diytube ST70 driver docs & mods (discontinued)

knowledge base for the classic Dynaco ST70

diytube ST70 driver docs & mods (discontinued)

Postby Shannon Parks » Wed Jan 25, 2006 8:28 pm

Manual:
diytube ST70 manual Rev B (with schematics and parts list)
Ned Carlson's tubezone ST70 manual

Cut-N-Paste BOM:

Mouser compatible Bill of Materials list Rev B
or
Mouser Direct Link *NEW*

New Addition:
Full Original Dynaco ST70 Manual hosted at Dynakitparts.com


Rev History
1/28/12 - Updated Mouser BOM. Replaced the 43K, 2W plate resistors and fixed the new LM334Z part number.
4/6/09 - Added a Mouser Bill of Material direct link.
3/6/07 - Added an assembly page and a phase note warning to the manual.
7/20/06 - BOM only changes:
-updated the 1M resistor to be compliant with the Mouser database
-subbed the LM234Z for the LM334Z (RoHS backorder)

These were updated January 25th, 2006 and introduce a revision of the ST70 parts list containing a some major changes to improve performance and stability. PCB is still Rev A, but the build is now Rev B. This work was done as research for the Mark IV driver.

Changes include the following (mpn# stands for Mouser part number):
Code: Select all
-Changed R13,R14 to 5.6K (mpn#71-RN60D-F-5.62K)
-Changed R15,R16 to 1M (mpn#71-RN55D-F-1.0M)
-Changed C1,C2 to 0.1uF,600V (mpn#75-715P600V0.1)
-Changed C3,C4 to 220pF (mpn#5982-15-500V220)
-Changed C5,C6 to 330pF (mpn#5982-15-500V330)
-Changed C7,C8,C9,C10 to 0.047uF,600V (mpn#75-715P600V0.047)
Last edited by Shannon Parks on Sat Jan 28, 2012 6:10 am, edited 10 times in total.
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Postby WA4SWJ » Thu Jan 26, 2006 7:05 am

Shannon,

What did you find with these changes? I haven't looked at the old vs new distortion figures, but I'm guessing those improved. Curious about the stability improvement too. What did you find there? Looks like I'll be sending out another Mouser order!

Thanks and keep up the absolutely great work!!

Regards,
Ed Long
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More Rev B Info

Postby Shannon Parks » Thu Jan 26, 2006 1:07 pm

Hi Ed,

I thought my previous tuning method was sufficient for stability with difficult loads - I was wrong. Thanks to Eric Hayes for calling me on it and encouraging me to get better at this. I've spent a considerable amount of time studying this over the last several weeks and am very pleased with the results.

The back half of the manual with distortion numbers hasn't been updated yet, nor have the square wave shots. Not sure what the THD+N will bear out (I've got a new audio analyzer), but the 10kHz square wave looks great.

- The step circuit at R13/R14 and C5/C6 along with the feedback capacitor at C3/C4 was changed to alter the high frequency cut off. The steepness of the roll-off of the slope is much improved and our cut-off is extended past 50kHz as well. The new procedure I used was different that my previous tuning method, and I will call it the 'Turner Method', named after Patrick Turner (note: others may have tuned using this exact method for eons, but Mr Turner is the only one to document it that I've found).

The Turner Method involves using a .22uF capacitor as the tuning test load during 10kHz square wave tests from the very start. It helps size the output capacitor appropriately, then you can tune the step circuit. Then you end up with a perfect square wave with a resistive load along with rock solid stability. Finally, the audio analyzer helps further tweak it for super steep slope.

- Increasing R15/R16 to 1M (from 475K) lowers the low cut-off for this circuit while the C7-C10 coupling caps going to .047uF makes that response higher. The poles are 'in tune' this way, and ends up giving us a nice flat response to 5Hz with a -3dB cut-off at 2.5Hz. The previous values caused some low frequency ripple that was not evident until you peeked under 10Hz.

Williamson lovers take note! If you plan a Williamson design with four stages be sure to pay close attention and have the appropriate function generator at hand. Proper sizing of the coupling caps is very important and bigger is not always better.

Shannon
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Postby TerrySmith » Sat Jan 28, 2006 7:02 am

Will any of these mods be relevant to the Eiclone, since it's basically the same circuit?
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Ike Changes

Postby Shannon Parks » Sat Jan 28, 2006 8:56 am

TerrySmith wrote:Will any of these mods be relevant to the Eiclone, since it's basically the same circuit?


Hi Terry,

Working on the Ike right now. Yellow_Light_Colorz_PDT_06

Shannon
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Postby mesherm » Sat Jan 28, 2006 3:33 pm

Shannon, do you perform the tuning on the entire amp or just the driver section? If the entire amp, what happens when different OP tubes and transformers are used? I ask because I am using your driver board in an amp I am building with 6BG6GTs with non-Dynaco OPTs.
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amp tuning

Postby Shannon Parks » Sun Jan 29, 2006 8:23 am

mesherm wrote:Shannon, do you perform the tuning on the entire amp or just the driver section? If the entire amp, what happens when different OP tubes and transformers are used? I ask because I am using your driver board in an amp I am building with 6BG6GTs with non-Dynaco OPTs.


Hi Mesherm,

The tuning is closed loop, so everything affects it, though I guess these mods will be useful for your design. Ultimately, modding DIYers can err on the side of caution and just apply less negative feedback, for example 12dB instead of 20dB, which makes it much easier for us to mod away and have fun.

Shannon
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Mods

Postby leadtower » Sun Jan 29, 2006 3:11 pm

Hi Shannon; the ST 70 Rev B; that is applicable with the SDS modded PS board do you know? Also can I get a recommendation for good 10ohm bias resistors?
Thanks Much Guys
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Re: Mods

Postby Shannon Parks » Mon Jan 30, 2006 7:08 am

leadtower wrote:Hi Shannon; the ST 70 Rev B; that is applicable with the SDS modded PS board do you know? Also can I get a recommendation for good 10ohm bias resistors?
Thanks Much Guys


Hi leadtower,

The changes don't affect the power supply directly. But with the smaller .047uF coupling caps we can now use the 600V rated ones more easily. This in turn permits easier DIY use at higher voltages or SS diode operation in the ST70, which the SDS board supports, I believe.

I have a huge roll of carbon comp 10 ohm resistors I think I received from a mixed Ebay lot - this will keep me busy for years.

Shannon
Last edited by Shannon Parks on Sat Jan 28, 2012 6:16 am, edited 1 time in total.
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Postby erichayes » Sat Aug 25, 2007 12:38 am

Hi All,

Just finished stuffing and installing the Rev B board in a friend's ST70, and discovered a typo in the resistance measurements chart: pin 2 to ground of V2 and V3 are stated as 475KΩ. They should be 1 meg.

This project took close to a year to complete--it originally was going to be a simple rebuild--and I discovered another phenomenon. The colors of the cloth leads of the output transformer secondaries can change and fade to the point of being non-recognizable. Fortunately, I had the foresight to mark the impedance on a couple of leads that were the same color on one OT, but I had to use the impedance bridge to figure out the other. Lesson learned: label ALL transformer leads, even if you're sure you're going to be reinstalling the transformers the next day. You'll save yourself a lot of grief.
Eric in the Jefferson State
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ST 70 leads

Postby leadtower » Sat Aug 25, 2007 8:36 am

Thats true; the colors are so faded I had to use a powerfull magnifier to take small samples of each lead and match them up. I also took resistance measurements of each lead to get two of the same as a reference point. It was crude but it worked.
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Re: OUT OF STOCK - diytube ST70 driver docs & mods *2/10/16

Postby EWBrown » Sat Sep 10, 2016 8:00 pm

I have three fully assembled Original Revision ST-70 driver boards, these are the brown "Rev A" boards, and not the newer red "Rev B"boards.
The major differences are a few resistor and capacitor values in the NFB loop and frequency compensation between the input stage (12AX7)
and the phase splitters (12AU7s), and the coupling caps to the EL34 grids (.22uF vs .047 uF).

I built these about 12 years ago, for a run of three ST-70s that I was planning to build for a couple of customers, but they changed their minds,
and went with factory built or solid state gear instead. So, I have three "orphans" looking for a good home(s).

I have two copies of the original manual, plus a couple of rev B manuals, including one from The Tube Zone.

/ed B in NC
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