CV4085

The CV4085 is a low-noise audio pentode for low-level, low-frequency amplifiers under demanding mechanical conditions. The M-O Valve Company, GEC group, emphasizes low hum and microphony, shock/vibration resistance and statistical parameter control. Our sources identify the special-quality M8195 version of the EF86 family under the British Common Valve code and historically associate it with 1959. The primary sheet is Issue 2, April 1977: its edition date is not the type’s introduction.

The heater requires 6.3 V, 0.2 A, with stated current tolerance ±7.5%; retained descriptions specify indirect heating. The M-O pentode test point uses plate 250 V, screen 140 V, suppressor zero, control grid −2 V. Mean plate current 3.0 mA, range 2.15–3.85 mA; mean screen current 0.60 mA, tabulated maximum 0.85 mA; mean transconductance 2.0 mA/V, range 1.55–2.45 mA/V. Mean internal plate resistance 2.5 MΩ. About 99% of samples lie within starred ranges; internal amplification factor mean 38, range 34–42, covers about 94%. These are not all absolute individual limits.

The same statistical distinction accompanies noise measurements. Grid hum 8 µV, cathode hum 60 µV and hiss 5 µV are listed maxima of the marked groups, covering about 99%. They are equivalent RMS voltages referred to g1, measured in the Government Authority’s specified amplifier, not universal noise for every installation. Test-point grid current mean 0.05 µA, maximum 0.40 µA, has a note that all samples meet that limit. The test amplifier and conditions are part of the numbers’ meaning.

Triode connection places plate, screen and suppressor at 250 V. At bias about −5 V, cathode current is 4 mA, transconductance 2.0 mA/V, internal resistance 16.5 kΩ, amplification factor 33, distinct from pentode data. An audio pentode example at supply 300 V, load 100 kΩ, screen resistor 0.39 MΩ and cathode resistor 1 kΩ gives cathode current 2.45 mA, gain 116, output 64 V at 5% total distortion, following-stage grid resistor 330 kΩ. M-O specifies output/distortion at onset of positive grid current. The triode table at supply 300 V, load 100 kΩ, cathode resistor 2.2 kΩ, following load 330 kΩ gives 1.5 mA, gain 28.5, output 50 V at 3.8%. The table does not label those output voltages RMS; no unstated convention is added.

Absolute maxima distinguish supply from actual electrode voltage: plate/screen supplies 550 V, plate 300 V, screen 250 V. Plate dissipation 1 W, screen 0.2 W, cathode current 6 mA, DC heater-cathode voltage 100 V. External grid-cathode resistor maximum 3 MΩ above plate dissipation 0.2 W, 10 MΩ below 0.2 W; equality is not specified in that row. Bulb maximum 165 °C, continuous vibration 2.5 g, brief shock 500 g. Cold capacitances without external screening: grid-plate maximum 0.05 pF, grid to all except plate 3.8 pF, plate to all except grid 5.1 pF.

Construction is internally screened. Photographs show mesh around electrodes and a screen plate below the lower mica; close descriptions identify a double-helix heater to reduce external AC field. B9A connections viewed from underside: 1 g2, 2 screen, 3 cathode, 4/5 heater, 6 plate, 7 screen, 8 g3, 9 g1. M-O permits any mounting position and recommends a retaining device. Primary maxima are overall 56 mm, seated height 49 mm, diameter 22.2 mm; individual body measurements 47/48 mm and diameter 20 mm do not replace them.

Quality control uses discrete lots, comprehensive electrical/mechanical tests, limits normally tighter than ordinary domestic/commercial valves, and random samples for destructive and life testing. A lot is released only after required tests including life tests; no numerical guaranteed life in hours is assigned here. Our sources interpret specimen KB/Z markings as British qualification and M-O Valve Company, Hammersmith. CV4086 is described as a flying-lead version, distinct from CV4085’s B9A base. Documentation: The M-O Valve Company Limited, GEC, CV4085, Rugged Low Noise AF Pentode, Issue 2, April 1977, six pages of characteristics, applications, connections and curves.