CV4085 (Special EF86) LOW-NOISE AUDIO PENTODE

The original GEC CV4085 is a special quality version of the EF86 audio voltage amplifier made by the company which produced the famous British KT beam-tetrodes.

Even compared with the classic EF86s from Telefunken and Mullard, the CV4085 is considered the best of all the EF86s equivalents. The original tubes are now rare and expensive.
The Phædrus Audio CV4085 Supertube™ matches the dimensions and performance parameters of the CV4085 tube and is supplied as a standard B9A substitute device.

It has a wide range of “plug-and-play” applications in: microphone preamplifiers; hi-fi equipment; and guitar amplifiers. In fact, almost anywhere a low-noise, low-distortion and zero microphony EF86 type pentode is required - see application notes.

For more information contact: sales@phaedrus-audio.com. Click to download the CV4085 (EF86) datasheet.

Applications

Three typical CV4085 (EF86) applications - circuits A, B & C

Three example CV4085 (EF86) pentode circuit schematics are given above. The Phædrus Audio CV4085 (Special EF86) device is a suitable substitute in all these circuit arrangements. Contact sales@phaedrus-audio.com for more information and advice.

• Circuit A is a typical guitar amplifier application with no negative feedback (see examples in Footnote. 2). The HT value and screen-dropper resistor is typical for these applications.

• Circuit B illustrates a typical "hi-fi" preamplifier application with the tube as the gain block in a virtual earth arrangement; the feedback voltage being returned to the control grid (see examples in Footnote. 3). HT voltages in preamplifiers are usually much lower than in guitar amplifier preamp’s: sometimes below Mullard operating envelope (HT <150V).

• Circuit C illustrates a typical application as the first vacuum tube in a power amplifier in which the tube works as a differential amplifier of the voltage appearing between its grid and cathode pins (see Footnote. 4). The CV4085 was also used in microphone preamplifiers in this arrangement too.⁵

Finally..... a quiet EF86 pentode!

Noise and microphony

The specification for a NOS CV4085 (EF86) under conditions of an HT voltage of 250V and an anode load of 100kΩ is for an equivalent input noise voltage of 2µV (RMS) in the frequency range 25 to 10kHz. The Phædrus Audio CV4085 (Special EF86) Supertube™ measures <1.5µV (RMS) under the same conditions. More importantly, tube microphony (the cause of feedback “howl” and “clang”) is not present in the Phædrus Audio CV4085 (Special EF86) Supertube™.

    Here is a recording of the noise level of a typical EF86 in our collection (tapped at one point to reveal the microphony). After the tone is a recording of the noise level of a Phædrus Audio CV4085 (Special EF86) at the same gain.

Footnotes

1. Valve Circuits for Audio Amplifiers. Mullard Ltd. 1959
2. GUITAR AMPS: Vox AC15/3 and above; Matchless DC 30 Series and others; Bad Cat Black Cat and others; Dr. Z; Blackstar Artisans; Ceriatone EF86 models.
3. Quad: 22 preamp (V1), Leak Point 1 preamp series (V1)
4. Leak TL/12 Plus (V1), TL/25 Plus (V1), Radford STA (& MA)12, 15, 25; Mullard: 20W (ultra-linear) amplifier - reference design (V1), Quad II V1 & V2.
5. EMI REDD.47 line amplifier (V1), Mullard: mixing preamplifier
6. A simple way to try a Phædrus Audio CV4085 in a socket in which the electrostatic shield is commoned with the cathode (and g3) is to short the cathode resistor by soldering a wire across it. This brings the whole shield, cathode, suppressor-grid network to 0V. The Phædrus Audio CV4085 does not require any volts on the cathode to operate correctly. This mod', of course, must be undone if an original tube is subsequently fitted.

Links

Address all mail to sales@phaedrus-audio.com

Maidstone . Kent . UK .