ST-70 Welborne labs

knowledge base for the classic Dynaco ST70

ST-70 Welborne labs

Postby AAaldo » Sun Mar 29, 2020 8:50 am

Hello there, am so happy to post here!
My name's Aldo and I live in Paris/France
For some reason am in love with the Dynaco ST-70 amp which in my opinion is one of the best and when "properly" modified, it sounds even better!
I have listened to at least many different variations of theST70 including stock, and the ST70 with the Welbourne Labs mod is, to my ears, the best sounding,
A few years ago I purchased a Welborne ST70 kit with the transformer upgrade, it Took 9 months to arrive but got everythingI also purchased a new chassis, a new output transformer and few others stuff fromDynakitparts.
This has been my first build ever and all I can say is that this is not a game for the beginner or the faint-hearted The circuit is a mu-follower using a 6U8 and a 5751 per side. Installing the circuit board was so painful The results are well worth it. Everyone that heard it thought it sounded terrific hat's a real powerhouse !
With deeper, tighter bass, greatly extended high frequency response, and a much cleaner midrange
This amp also features the Curcio Audio Engineering bias control module, which uses an op-amp to set the EL34 bias precisely and holds it as set.

Here is what Welborne Labs says about this amp: "Alan Kimmel designed this modification to address these shortcomings of the ST70. Our mod 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 800uF 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 and unlike our competitor's mods, our's has no dangerous, high voltage, ugly, dust collecting parts exposed to the outside world. 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 a vintage ST70"

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#stayhome #besafe
as we say in French "Bon courage à tous ".
Last edited by AAaldo on Sun Mar 29, 2020 8:55 pm, edited 1 time in total.
AAaldo
 
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Re: ST-70 Welborne labs

Postby Auricle » Sun Mar 29, 2020 8:13 pm

Very interesting. Where did you find the Welborne PCB?

BTW the images you posted do not display.

Best,
Auricle
Auricle
 
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Location: Vancouver BC Canada

Re: ST-70 Welborne labs

Postby AAaldo » Sun Mar 29, 2020 8:56 pm

Auricle wrote:Very interesting. Where did you find the Welborne PCB?

BTW the images you posted do not display.

Best,
Auricle


Hey thank you I changed the images
the PCB is from when Melbourne labs was still a shop like 5/6 years ago
AAaldo
 
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Re: ST-70 Welborne labs

Postby AAaldo » Mon Mar 30, 2020 12:13 pm

huit found this today to go with the dynakit

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ECC83 / 12AX7A Mazda Belvu INOX made in France brand new 45 for the both tubes (banana)
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