Pre-amplifier (discontinued)
Power Supply.
The
preamp has a relatively simple power supply. Although separate, it features
a Mullard IW1 rectifier, 8mF Sangamo Oil, 10He200mA choke and again a 8mF Sangamo
oil capacitor. The rectifier is an old European tube globe, 4pin socket and
indirectly heated, providing a slow turn on... In my experience is the lesser
capacity in the power supply, the faster the sound. The choke is divided into
6 "chambers" to decrease the capacity on its windings. Then a 1 meter
cord feeds the actual pre-amp. The powersupply for the filaments of the 7788
is rectified/stabilised in the power supply chasis and travels the same cord
to the pre-amp. The connector used for this cable is made out of an old metal-type
tube, the receptical is an octal tube socket. It also has a stabilised power
supply, specially for the RIAA-correction pre-pre. This one is diodes rectified,
using Nichicon electrolytics. The tubes used for this circuit are 6080 and 12ax7.
The Pre-amp.
This is a mu-stage configuration around the VT67, the military version of
the 30. Its load is the 7788(E810F), a high transconductance penthode, thus
providing a very low output impedance. Coupling capacitor are UtraTone's, 0,1mF
for the output, and ICAR, 0,22 mF for "feeding" the 7788. The filament
of the VT67 is fed through a lead-acid battery, which makes it as quiet as can
be. The filament of the 7788 is being lifted in the power-supply chassis, and
is stabilised using a 78S05 with two diodes to ground, lifting the voltage
up to 6,2V. This is a VERY fine souding pre-amplifier! Normally used (with no
mu-stage), the VT67 can sound a little "thin", the mu-stage
does all the work, resulting in a tight bass....
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