Rebuild for Dynaco ST-35

High Performance Power Supply Modifications to an ST-35 Including Individual EFB Bias

By Arthur Grannell

David Hafler released the Dynaco ST-35 two-channel power tube amplifier nearly 50 years ago as a lower cost alternative to the popular ST-70. Ignored for years by many tube enthusiasts, due to modest power output, it’s become a hot item since the start of the new millennium, with new releases of complete amplifiers, clone boards, transformers, tubes and other parts. This complete revision of an original illustrates various techniques to bring this diminutive classic to it’s full potential.

PS Mods EFB ST-35 icon

PS Mods Individ EFB Bias ST-35.pdf (1.4MB)

ST-35 caps

Handcrafted circuit boards and large reser- voir capacitors figure prominently in this rebuild of the ST-35.

Dynaco ST-35 EFB Individual adjustment schematic low

Includes detailed instructions for imple- menting EFB with separate adjustments for each output tube.


Reader Comments

Posted by Bill December 14, 2016 - 05:49 pm
I see them now. Thanks.

Posted by Arthur Grannell December 14, 2016 - 10:27 am
I usually assume that they will not have further use, so I cut them quite short, and insulate the ends with heat shrink. If you slide a piece of heat shrink over a wire end leaving 1/2 to 3/4 inch extra, you can press the extra amount closed with pliers, while heating the tubing. After it cools, release the pliers, and the end will stay closed.
If you look closely at the picture on page 5, you will see where I did this, and tucked the insulated ends under the (brown) AC cord.
Another possibility is to insulate the ends with Liquid Electrical Tape that seems to do a good job.

---Art Grannell

Posted by Bill December 13, 2016 - 10:02 pm
What did you do with the unused 6.3 VAC filament taps on the PA774 when you went with an outboard supply? Isolate the ends with heat shrink? Attach them to an unused terminal strip?

Posted by Art Grannell March 26, 2016 - 02:48 pm
The 12DW7/7247 actually has two 6.3 V heaters, that is, one for each triode section. If you wish to use 12.6 volts then you use pins 4 and 5, leaving pin 9 unconnected. In certain circuits, pin 9 may be connected either to ground or some other place, but that has nothing to do with actually powering the heaters.

If you wish to power the tube heaters with 6.3 volts, as in the original Dyna ST-35, then pins 4 and 5 are connected together. One 6.3 volt supply lead is attached to that point and the other supply lead is connected to pin 9.

Normally the two “outer leads” of a 12.6 volt or 6.3 volt transformer are used. The center tap of a transformer will only deliver one-half the current of the full winding. Sometimes the center tap is grounded or attached someplace else again, depending upon the actual circuit.

---Art Grannell

Posted by Bill March 24, 2016 - 11:04 pm
Oops, that would be one lead to pin 4, the other lead to pin 5, and the center tap to pin 9. Since this would be a series connection, then a 12.6VAC transformer would be required. Does this sound better?

Posted by Bill March 23, 2016 - 03:56 pm
Speaking of centre taps, I wonder how you would wire a 12DW7 with a separate filament transformer with centre tap. One wire to pin 4 (which is internally connected to pin 5, so each heater would have voltage) and the other wire going to pin 9, and the center tap wire going to ground. With a non-tapped transformer, you would use a 6.3V rating. Would you use the same with the centre tapped version? Or would you use 12.6V rating?

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