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The Dynaco ST-70 is no doubt the most popular tube amp ever made and we've got the
most popular ST70 mod ever sold for it. After a year in seclusion the Welborne Labs/Alan Kimmel mod is back. We've upgraded and expanded the circuit
board design, upgraded the components, re-written the manual and improved the performance to a whole new level!
Why modify an old Dynaco ST70? While the original ST70 was a decent amplifier, by
today’s standards its sonic performance is somewhat mediocre. Its bass response is loose and the top-end lacks the detail required to make it a real
contender. These deficiencies can be attributed to its less than adequate power supply design and its simple but ineffective driver stage.
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).
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.
Plus a new and detailed assembly manual with step-by-step instructions. If you can solder and follow
directions, you can do this mod!
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. |
