Alan Kimmel designed this
modification for us, back the mid
'90's, to address these shortcomings of
the ST70. It incorporates an
advanced constant-current technique
(mu-stage) using a 6U8A tube to operate
a 5751 (or 12AX7) triode voltage gain
stage in the most linear manner
possible. In addition, the output of
the 5751 is optimally coupled to the
rest of the circuit and this coupling
is such that the 5751 sees virtually no
capacitance and no loading at all. The
amount of negative feedback is modest,
and is taken from the 8 ohm output tap
which is more effective than taking it
from the 16 ohm tap, as most do. This
circuitry results in a superior level
of detail in both the midrange and
top-end extension. Next, the output
tube's bias supply is regulated
resulting in a constant bias point with
minimum drift. And finally, over 500uF
of capacitance is added to the main
power supply circuit resulting in a
much tighter bass response and lower
distortion throughout the frequency
spectrum. Of course high quality parts
are used in our design which is all
conveniently mounted on a single
circuit board that fits snugly inside
the existing chassis. No
dangerous, ugly, dust collecting parts
exposed to the outside world.... like
everyone else's mod does. To say
the least, we are very happy with our
new release. The layout is
cleaner and easier to build, the noise
level is lower (less than 1mV measured)
and it just sounds better.
The result is not just another
ST70 modification, but a true
innovation that brings you closer to
the music than you ever thought
possible from an ST70. So pimp
on!
Our ST-70 Mod
Design
Our mod
includes the following
features:
The 5751/6U8A
mu-stage driver circuitry with
AudioCap, Solen and Wima polypropylene
film capacitors. 1% metal film
Dale/Vishay resistors.
New separate bias supplies for each
channel including new external
adjustment pot and test
point.
A redesigned power
supply featuring over 500uF of low
impedance capacitance while still
retaining the vacuum tube
rectification. The Dyna sounds
especially good with tube rectification
so leave the solid-state voltage
regulated supplies out of this
box!
All new wiring (Teflon)
and new ceramic tube sockets w/mounting
hardware (we include new sockets for
the output stage too). Plus we
include brand new replacement terminal
strips for the output
connections.
An aluminum plate, with
black anodized finish, to cover and
protect the circuitry. This cover
looks especially good with the chrome
chassis (New photos will be
uploaded soon).
Plus a new and detailed
assembly manual with step-by-step
instructions. If you can solder
and follow directions, you can do this
mod!A high grade, silk
screened, circuit board with all driver
stage, bias and power supply components
mounted onto it, including the tube
sockets. This layout minimizes
noise and eliminates the rat's nest
wiring found in most other mods.
It's an extremely clean layout, see
diagram below.
Bottom
View
A pretty
clean layout for an ST70
An easily made
hardwood strip to cover the original
holes. Looks nice, eh?
Yeah
baby, Pimp My 70
Our mod pretty
much replaces everything except for the
connectors, choke, output transformers
and power transformer. We
therefore recommend you perform the
following optional upgrades:
After you yank
out all of the existing components, why
not give your new amp a cosmetic
makeover? At a minimum we
recommend that you remove the
transformers, clean off the rust, give
them a new paint job (spray paint works
great), clean and polish the chrome and
replace the hardware.
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