Lo V - Episode I - Vanilla Overdrive
(englisch Text)
Thanks to Steve Husting for the Translation
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Design:
The "Vanilla Overdrive" is designed to run completely
on a 12V (or 12.6V) power supply. As such it is also interesting
for beginners, as no fatal voltages are present as they are
in "normal" amplifiers, and one can safely begin
with tube technology. The tubes used include the 12U7 (space
charge double triode) or the 12AU7. Other tubes (e.g. 12AX7)
which at 12V anode voltage run in 'starved plate mode,' perform
poorly or not at all and are not considered.
The voltage should be supplied via a stabilized, external
power adapter, which is capable of providing at least 0.35A.
0.5A or more is strongly recommended, so that enough reserve
is available. A supply voltage of 12.6 volts would be ideal,
but 12 volts works without problem - but it should not drop
below 12 volts! Of course, there is no reason not to build
a power supply for the unit.
The resistor values may appear "unusual," but they
are specified for the 12 volt power supply and are optimized
for the design objectives of the overdrive. Changing the resistor
values does not result in any tonal improvement, and in some
positions actually degrades the tone. Every unnecessary feature
has been deliberately omitted, as what isn't there can't cause
problems, and the circuit itself is as simple and effective
as possible.
As a note, the last stage is a cathode follower, which provides
no additional gain and as such the maximum overdrive possible
from 4 triodes is not obtained. Distortion is however not
the goal of this project; a separate circuit which does so
will be presented in the next project.
Sound:
From clean to crunch, classic overdrive tone a la AC/DC is
possible. The Vanilla Overdrive reacts very well to changes
to the volume on the guitar, which permits a very effective
control of the overdrive characteristics.
Use of 12U7 tubes produces a bit more bass - the sound has
a more solid foundation than that produced by 12AU7 triodes.
As such, when a heavy bass tone is desired, the 12U7 should
be used, otherwise the 12AU7 is fine.
Usage:
The Vanilla Overdrive can be used as a standalone preamp,
or as a lead boost, or to drive tube amps, which have little
or no overdrive themselves.
The Overdrive was tested with a variety of tube amps, including
a JTM45, with which the overdrive works very well. Finally,
it is possible to coax a few proper rock sounds out of the
amp and moderate volumes. The Overdrive should also work well
with old Fender (or similar) amps.
Construction:
Construction is in classic true point-to-point style, in
which most of the components (not as many as usual in "real"
tube circuit) can be mounted directly on the tube sockets.
The plate voltage B+ can be tapped directly from the heater
supply. Twisting the heater wires is unnecessary. Prior to
soldering, the components should be mounted on the sockets
to make sure that everything fits. Once all components are
placed properly, they may be soldered firmly in place. No
special order or layout is required, but as a rule, the connections
should be kept as short as possible, as in all tube circuits.
All resistors are 1/4 watt metal film. Carbon composition
resistors can be also used.
Tests have not shown any need for shielded cable. All test
overdrives worked without problem and without noise using
normal hookup wire.
There are no special requirements for the case, as the circuit
does not run hot and there are no heavy components. The box
must simply be large enough.
Mods:
The basic design does not includes a tone stack, so that
pure tube sound is produced. Adjusting C1 affects the bas
response of the signal. Larger values for C1 result in more
bass. Omitting C1 entirely will ensure the maximum bass reproduction.
If a tone stack is desired, it can best be added between
the points marked TS-A and TS-B on the schematic. The design
of the tone stack can be as desired; as a starting point,
the same tone stacks as those used for the TT-SAM can be used.
The Vanilla Overdrive is designed as a floor pedal with true
bypass. In bypass mode, the signal is routed around the tube
circuitry. It is also possible to place the in front of the
pot P1 or after C4, so that 1 or 2 gain stages remain in the
signal path. Try it and see if you like it.
Parts:
a basis kit is available in the
online-shop part-number "kit-vanilla".
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