Comments for page: PS Mods for ST-35

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Posted by Art May 27, 2014 - 10:43 pm
“Sounds great but I would like to install your EFB modification to it.”

A clarification is necessary: The EFB modification is entirely Dave Gillespie’s innovation, and a very significant one, I might add! I only have illustrated a couple of different ways to add it to a particular circuit.

---Art Grannell

Posted by Jeremy May 27, 2014 - 10:17 pm
Thank you.. I will see what they have to say over at diy tube.

Posted by Art May 27, 2014 - 05:46 pm
I think the best place to find some help with this would be at Shannon Park’s “diytube” site. I do not currently have one of these DIY35 boards as a reference, so I would be only guessing, as far as parts layout and specific parts placement.

There are many experienced “builders” that frequent the site who are quite helpful when it comes to questions such as this. I suspect you will find at least one who has already accomplished exactly that which you are attempting.

---Art Grannell

Posted by Jeremy May 26, 2014 - 10:21 pm
I just built an ST-35 From A Triode Electronics Revision E board. I love it. Sounds great but I would like to install your EFB modification to it. My electronic schematic following skills are not necessarily great however I can soldier and build stuff following a diagram without too much of a problem.
The board itself was suppose to have been built with this mod in mind however my skills are just not at the point where I can figure it out. Is there a way you could mark up a "Parks Audio diy tube ST35 Rev E" Schematic for me so I can piece together this mod on the board? I have been trying to compare the REV E board with the standard ST35 schematic but I am getting thrown off by the individual bias adjustments on the Rev E board.

Posted by Bill May 26, 2014 - 01:11 pm
I use a CL90 in my amp, or actually before the amp in a box with a make-before-break switch kinda' thing, so I can switch it out of the circuit if needed. The added resistors is probably too much as you point out.

Posted by Art May 25, 2014 - 08:10 pm
Using resistors in series with rectifier diodes will indeed limit current through the power supply---at all times, which makes them a less than optimal choice. While they will help limit inrush current at start-up, they will also limit current to the output tubes when they require maximum current. A better solution is the inrush limiter that I showed in the article. These devices present maximum resistance when cold ( at amplifier turn on ), but quickly warm up, presenting a low series resistance, which helps to maintain good power supply regulation. An overview and selection of these devices can be seen at:

I used a CL-200 in the line on one side of the power transformer primary. Others use them in the secondary, either between the center tap and ground or in series with full wave or bridge rectifiers. While I didn’t have it in place in the photo, I placed a piece of heatshrink tubing over the device as an insulator to help retain heat and maintain low resistance.

The biggest disadvantage to these devices is that during a short power outage, such a “glitch” during a thunderstorm, the inrush limiter might not cool down, and develop the high resistance necessary to protect the amplifier components at a quick restart.

Despite this limitation, they are cheap and widely used today. Other solutions to controlling inrush current are more complex, involving control circuits and relays or triacs.

---Art Grannell

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