMP 2 DIGITAL TUBE TESTER

TEST SPECS ARE AS PICTURED AND AS FOLLOWS:

TUBE 1

GAIN A (Mu)          100.1  
GAIN B (Mu)          98.5

CONDUCTANCE (Gm) A 1599
CONDUCTANCE (Gm) B 1619

RMS NOISE A         -84.7 db
RMS NOISE B         -83.7 db

🧠 Real-world takeaway

• The M8137 = one of the best 12AX7 variants ever made
• Highly sought after for:
  
• • Guitar amps
  • Hi-fi systems
• 🔌 What the Mullard M8137 actually is

• Type: Dual triode preamp tube
• Equivalent to: ECC83 / 12AX7 / CV4004  
• Origin: Made in the UK by Mullard (often Blackburn or Mitcham factories)
• Designation meaning:
  
• • M8137 = Mullard’s internal “special quality” version
  • CV4004 = British military spec number
• 👉 In plain terms:

This is a premium, military-grade 12AX7.

🧪 What makes it special

1. “Special Quality” (SQ) build

• Built for military/industrial use
• Tighter tolerances, longer life
• Lower noise and microphonics than standard ECC83  

2. Construction (why people chase these)

• Short box plates (most M8137)
• Thicker mica supports
• Extremely consistent triode sections

👉 Result:

• Less noise
• More stable gain
• Handles high-gain amps better

3. Tone (this is why guitarists care)

Compared to modern 12AX7s:

• Warm, rich mids (classic Mullard signature)
• Smooth top end (never harsh)
• Thick, musical overdrive
• Slightly compressed feel vs modern tubes

💡 In guitar terms:

• Makes amps sound more “vintage British”
• Think early Marshall / blues-rock tones

🎸 Why it matters for your gear

If you’re running:

A tube amp (especially Marshall-style)

• A vintage preamp
• Hi-fi gear

👉 The M8137 is often used in:

• V1 position (first gain stage) for tone shaping
• Phase inverter (for smoother breakup)

💰 Current value (this is where it gets interesting)

NOS / vintage originals (what collectors want)

Typical pricing:

• $80 – $150 → used / tested pulls
• $150 – $300 → strong NOS single
• $250 – $600+ → matched pairs / premium testing
• $800+ → rare collector-grade listings (often overpriced)

Modern reissues (NOT the same thing)

👉 These are:

• Made in Russia (New Sensor)
• Good tubes—but not the same tone or value

👍 Bottom line

• If yours is a real Mullard M8137 (UK-made):
  👉 Expect roughly $100–$300 per tube depending on condition
• 
  
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