Matched pairs of Mullard Vacuum Tubes 12AU7 (2 units) and 12AX7 (2 units)

1. Mullard 12AU7 / ECC82

The 12AU7 is a medium-mu (gain) dual triode. In audiophile gear, it is rarely used for high-voltage gain; instead, it shines as a low-noise line amplifier, cathode follower, driver tube, or phase inverter because of its high current capability and low output impedance.

Core Electrical Specifications (Per Section)

• Amplification Factor ($\mu$): 17 to 20

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• Plate Resistance (rp​): ~7,700 $\Omega$ (at nominal operating points)

• Transconductance (gm​): 2,200 $\mu \text{mhos}$ ($2.2\text{ mA/V}$)

• Nominal Plate Current (Ia​): $10.5\text{ mA}$

• Max. Plate Voltage (Va​): 300 V

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• Max. Plate Dissipation (Pa​): 2.75 W

  Wikipedia

   

• Grid Voltage (Vg​): -8.5 V (Nominal bias)

Filament / Heater Ratings

• Series Wiring: 12.6 V @ 150 mA (Pins 4 and 5)

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• Parallel Wiring: 6.3 V @ 300 mA (Pins 4+5 tied, Pin 9 center tap)

Audiophile Design & Structural Variants

Vintage Mullards from the Blackburn factory (look for the “B” in the etched glass etched code) are prized for their incredibly rich mid-range and low microphonics.

• Long Plates (17mm) vs. Short Plates (14mm): The 1950s “Long Plate” versions have a massive soundstage and deep low-end, but can be slightly more prone to microphonics in high-gain positions. The 1960s “Short Plate” versions are structurally stiffer, offering lower background noise and an incredibly punchy, ink-black quiet background.

• CV491 / ECC802S: These are military/ruggedized premium versions of the Mullard 12AU7. They feature tighter factory matching between the two internal triodes and ultra-low noise floor, making them the holy grail for high-end preamps.

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2. Mullard 12AX7 / ECC83

The 12AX7 is a high-mu (gain) dual triode. This is the quintessential voltage-multiplication tube found in phono stages, line-stages, and the input positions (V1​) of power amplifiers.

Core Electrical Specifications (Per Section)

• Amplification Factor ($\mu$): 100

• Plate Resistance (rp​): ~62,500 $\Omega$

• Transconductance (gm​): 1,600 $\mu \text{mhos}$ ($1.6\text{ mA/V}$)

• Nominal Plate Current (Ia​): $1.2\text{ mA}$

• Max. Plate Voltage (Va​): 300 V

  Wikipedia

   

• Max. Plate Dissipation (Pa​): 1.0 W

• Grid Voltage (Vg​): -2.0 V

Filament / Heater Ratings

• Series Wiring: 12.6 V @ 150 mA

  Wikipedia

   

• Parallel Wiring: 6.3 V @ 300 mA

  Wikipedia

   

Audiophile Design & Structural Variants

Because the 12AX7 deals with ultra-low signals (especially in phono preamps), the structural era of the Mullard drastically changes the noise floor and presentation.

• The “Mullard Warmth”: Structurally, vintage Mullard 12AX7s utilize heavily coated unipotential cathodes and a unique grid geometry that rolls off harsh, sterile high-frequency transients while emphasizing second-harmonic richness.

• Mullard CV4004 / M8137: This is the box-plate, ultra-premium military variant. Instead of ladder or ribbed plates, it features a rigid “box” structure around the grid. For audiophiles, this tube offers the absolute lowest microphonics and the tightest imaging of any 12AX7 variant ever built.

• 10M Series: Released by Mullard as a gold-standard consumer line, these guaranteed low noise and a 10,000-hour lifespan with gold pins to reduce contact resistance.

Quick Reference Comparison for Circuit Tuning