Posted by Art Grannell

January 20, 2012 - 09:13 pm

Lydia, Since the output stage of the ST-35 is electrically identical to the output stage of the SCA-35, the parts values I used are essentially the same as those described by David Gillespie in the his original article “A New Look at an Old Friend” that appears on this same website. On page 9, Figure 2 the schematic diagram of the SCA-35 is shown with all the component changes shaded in yellow. The only parts changes I made were: I substituted 2 precision 5 ohm resistors, one for each EL84 cathode pair, in place of the two pairs of parallel 10 ohm resistors that David used in the SCA 35. I replaced the 100 mfd-25 volt output tube-pair cathode capacitor, that’s enclosed in the 3 section aluminum can, with a small 33mfd 25 volt capacitor on the board that I made. The value isn’t critical; nearly any 25 to 100 mfd, 25 volt electrolytic capacitor should work well here. This is the capacitor that is shown next to the “removed” 95 ohm power resistor on the diagram. However, if you prefer, this change is not necessary. You can continue to use the original enclosed capacitor. The “trimmer” potentiometer, the voltage regulator, and the precision 5 ohm resistors are listed at the end of the article. Additionally, you will need (1) 4.7 mfd, 25 volt electrolytic capacitor between the LM337 adjustment pin and ground, and (1) 2 Watt, 360,000 ohm resistor between the high voltage supply and the trimmer top terminal and (1) 15,000 ohm ˝ watt resistor between the bottom trimmer terminal and ground. A 1000 ohm resistor must be placed between pin 2 of each output tube and the corresponding circuit board numbered eyelet (the eyelet numbering will vary between the SCA 35 and ST-35). Also a 100 ohm resistor needs to be placed between pin 9 of each output tube and the individual green or green/white wires from the output transformer. Pin 8 can be used as a “tie-point”. The (4) 1000 ohm and (4) 100 ohm resistors can be either ˝ or 1 watt metal film or carbon composition construction, if you prefer. Please note that V1, V2, and the #47 light bulb are not used in the ST-35, and there are a couple extra filter capacitors, with series resistors used in the SCA-35 power supply. These items do not affect this modification in any way, and can be ignored. I hope this helps you to complete this circuit modification! If you have specific questions about any part of these instructions, let me know.

Posted by Lydia Pawlak

January 19, 2012 - 06:42 am

Hi Art, I need to know what parts list is needed for the st-35? I have seen photos but not enough to justify all of the parts.... Can you help me walking me thru the EFB installation and how are those parts of the EFB wired up? I would sincerily hope you can help me. Thank You, Lydia

Posted by Art Grannell

June 28, 2011 - 05:20 pm

Rob, Thanks for bringing up this important question. Because others have asked the same, I've included the results of my listening tests as a addendum to the article. It appears directly above. ---ART---

Posted by Rob

June 25, 2011 - 01:21 pm

So how does it sound to you?

Posted by Art Grannell

March 31, 2011 - 08:13 pm

Tom, I understand JB Weld contains powdered steel which will give it some conductivity as well as extra strength. I measured it years ago and found that it is slightly conductive, so I thought I'd caution against an accidental unwanted problem. Of course, with an SCA-35 you would follow the procedure outlined by Dave. The fun thing about this hobby, though, is that you are free to devise your own solution to parts mounting and wiring as long as you follow the electrical diagram. The only exception to that might be to mount the new grid resistors directly on the tube sockets. Good luck with the project and enjoy the results! ---ART---

Posted by Tom Dickinson

March 31, 2011 - 07:26 am

Hello Arthur, Very interesting write-up with some good suggestions, ideas, and tips! I didn't know JB Weld was conductive!! I'm about to attempt this MOD on one of my 5 SCA-35s. I've been in touch with Dave Gillespie, but your write-up and your test results have further inspired me. I'm not 'up' to the level of you and Dave, either mentally or technically.....still got a LOT of learning to do! But, my 'enthusiasm' knows no bounds.....and I'm having a GREAT time finally doing some of the projects I've wanted to learn about since I was a kid (nearly 60 years ago). Anyway, thanks for your info.....it's a big help!! Tom - Wharf_Creek at Hotmail dot com. .