Phædrus Audio VF14M Electronic TubeTM is suitable for direct replacement of the Telefunken VF14 (VF14M) tube in Neumann U47 and U48 microphones and other derivative microphones originally specifying this device. See technical specification below. For more information and enquiries, email sales@phaedrus-audio.com.
Phædrus Audio Electronic TubesTM are designed so as to not require any modification to these valuable microphones. Download data sheet.
The full story of the VF14M and the development of the Phædrus Audio VF14M Electronic TubeTM equivalent is given here. And read this warning about fake VF14s on the market.
Phædrus Audio VF14M Electronic TubesTM are now available. Contact sales@phaedrus-audio.com for more information.
Here's an audio file to hear the difference between an alternative VF14M replacement (recorded first) and the Phædrus Audio VF14M Electronic TubeTM (recorded second).
Here's a second example which demonstrates the difference between a new VF14M Electronic TubeTM and an old German tube. In this example audio is fed to amplifier via a capsule simulator as explained here. The audio captured with the German tube as the amplifier (played first) has been further amplified in the DAW to bring it closer to the gain of the Phædrus Audio VF14M Electronic TubeTM (which appears second). Low mutual conductance (gm) is a classic fault in an old VF14 tube and this causes the impedance-converter gain to fall far below the Neumann specification for the U47 (U48). Unfortunately, noise performance in a tube is related to the reciprocal of mutual conductance, so low gm means lower-gain and higher-noise.
Phædrus Audio Electronic TubesTM are now available in two grades: a value grade - ideal for DIYers; and the Phædrus RED DOT Electronic TubeTM grade. Read more about RED DOT tubes here. Prices are here.
The curves below illustrate the frequency response* of the U47 with the Phædrus Audio VF14M Electronic TubeTM fitted (red curve). Note that the bass response does not fall away in relation to the original Neumann response (black curve - measured with an original VF14M tube).
Unfortunately, old VF14 tubes and the various substitutes based on "button" tubes both display this falling-bass effect (blue trace). This is because the grid-current is too high with the majority of these tubes and this "sucks charge" off the capsule - thereby reducing bass-response. The effect of this is quite audible in both the audio examples above. The lack of bass response is quite enough to modify the sound perceptibly.
The grid-current of the Phædrus Audio VF14M Electronic TubeTM on the other hand, is carefully designed to match that of Neumann's selected VF14 tubes (marked with the all-important M). Read more about the difference between VF14 and VF14M tubes here.
*Frequency response measured relative to a reference pressure microphone: sound source presented directly on axis with U47 in cardioid condition. The curves are offset by 2dB for clarity only.
Full measured specifications of the Phædrus Audio VF14M Electronic TubeTM are given here.
Having experimented with all the known substitute tubes for the VF14M, our ideas turned towards developing a circuit in which the characteristics of a brand-new tube were simulated rather than relying on the compromise of a vacuum tube "somewhat like" the original.
The idea was to develop a "black-box" approach in which a circuit would behave exactly as the VF14M does in terms of its input and output impedance, transfer characteristic and transfer-function and use this to substitute for the original tube.
Nowadays we are familiar with mathematical simulations of analogue circuits being employed to aid circuit-designers (as in the SPICE circuit analysis tools) and in software plug-ins for the musician. Why not apply the same methodology to emulate a tube?
The usual approach for the mathematical models for tubes is to treat them as voltage-controlled current sources whose output current is a weighted sum of controlling element voltages raised to the power of 1.5. The equation for anode current (Ia) in a triode looks like this,
Ia = K × (µ.Vgk + Vak) 3/2
Where K is a parameter known as perveance, µ has the usual sense of an amplification factor and Vgk and Vak are correspondingly the voltages on the grid and anode with respect to the cathode. (Remember that the VF14M, despite being a pentode tube, is wired as a triode inside the U47/48 microphones.)
This is a powerful model for a tube which is operating with a high anode voltage and which is well away from grid-current. In other words, when a tube is operating well within its operational envelope. Unfortunately, those are not the conditions under which the VF14M is operating inside the Neumann microphones. Instead, the tubes operate in a region where the exponent is not 1.5 and the effect of initial electron velocity (one of the components of grid current) is not insignificant in relation to anode-current. So, the eventual simulation is arranged so that the output signal current (Ia) is a function ( H ) of µ.Vgk (the AC signal voltage), Vak and a third term (Vi) such that,
Ia = H (µ.Vgk + Vak + Vi )n
where Vi is an equivalent voltage related to the initial electron velocity and n is a suitable exponent.
_small.png)
For applications information download data sheet for Phædrus Audio VF14M Electronic Tube
The full manual for the VF14M (complete with fitting instructions) is here.
Phædrus Audio VF14M Electronic TubesTM are now available. Contact sales@phaedrus-audio.com for more information.
Heater voltage: Underrun at 35V: fed by
1800Ω resistor from HT (Note 1.)
Heater current: 40mA
HT Supply: 90V (nominal top of R5)
Anode load: 100k
Anode volts: approx. 35V
Grid voltage: 0V
Grid resistor: 60 - 100MΩ
Cathode circuit: approx. 1V (derived across R3)
Notes:
1. Heater polarity must be respected. Thus terminal h+ must be positive with respect to terminal h-.
Address all mail to sales@phaedrus-audio.com
