Comments for page: EFB in a Dynaco ST-35

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Posted by Steve L. January 29, 2012 - 08:32 am
Hi Lydia, I guess instead of "cathode resistors," you mean grid resistors. These 470K resistors go to ground in the original Dynaco circuit. For the mod, they are disconnected from ground and connected to the wipers of the 2K pots shown in Dave's Fig.10). A good way to do this is to remove the 470K resistors from the Dynaco circuit board and put them (preferably new ones) in your mod board, with the pots. Wire the ends of the resistors to points "8" and "9" on the Dynaco circuit board.

Please be very careful about the dangerous high voltage. If the unit has been on and you need to work on it, discharge the filter caps, after powering down.

Posted by Lydia January 29, 2012 - 03:06 am
Thanks, Steve! this is starting to make sense now! I appreciate that!. I am a beginner at Hi Fi designing/building, however that's what I got my Degree in Design/Development; so I hope I can do this....The only other question, I have is when I hook up the cathode resistors to the EFB trimmers, I am a little fuzzy on how to wire this, even though I have read the articles, I guess once I get into it, I will go as far as I can and get help by showing examples also to other DIY members that have already built their assemblies and so forth. Again thanks. Lydia

Posted by Steve L. January 28, 2012 - 09:57 am
Hi Lydia, Well, Art thought that I should give this to Dave and he can't get to it for a few days, so I will answer. Let's refer to Dave's Fig.10 on the last page of his article ("A New Look..."). The LM337 is a negative regulator. Normally, the IN terminal is at a negative voltage and the ADJ terminal is at a negative reference voltage. The job of the regulator is to keep OUT about 1.2V more negative than ADJ. So OUT "follows" ADJ in voltage, but offset by 1.2V. You could think of the LM337 as a buffer, which lets a weak voltage at ADJ, control the OUT pin.

In this circuit, ground is the most negative voltage, so IN is connected to that. The positive supply, through the output tubes, is the load on the OUT of the LM337. The chip will make OUT follow the voltage on the ADJ terminal (offset by 1.2V), as long as the ground is more negative. It will be more negative, because the positive supply, through output tubes, is trying to pull OUT positive.

So the voltage set by the 5K pot will set the cathode voltage. To implement the individual tube adjustments, Dave has inserted the 2K pots, which have a small amount of voltage (maybe 1V) across them. This let's you tweak the grid voltages enough to balance the individual tubes. Mostly, the bias is set by the 5K pot, though.

Note that the tubes draw almost no current from the wipers of the 2K pots. Thus, the voltage across the pots does not change with the pot settings. This means that the pots cannot interact.

We will be happy to help if you have further questions.

Posted by Lydia Pawlak January 27, 2012 - 07:13 am
Hi Art,
thanks for the info "very detailed as needed" but I want to implement Dave's circuit for the biasing of each channel also. look at the article in dave's and see the #12 modification and see that you can add after the 13k resistor; a parallel (2) 2k trimmer pots to control biasing to a pair each channel 6BQ5's. I am concerned about this circuit because for one there is no voltage/current going into the input of the LM337 Regulator, and the parallel after the 13K resistor with the (2) 2K Trimmers that, if you change the bias on one channel, then it will affect the other at the same time?.... Can you elaborate and help me to understand this further.... I would appreciate that, Lydia

Posted by Art Grannell January 20, 2012 - 09:13 pm
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

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