Comments for page: PS Mods for ST-35

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Posted by Art Grannell March 16, 2016 - 12:16 pm
I hear from a couple sources that Triode Electronics continues to be out of stock on the PA-774. Triode’s claim is that they run cooler due to being wound with larger wire than the original Dynaco units is correct. It’s easily confirmed by simple resistance measurements of the primary and secondary windings.

Since I needed two of these transformers, I contacted Dynakit to inquire about their PA-774. Kevin stated that he “did not believe” that their transformer supplier was the same as Triode’s. He then took resistance measurements and gave me the information. This was quite similar to the data I had on the Triode transformers, so I ordered two.

When they arrived, I repeated the resistance tests, using a somewhat more accurate method, and found that the DynaKit transformers match or very slightly exceed the readings of the Triode transformers. Additionally, measured at 120 VAC input, the unloaded output voltage across the secondary windings are: Dynaco 660VAC, DynaKit 662 VAC, Triode 670 VAC.

Of course, at the center tap these voltages will be half of the above voltages.

---Art Grannell

Posted by Art Grannell February 14, 2016 - 08:13 pm
The EL84 heaters draw .76A each, 1.52A per pair, or just over 3A for a two channel amplifier. The 12DW7/7247 draws .3A each at 6.3V or .6A for a two channel amplifier. Altogether, this calculates to about 25% of the 100Watt power consumption shown by Dynaco.
Adding a separate filament transformer will reduce the load on main transformer allowing it to run cooler. My solution, if you haven’t previously seen it, is on this site at:

You mention building on a larger chassis which will help, since the original ST35 chassis was very cramped, hindering ventilation.

One other possibility is to use Triode Electronics PA774 replacement power transformer which is wound with larger wire than the original, (I have confirmed this) and results in cooler operation. I understand however, that this item is now temporarily “out-of-stock”.

I use both the Triode Electronics PA774, and the outboard SMPS heater supply. This combination results in a considerably cooler running power transformer.

---Art Grannell

Posted by Bill February 13, 2016 - 10:19 am
Actually, I can get 1,2,3,4,5,6, and 8 amp versions. 3 is probably enough and 4 would be overkill.

Posted by Bill February 13, 2016 - 10:08 am
One year later and I haven't done anything to my ST-35. I am scrounging some parts for a similar amp but housed in a much bigger chassis. (I'm not going to do mono blocks.)

As most people have noticed, the PA-774 runs blazing hot; and you mentioned that the output tubes consume 25% of the power. I will have room for a separate filament transformer and thought I would use the existing pair of 6.3VAC leads of the PA-774 for the 12DW7 heaters, and use a new Hammond 6.3VAC dual secondary filament transformer wired in parallel (as per the original) for the EL84 heaters. They have a 2 amp and 4 amp available.

I don't want to get this to get too complicated, but is this a sound idea?

Posted by Art Grannell February 16, 2015 - 07:46 pm
If you start on the mono, you can continue to enjoy your present amp. Build the first, get everything adjusted the way you want it, and then simply copy it for the second. I’d plan for a two tube socket VA and PI stage (ala DIY ST35) so that you are not committed to a 7247/12DW7.

---Art Grannell

Posted by Bill February 12, 2015 - 02:26 pm
Hey Art, by reading between the lines, I see where you're coming from.The 420 VDC did seem rather high but I was assuming that the first stage was higher still!

I would certainly use the EFB in any ST-35 version I build - that's a given. It's a little puzzling why SP isn't all on-board with this.

It's possible that I may disable one channel of my amp, double the 6k8 resistor and see what happens, or start with one mono and see how it goes.

Thanks, Bill

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