User's Manual

Technical
The Bottle vacuum tube amplifier consists of a classic common cathode circuit using a current source
for the plate load, rather than the more commonly used simple resistor. The amplified signal is taken
from the plate and fed through a high-quality polypropylene capacitor, bypassed by a smaller value
polystyrene capacitor (Russian MIL spec), and output to our Blue custom hand-built transformer. Both
of these capacitors have a low dielectric absorption coefficient and a low equivalent series resistance.
These parameters are essential to high end audio and are not present in lower priced microphones.
D.A. (dielectric absorption) is reluctance on the part of the capacitor to give up stored electrons when
the capacitor is discharged. E.S.R. (equivalent series resistance) is the resistance composed of the
capacitor plate, lead, and termination resistances. If the capacitors used in the audio path possess high
values of D.A. and E.S.R., the result is a loss of accuracy and dynamic structure when reproducing the
finer details of the recorded source. Under these conditions, a definite “grunge” or hashy distortion is
added to the reproduced signal.*
The Blue hand-built Bottle microphone transformer is balanced, using a symmetrical two-bobbin
design (i.e. humbucking), with a transforming ratio of 13:1. With this ratio, the microphone achieves a low
output impedance, typically 110 ohms. The primary transformer windings are connected in series. The
transformer’s secondary windings are connected in parallel, and connect directly to the XLR output
pins. The transformer lamination has a high relative permeability, which is one of the factors contribut-
ing to low distortion and higher dynamic range. A permalloy circular housing covers the transformer,
thus providing additional isolation from external magnetic fields.
The Bottle microphone employs an EF86 pentode vacuum tube connected in a triode mode. The third
grid of the EF86 is connected with a cathode and is grounded. The plate voltage is at 65.0 VDC, and
the plate is 0.6 mA. The heater voltage is kept at an optimum 6V to ensure the longevity of the tube.
Each EF86 is tested by Blue engineers for self noise level, AC amplification factor, and THD distortion.
Measurements are taken twice on each tube— first after a 24-hour burn-in period. The tube is mounted
on custom made dampers to minimize microphonics. A custom rubber string is applied for dampening
and holding the tube firmly in its socket.
A Wilson current mirror, the most accurate solution for a current source circuit, uses four thermo-
coupled solid state devices. These discrete devices are separate from the signal path.
A precise servo system is used to maintain the DC plate voltage at a constant level. This keeps
the tube in peak condition, despite its natural aging. If a new tube is installed, this servo circuit keeps
the tube working optimally. This unique circuitry, at the time of this writing, has not been used in
any commercially available microphone. The servo system uses an integrated circuit with high time
constant value, and is external to the signal path.
The amplifier input is separated from the microphone capsule with a capacitance consisting of two
styroflex caps (one ten times the value of the other) mounted on special Teflon isolators. Both the
tube grid and the microphone capsule are controlled through high-quality, low-noise 0.5W 400Mohm
resistors. The capsule’s polarization voltage is fed through two low-pass filters to enable voltage is
adjustable by a multi-position switch on the power supply, with ranges from 30.1V (-6dB) up to 95.1V
(+4dB), with a mid-point of 60V (0dB).
All of the Bottle’s internal wiring is oxygen-free copper Teflon-insulated.
Power Stream Power Supply
No other commercially available tube mic power supply offers the unique features of the Blue Power
Stream. To assure the longevity of the vacuum tube and the stability of the tube microphone circuitry,
Blue has developed the Power Stream power supply with the new SOFT START feature.
In the past, power supplies have been designed to use both heater and plate voltages applied
simultaneously once power has been switched on. In this case, the high voltage potential on the plate