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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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denny9167
Bob Latino
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    The Dan Steele ST-70 II amp rebuild

    Bob Latino
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    • Post n°1

    The Dan Steele ST-70 II amp rebuild Empty The Dan Steele ST-70 II amp rebuild

    Post by Bob Latino Wed Apr 03, 2013 11:58 am

    In the mid 1990's California tube tech Dan Steele was rebuilding Dynaco ST-70's with an interesting look. The amp had no driver board - everything was hard wired into the circuit. The circuit used a combination of Dynaco, Scott and Williamson circuits. This amp used an EF86 voltage amplifier and a 6J5 phase splitter/inverter. He also added more capacitance to bring the DC storage from 90 uF to 290 uF. The amp had a front switch, a bias meter and two "eye" tubes that indicated output level for each channel. Photos below ...

    A larger view of Dan's schematic seen below may be found here > Dan Steele ST-70 schematic

    Bob

    The Dan Steele ST-70 II amp rebuild SteeleST-70front_zps7aef0a00

    The Dan Steele ST-70 II amp rebuild Steeleinteriorwiring_zps12530848

    The Dan Steele ST-70 II amp rebuild Steeleschematic_zpse144914f


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    denny9167
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    • Post n°2

    The Dan Steele ST-70 II amp rebuild Empty Re: The Dan Steele ST-70 II amp rebuild

    Post by denny9167 Wed Apr 03, 2013 8:05 pm

    Who needs to spend loads of money for AR gear,when you have cool amps like that out there!!!
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    Roy Mottram
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    • Post n°3

    The Dan Steele ST-70 II amp rebuild Empty Re: The Dan Steele ST-70 II amp rebuild

    Post by Roy Mottram Wed Apr 03, 2013 9:10 pm

    I'll bet that rectifier tube didn't last very long with 100uF attached to it !!! pale
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    DesignInc
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    • Post n°4

    The Dan Steele ST-70 II amp rebuild Empty Re: The Dan Steele ST-70 II amp rebuild

    Post by DesignInc Sat Jun 06, 2020 7:19 pm

    I have a Dan Steele MKII and it does sound very good I'm running it through a Schiit's Saga+ pre-amp (c.2019) and Allison One speakers (c.1975)
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    DesignInc
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    • Post n°5

    The Dan Steele ST-70 II amp rebuild Empty Rectifier Tube

    Post by DesignInc Sat Jun 06, 2020 7:41 pm

    ... and yes the rectifier tubes are burning.

    Original Sylvania/USA 5AR4/GZ34 the original use is unknown and was replaced in
    1/2020 Electron/China 5AR4 new in was replaced in
    6/2020 Tung-Sol/Russian 5AR4 new

    I was kind of bummed when the Sylvania went, a friend gave me the China fire cracker that lasted 1100 hours (55hrs/week) I'll be interested to see how long the better quality Russian Tung-Sol lasts.
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    Bob Latino
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    • Post n°6

    The Dan Steele ST-70 II amp rebuild Empty Re: The Dan Steele ST-70 II amp rebuild

    Post by Bob Latino Sun Jun 07, 2020 9:17 am

    The problem with the Dan Steele version of the ST-70 is that Panor (who owned the Dynaco trade name at the time) came out with their own version of a redesigned ST-70 at virtually the same time. The mid 1990's Dynaco Stereo 80 (and Stereo 160) from Panor was a much more sophisticated version of the original Dynaco ST-70. These amps were the first Dynaco type amps that had triode/pentode ultralinear switches and is where I got the idea of using triode/pentode ultralinear switches on all the VTA amp kits. The Stereo 80 was rated at 40 watts per channel and got there by raising the 435 VDC B+ used on the original Dynaco ST-70 to 480 VDC. The Stereo 160 used a B+ of 540 VDC and if you read the fine print, the Stereo 160 was rated at just 75 watts per channel. Why didn't they call the Stereo 160, the Stereo 150 ? Marketing hype ? Check the 2014 forum link below for more info on the Panor Stereo 80 and the Stereo 160 ?

    Panor mid '90's Stereo 80 and Stereo 160 post

    Bob

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    rjpjnk
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    • Post n°7

    The Dan Steele ST-70 II amp rebuild Empty Re: The Dan Steele ST-70 II amp rebuild

    Post by rjpjnk Mon Jun 08, 2020 12:23 pm

    DesignInc wrote:... and yes the rectifier tubes are burning.

    Original Sylvania/USA 5AR4/GZ34 the original use is unknown and was replaced in
    1/2020 Electron/China 5AR4 new in was replaced in
    6/2020 Tung-Sol/Russian 5AR4 new

    I was kind of bummed when the Sylvania went, a friend gave me the China fire cracker that lasted 1100 hours (55hrs/week) I'll be interested to see how long the better quality Russian Tung-Sol lasts.

    Not sure why they went with the 100uf. I'd knock that first cap down to at least the rated max of 60uf, probably 40uf with today's tubes.
    Or... toss in a couple diodes and just leave the tube for effect. Wink
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    boeingpilot
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    • Post n°8

    The Dan Steele ST-70 II amp rebuild Empty Re: The Dan Steele ST-70 II amp rebuild

    Post by boeingpilot Mon Jun 08, 2020 7:07 pm

    Wonder what the life expectancy of the power transformers were, especially as it look likes the short stack one in the pics
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    • Post n°9

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