Comments for page: A New Look

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Posted by Dave March 01, 2012 - 01:28 pm
Hi Steven --

Because the ST-35 remains predominantly a class AB1 amplifier even when operating in triode mode, there will still be significant benefits from using the EFB(tm) modification when operated that way.

While there will not be nearly the power supply concerns for the "Enhanced" element of EFB to address, the basic conversion to fixed bias operation that EFB affords will still provide for increased power output and lower distortion in triode mode over that of a conventional cathode bias installation. This is due to the greater current flow that EFB allows the output stage to draw when needed to.

Without EFB, most of the increased current needs become increased bias applied to the output tubes as well when cathode bias is used. This reduces the available current the stage can draw as it tries to faithfully follow the input signal, resulting in the increased distortion and reduced power output as mentioned above. Using the EFB modification will eliminate these concerns, and provide for optimized performance in triode mode, just as it does in UL mode.

Also, you will still enjoy the increased tube life that EFB allows for with a lower quiescent current operating point as well.

Thanks for your interest in the EFB modification, and good luck with your amplifier!


Posted by Steven Wood March 01, 2012 - 12:31 pm
I am going to buy the Autoregenisis ST-35 EFB/Cap board as soon as it reaches the market. I am wondering if you can run the upgraded ST35 in Triode mode, speakers are very efficient. Thanks

Posted by Dave November 09, 2011 - 10:20 pm
Hi Hrudu -- The EFB(tm) mod is just as appropriate for the ST-35 as it is for the SCA-35. On the ST-35, R-42 is located near the bottom of the can cap, and connects between one of its main terminals, and one of its ground lug terminals. If the original R-42 component is still installed in your amplifier, it is a 95 ohm 5 watt resistor.

While the article itself shows installation in an SCA-35, there are numerous examples on both the and sites showing how folks have installed the modification in their ST-35 amplifiers as well. If you go to those sites and search under "EFB", you will find plenty of discussion and very good pics showing different ways the EFB mod has been installed in these amplifiers. You can use them and the discussion to help with your installation. If I can help anymore, don't hesitate to ask. Good luck with it!


Posted by Hrudu November 09, 2011 - 11:41 am
Can I do this mode on a ST-35?
Where is the R42?

Posted by Bill February 09, 2011 - 10:17 pm
I thought I noticed the tie between the resistors before but wasn't sure. It makes perfect sense now.


Posted by David Gillespie February 09, 2011 - 07:20 pm
Hi Bill --
You got it correct in your last post. I tried to maintain the original Dynaco color code with the modification, so in the pic with the 5 ohm resistors, the green wire on the left goes to the cathodes of the left channel, the red wire on the right goes to the cathodes of the right channel, and the red wire in the middle goes to the output of the EFB regulator.

In the pic of the filter cap wiring, the old cathode bypass cap is the terminal at 11:00, and has three wires connected to it: a red wire identified as "to EFB output" which comes from the 337 regulator, a black wire which goes to the two hum balance controls, and an additional red wire which is the red wire coming up between the two 5 ohm resistors. It is not easy to see, but careful inspection will show that the two center terminals where the 5 ohm resistors are mounted are tied together to make for a common connection to both resistors at that point.

I apologize for the confusion, and hope this helps!


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